3.1 使用 Spring AMQP

这一节我们将探索使用Spring AMQP开发应用所用到的基础的接口和类。

3.1.1 AMQP 抽象类

简介

Spring AMQP包含若干个模块，模块在发行包中以一个jar包的形式存在。这些模块是：spring-amqp, and spring-rabbit。spring-amqp这个模块包含了org.springframework.amqp.core这个包。在这个包里，你会发现代表AMQP核心模块相关的类。我们的意图是提供一个通用的抽象模块，不依赖具体的AMQP broker 和客户端的实现。终端用户只需基于抽象层进行编码就能应对各种AMQP的实现。Broker具体的模块将使用这些抽象类实现具体功能，比如spring-rabbit。目前只有RabbitMQ的实现；然而除了RabbitMQ之外，这些抽象类也在Apache Qpid 这个.NET用的AMQP项目中得到验证。原则上，RabbitMQ的客户端可以用来使用相同版本协议的AMQP上，但是我们没有测试过其他的broker。

这里假设您已经熟悉了AMQP规范的基础知识了，如果不熟悉，需要看一下第五章“其他资源”中列出的资源。

Message（消息）

在0-8和0-9-1的AMQP规范中没有定义Message这个类或接口。相反，在执行一个操作，如basicPublish()时，内容是作为一个字节数组参数传递，消息属性是作为另一个单独的参数传递。Spring AMQP 定义了一个 Message 类 作为 AMQP 领域模型的一部分，Message 类为了让API可以变得简单，在一个实例中封装了body和properties。 Message类定义得非常简单。

public class Message {

    private final MessageProperties messageProperties;

    private final byte[] body;

    public Message(byte[] body, MessageProperties messageProperties) {
        this.body = body;
        this.messageProperties = messageProperties;
    }

    public byte[] getBody() {
        return this.body;
    }

    public MessageProperties getMessageProperties() {
        return this.messageProperties;
    }
}

MessageProperties接口定义了几个公用的属性，如messageId,timestamp, contentType等。这些属性可以通过调用setHeader(String key, Object value)这个用户自定义头方法来扩展。

Exchange（交换器）

Exchange表示为一个 AMQP Exchange,消息生产者将会把消息发送到Exchange。每个broker 的virtual host中的每个Exchange会有一个唯一的名字和一些其他属性:

public interface Exchange {

    String getName();

    String getExchangeType();

    boolean isDurable();

    boolean isAutoDelete();

    Map getArguments();

}

正如你所看到的，一个Exchange也有一个类型，通过ExchangeTypes这个类表示的，基础的类型有：Direct, Topic, Fanout和Headers。在核心包中，你会发现Exchange接口的这些类型的实现类。 不同的exchange类型，处理bindings和 Queues的行为不一样。举个例子，Direct exchange允许一个队列按固定的routing key（通常是队列的名称）绑定过来。Topic exchange支持routing key中使用和#通配符，表示不限字符，#表示一个字符。Fanout exchange 不需要考虑任何routingkey，它会把消息发给所有绑定在这个exchange上的所有队列。更多关于这些的信息和其他Exchage类型，可以查看第五章的其他资源 。

Queue （队列）

消息消费者将从Queue中接受消息。和exchange类一样，Queue这个AMQP的核心类也是设计成抽象模式。

public class Queue  {

    private final String name;

    private volatile boolean durable;

    private volatile boolean exclusive;

    private volatile boolean autoDelete;

    private volatile Map arguments;

    **/**
     * 这个队列是持久的，不排他，不自动删除.
     *
     * @param name the name of the queue.
     */**
    public Queue(String name) {
        this(name, true, false, false);
    }

    // Getters and Setters omitted for brevity

}

Queue的构造函数中接受一个队列名称的参数，依赖这个构造函数，admin template可以生成唯一名称的队列，这种队列可以作为“reply-to”的地址或者其他临时队列。这种自动生成的队列中，exclusive（排他）和autoDelete（自动删除）的属性都被设置成true。

Binding（绑定）

Given that a producer sends to an Exchange and a consumer receives from a Queue, the bindings that connect Queues to Exchanges are critical for connecting those producers and consumers via messaging. In Spring AMQP, we define a Binding class to represent those connections. Let’s review the basic options for binding Queues to Exchanges.

你可以通过一个固定routing key 把一个队列绑定到DirectExchange上

new Binding(someQueue, someDirectExchange, "foo.bar")

可以通过带通配符的routing key 把一个队列绑定到一个 TopicExchange

new Binding(someQueue, someTopicExchange, "foo.*")

可以通过不指定routing key 的方式，把一个队列绑定到FanoutExchange上

new Binding(someQueue, someFanoutExchange)

我们也提供一个支持流式api的BindingBuilder

Binding b = BindingBuilder.bind(someQueue).to(someTopicExchange).with("foo.*");

By itself, an instance of the Binding class is just holding the data about a connection. In other words, it is not an "active" component. However, as you will see later in Section 3.1.9, “Configuring the broker”, Binding instances can be used by the AmqpAdmin class to actually trigger the binding actions on the broker. Also, as you will see in that same section, the Binding instances can be defined using Spring’s @Bean-style within @Configuration classes. There is also a convenient base class which further simplifies that approach for generating AMQP-related bean definitions and recognizes the Queues, Exchanges, and Bindings so that they will all be declared on the AMQP broker upon application startup.

The AmqpTemplate is also defined within the core package. As one of the main components involved in actual AMQP messaging, it is discussed in detail in its own section (see Section 3.1.3, “AmqpTemplate”).

3.1.2 Connection and Resource Management

Introduction

Whereas the AMQP model we described in the previous section is generic and applicable to all implementations, when we get into the management of resources, the details are specific to the broker implementation. Therefore, in this section, we will be focusing on code that exists only within our "spring-rabbit" module since at this point, RabbitMQ is the only supported implementation.

The central component for managing a connection to the RabbitMQ broker is the ConnectionFactory interface. The responsibility of a ConnectionFactory implementation is to provide an instance of org.springframework.amqp.rabbit.connection.Connection which is a wrapper for com.rabbitmq.client.Connection. The only concrete implementation we provide is CachingConnectionFactory which, by default, establishes a single connection proxy that can be shared by the application. Sharing of the connection is possible since the "unit of work" for messaging with AMQP is actually a "channel" (in some ways, this is similar to the relationship between a Connection and a Session in JMS). As you can imagine, the connection instance provides a createChannel method. The CachingConnectionFactory implementation supports caching of those channels, and it maintains separate caches for channels based on whether they are transactional or not. When creating an instance of CachingConnectionFactory, the hostname can be provided via the constructor. The username and password properties should be provided as well. If you would like to configure the size of the channel cache (the default is 1), you could call the setChannelCacheSize() method here as well.

Starting with version 1.3, the CachingConnectionFactory can be configured to cache connections as well as just channels. In this case, each call to createConnection() creates a new connection (or retrieves an idle one from the cache). Closing a connection returns it to the cache (if the cache size has not been reached). Channels created on such connections are cached too. The use of separate connections might be useful in some environments, such as consuming from an HA cluster, in conjunction with a load balancer, to connect to different cluster members.

	Important
When the cache mode is CONNECTION, automatic declaration of queues etc. (See the section called “Automatic Declaration of Exchanges, Queues and Bindings”) is NOT supported.

It is important to understand that the cache size is (by default) not a limit, but merely the number of channels that can be cached. With a cache size of, say, 10, any number of channels can actually be in use. If more than 10 channels are being used and they are all returned to the cache, 10 will go in the cache; the remainder will be physically closed.

Starting with version 1.4.2, the CachingConnectionFactory has a property channelCheckoutTimeout. When this property is greater than zero, the channelCacheSize becomes a limit on the number of channels that can be created on a connection. If the limit is reached, calling threads will block until a channel is available or this timeout is reached, in which case a AmqpTimeoutException is thrown.

	Warning
Channels used within the framework (e.g. RabbitTemplate) will be reliably returned to the cache. If you create channels outside of the framework, (e.g. by accessing the connection(s) directly and invoking createChannel()), you must return them (by closing) reliably, perhaps in a finally block, to avoid running out of channels.

CachingConnectionFactory connectionFactory = new CachingConnectionFactory("somehost");
connectionFactory.setUsername("guest");
connectionFactory.setPassword("guest");

Connection connection = connectionFactory.createConnection();

使用XML时，配置文件看起来像这样的:

    
    
    
</bean>

使用 rabbit 命名空间，可以非常快速的创建出一个 ConnectionFactory :

In most cases this will be preferable since the framework can choose the best defaults for you. The created instance will be a CachingConnectionFactory. Keep in mind that the default cache size for channels is 1. If you want more channels to be cached set a larger value via the channelCacheSize property. In XML it would look like this:

    
    
    
    
</bean>

并且可以设置这个命名空间的 channel-cache-size 属性:

默认的缓存模式是 CHANNEL, 但是我们可以把缓存模式改成缓存连接;下面这个例子，我们设置了 connection-cache-size:

Host 和 port 数性也可以通过这个命名空间来设置

如果使用集群环境，可以使用 addresses 属性.

Here’s an example with a custom thread factory that prefixes thread names with rabbitmq-.

    
</bean>

配置基本的客户端连接工厂

The CachingConnectionFactory uses an instance of the Rabbit client ConnectionFactory; a number of configuration properties are passed through (host, port, userName, password, requestedHeartBeat, connectionTimeout for example) when setting the equivalent property on the CachingConnectionFactory. To set other properties (clientProperties for example), define an instance of the rabbit factory and provide a reference to it using the appropriate constructor of the CachingConnectionFactory. When using the namespace as described above, provide a reference to the configured factory in the connection-factory attribute. For convenience, a factory bean is provided to assist in configuring the connection factory in a Spring application context, as discussed in the next section.

Configuring SSL

Starting with version 1.4, a convenient RabbitConnectionFactoryBean is provided to enable convenient configuration of SSL properties on the underlying client connection factory, using dependency injection. Other setters simply delegate to the underlying factory. Previously you had to configure the SSL options programmatically.

    
    
</bean>

Refer to the RabbitMQ Documentation for information about configuring SSL. Omit the keyStore and trustStore configuration to connect over SSL without certificate validation. Key and trust store configuration can be provided as follows:

The sslPropertiesLocation property is a Spring Resource pointing to a properties file containing the following keys:

keyStore=file:/secret/keycert.p12
trustStore=file:/secret/trustStore
keyStore.passPhrase=secret
trustStore.passPhrase=secret

The keyStore and truststore are Spring Resources pointing to the stores. Typically this properties file will be secured by the operating system with the application having read access.

Starting with Spring AMQP version 1.5, these properties can be set directly on the factory bean. If both discrete properties and sslPropertiesLocation is provided, properties in the latter will override the discrete values.

Routing Connection Factory

Starting with version 1.3, the AbstractRoutingConnectionFactory has been introduced. This provides a mechanism to configure mappings for several ConnectionFactories and determine a target ConnectionFactory by some lookupKey at runtime. Typically, the implementation checks a thread-bound context. For convenience, Spring AMQP provides the SimpleRoutingConnectionFactory, which gets the current thread-bound lookupKey from the SimpleResourceHolder:

    
        <map>
            
            
        </map>
    </property>
</bean>

public class MyService {

    _@Autowired_
    private RabbitTemplate rabbitTemplate;

    public void service(String vHost, String payload) {
        SimpleResourceHolder.bind(rabbitTemplate.getConnectionFactory(), vHost);
        rabbitTemplate.convertAndSend(payload);
        SimpleResourceHolder.unbind(rabbitTemplate.getConnectionFactory());
    }

}

It is important to unbind the resource after use. For more information see the JavaDocs of AbstractRoutingConnectionFactory.

Starting with version 1.4, the RabbitTemplate supports the SpEL sendConnectionFactorySelectorExpression and receiveConnectionFactorySelectorExpression properties, which are evaluated on each AMQP protocol interaction operation (send, sendAndReceive, receive or receiveAndReply), resolving to a lookupKey value for the provided AbstractRoutingConnectionFactory. Bean references, such as "@vHostResolver.getVHost(#root)" can be used in the expression. For send operations, the Message to be sent is the root evaluation object; for receive operations, the queueName is the root evaluation object.

The routing algorithm is: If the selector expression is null, or is evaluated to null, or the provided ConnectionFactory isn’t an instance of AbstractRoutingConnectionFactory, everything works as before, relying on the provided ConnectionFactory implementation. The same occurs if the evaluation result isn’t null, but there is no target ConnectionFactory for that lookupKey and the AbstractRoutingConnectionFactory is configured with lenientFallback = true. Of course, in the case of an AbstractRoutingConnectionFactory it does fallback to its routing implementation based on determineCurrentLookupKey(). But, if lenientFallback = false, an IllegalStateException is thrown.

The Namespace support also provides the send-connection-factory-selector-expression and receive-connection-factory-selector-expression attributes on the <rabbit:template> component.

Also starting with version 1.4, you can configure a routing connection factory in a SimpleMessageListenerContainer. In that case, the list of queue names is used as the lookup key. For example, if you configure the container with setQueueNames("foo", "bar"), the lookup key will be "[foo,bar]" (no spaces).

Queue Affinity and the LocalizedQueueConnectionFactory

When using HA queues in a cluster, for the best performance, it can be desirable to connect to the physical broker where the master queue resides. While the CachingConnectionFactory can be configured with multiple broker addresses; this is to fail over and the client will attempt to connect in order. The LocalizedQueueConnectionFactory uses the REST API provided by the admin plugin to determine which node the queue is mastered. It then creates (or retrieves from a cache) a CachingConnectionFactory that will connect to just that node. If the connection fails, the new master node is determined and the consumer connects to it. The LocalizedQueueConnectionFactory is configured with a default connection factory, in case the physical location of the queue cannot be determined, in which case it will connect as normal to the cluster.

The LocalizedQueueConnectionFactory is a RoutingConnectionFactory and the SimpleMessageListenerContainer uses the queue names as the lookup key as discussed in the section called “Routing Connection Factory” above.

	Note
For this reason (the use of the queue name for the lookup), the LocalizedQueueConnectionFactory can only be used if the container is configured to listen to a single queue.

	Note
The RabbitMQ management plugin must be enabled on each node.

Caution

This connection factory is intended for long-lived connections, such as those used by the SimpleMessageListenerContainer. It is not intended for short connection use, such as with a RabbitTemplate because of the overhead of invoking the REST API before making the connection. Also, for publish operations, the queue is unknown, and the message is published to all cluster members anyway, so the logic of looking up the node has little value.

Here is an example configuration, using Spring Boot’s RabbitProperties to configure the factories:

@Autowired
private RabbitProperties props;

private final String[] adminUris = { "http://host1:15672", "http://host2:15672" };

private final String[] nodes = { "rabbit@host1", "rabbit@host2" };

@Bean
public ConnectionFactory defaultConnectionFactory() {
    CachingConnectionFactory cf = new CachingConnectionFactory();
    cf.setAddresses(this.props.getAddresses());
    cf.setUsername(this.props.getUsername());
    cf.setPassword(this.props.getPassword());
    cf.setVirtualHost(this.props.getVirtualHost());
    return cf;
}

@Bean
public ConnectionFactory queueAffinityCF(
        @Qualifier("defaultConnectionFactory") ConnectionFactory defaultCF) {
    return new LocalizedQueueConnectionFactory(defaultCF,
            StringUtils.commaDelimitedListToStringArray(this.props.getAddresses()),
            this.adminUris, this.nodes,
            this.props.getVirtualHost(), this.props.getUsername(), this.props.getPassword(),
            false, null);
}

Notice that the first three parameters are arrays of addresses, adminUris and nodes. These are positional in that when a container attempts to connect to a queue, it determines on which node the queue is mastered and connects to the address in the same array position.

Publisher Confirms and Returns

Confirmed and returned messages are supported by setting the CachingConnectionFactory's publisherConfirms and publisherReturns properties to 'true' respectively.

When these options are set, Channel s created by the factory are wrapped in an PublisherCallbackChannel, which is used to facilitate the callbacks. When such a channel is obtained, the client can register a PublisherCallbackChannel.Listener with the Channel. The PublisherCallbackChannel implementation contains logic to route a confirm/return to the appropriate listener. These features are explained further in the following sections.

	Tip
For some more background information, please see the following blog post by the RabbitMQ team titled Introducing Publisher Confirms.

Logging Channel Close Events

A mechanism to enable users to control logging levels was introduced in version 1.5.

The CachingConnectionFactory uses a default strategy to log channel closures as follows:

• Normal channel closes (200 OK) are not logged.
• If a channel is closed due to a failed passive queue declaration, it is logged at debug level.
• If a channel is closed because the basic.consume is refused due to an exclusive consumer condition, it is logged at INFO level.
• All others are logged at ERROR level.

To modify this behavior, inject a custom ConditionalExceptionLogger into the CachingConnectionFactory in its closeExceptionLogger property.

Also see the section called “Consumer Failure Events”.

3.1.3 AmqpTemplate

Introduction

As with many other high-level abstractions provided by the Spring Framework and related projects, Spring AMQP provides a "template" that plays a central role. The interface that defines the main operations is called AmqpTemplate. Those operations cover the general behavior for sending and receiving Messages. In other words, they are not unique to any implementation, hence the "AMQP" in the name. On the other hand, there are implementations of that interface that are tied to implementations of the AMQP protocol. Unlike JMS, which is an interface-level API itself, AMQP is a wire-level protocol. The implementations of that protocol provide their own client libraries, so each implementation of the template interface will depend on a particular client library. Currently, there is only a single implementation: RabbitTemplate. In the examples that follow, you will often see usage of an "AmqpTemplate", but when you look at the configuration examples, or any code excerpts where the template is instantiated and/or setters are invoked, you will see the implementation type (e.g. "RabbitTemplate").

As mentioned above, the AmqpTemplate interface defines all of the basic operations for sending and receiving Messages. We will explore Message sending and reception, respectively, in the two sections that follow.

See also the section called “AsyncRabbitTemplate”.

Adding Retry Capabilities

Starting with version 1.3 you can now configure the RabbitTemplate to use a RetryTemplate to help with handling problems with broker connectivity. Refer to the spring-retry project for complete information; the following is just one example that uses an exponential back off policy and the default SimpleRetryPolicy which will make three attempts before throwing the exception to the caller.

Using the XML namespace:

    
        
            
            
            
        </bean>
    </property>
</bean>

Using @Configuration:

_@Bean_
public AmqpTemplate rabbitTemplate();
        RabbitTemplate template = new RabbitTemplate(connectionFactory());
        RetryTemplate retryTemplate = new RetryTemplate();
        ExponentialBackOffPolicy backOffPolicy = new ExponentialBackOffPolicy();
        backOffPolicy.setInitialInterval(500);
        backOffPolicy.setMultiplier(10.0);
        backOffPolicy.setMaxInterval(10000);
        retryTemplate.setBackOffPolicy(backOffPolicy);
        template.setRetryTemplate(retryTemplate);
        return template;
}

Starting with version 1.4, in addition to the retryTemplate property, the recoveryCallback option is supported on the RabbitTemplate. It is used as a second argument for the RetryTemplate.execute(RetryCallback<T, E> retryCallback, RecoveryCallback&lt;T&gt;recoveryCallback).

	Note
The RecoveryCallback is somewhat limited in that the retry context only contains the lastThrowable field. For more sophisticated use cases, you should use an external RetryTemplate so that you can convey additional information to the RecoveryCallback via the context’s attributes:

retryTemplate.execute(
    new RetryCallback() {

        _@Override_
        public Object doWithRetry(RetryContext context) throws Exception {
            context.setAttribute("message", message);
            return rabbitTemplate.convertAndSend(exchange, routingKey, message);
        }
    }, new RecoveryCallback<Object>() {

        _@Override_
        public Object recover(RetryContext context) throws Exception {
            Object message = context.getAttribute("message");
            Throwable t = context.getLastThrowable();
            // Do something with message
            return null;
        }
    });
}

In this case, you would not inject a RetryTemplate into the RabbitTemplate.

Publisher Confirms and Returns

The RabbitTemplate implementation of AmqpTemplate supports Publisher Confirms and Returns.

For returned messages, the template’s mandatory property must be set to true, or the mandatory-expression must evaluate to true for a particular message. This feature requires a CachingConnectionFactory that has its publisherReturns property set to true (see the section called “Publisher Confirms and Returns”). Returns are sent to to the client by it registering a RabbitTemplate.ReturnCallback by calling setReturnCallback(ReturnCallback callback). The callback must implement this method:

void returnedMessage(Message message, int replyCode, String replyText,
          String exchange, String routingKey);

Only one ReturnCallback is supported by each RabbitTemplate. See also the section called “Reply Timeout”.

For Publisher Confirms (aka Publisher Acknowledgements), the template requires a CachingConnectionFactory that has its publisherConfirms property set to true. Confirms are sent to to the client by it registering a RabbitTemplate.ConfirmCallback by calling setConfirmCallback(ConfirmCallback callback). The callback must implement this method:

void confirm(CorrelationData correlationData, boolean ack, String cause);

The CorrelationData is an object supplied by the client when sending the original message. The ack is true for an ack and false for a nack. For nack s, the cause may contain a reason for the nack, if it is available when the nack is generated. An example is when sending a message to a non-existent exchange. In that case the broker closes the channel; the reason for the closure is included in the cause. cause was added in version 1.4.

RabbitTemplate只支持一个ConfirmCallback.

Messaging integration

Starting with version 1.4 RabbitMessagingTemplate, built on top of RabbitTemplate, provides an integration with the Spring Framework messaging abstraction, i.e. org.springframework.messaging.Message. This allows you to create the message to send in generic manner.

3.1.4 Sending messages

简介

可以使用以下的任一方法发送消息:

void send(Message message) throws AmqpException;

void send(String routingKey, Message message) throws AmqpException;

void send(String exchange, String routingKey, Message message) throws AmqpException;

我们从最后一个方法开始讨论，这个方法最好理解。允许运行时设置Exchange 和routing key，使用这个方法的例子看起来像这样的:

amqpTemplate.send("marketData.topic", "quotes.nasdaq.FOO",
    new Message("12.34".getBytes(), someProperties));

当大部分或全部的消息都发往同一个Exchange时，Exchange参数可以设置在template中，这样就可以用列表中的第二个方法发送消息。以下这个例子功能上和之前的一样:

amqpTemplate.setExchange("marketData.topic");
amqpTemplate.send("quotes.nasdaq.FOO", new Message("12.34".getBytes(), someProperties));

如果"exchange" 和 "routingKey"属性都设置在template中，可以使用只接受Message参数的方法:

amqpTemplate.setExchange("marketData.topic");
amqpTemplate.setRoutingKey("quotes.nasdaq.FOO");
amqpTemplate.send(new Message("12.34".getBytes(), someProperties));

A better way of thinking about the exchange and routing key properties is that the explicit method parameters will always override the template’s default values. In fact, even if you do not explicitly set those properties on the template, there are always default values in place. In both cases, the default is an empty String, but that is actually a sensible default. As far as the routing key is concerned, it’s not always necessary in the first place (e.g. a Fanout Exchange). Furthermore, a Queue may be bound to an Exchange with an empty String. Those are both legitimate scenarios for reliance on the default empty String value for the routing key property of the template. As far as the Exchange name is concerned, the empty String is quite commonly used because the AMQP specification defines the "default Exchange" as having no name. Since all Queues are automatically bound to that default Exchange (which is a Direct Exchange) using their name as the binding value, that second method above can be used for simple point-to-point Messaging to any Queue through the default Exchange. Simply provide the queue name as the "routingKey" - either by providing the method parameter at runtime:

RabbitTemplate template = new RabbitTemplate(); // using default no-name Exchange
template.send("queue.helloWorld", new Message("Hello World".getBytes(), someProperties));

Or, if you prefer to create a template that will be used for publishing primarily or exclusively to a single Queue, the following is perfectly reasonable:

RabbitTemplate template = new RabbitTemplate(); // using default no-name Exchange
template.setRoutingKey("queue.helloWorld"); // but we'll always send to this Queue
template.send(new Message("Hello World".getBytes(), someProperties));

Message Builder API

从 version 1.3 开始, 提供了MessageBuilder和MessagePropertiesBuilder来构建消息，他们提供了方便的流式方法创建消息或消息属性:

Message message = MessageBuilder.withBody("foo".getBytes())
    .setContentType(MessageProperties.CONTENT_TYPE_TEXT_PLAIN)
    .setMessageId("123")
    .setHeader("bar", "baz")
    .build();

或者

MessageProperties props = MessagePropertiesBuilder.newInstance()
    .setContentType(MessageProperties.CONTENT_TYPE_TEXT_PLAIN)
    .setMessageId("123")
    .setHeader("bar", "baz")
    .build();
Message message = MessageBuilder.withBody("foo".getBytes())
    .andProperties(props)
    .build();

MessageProperies 中的每一个属性都可以被设置. 另外还包括 setHeader(String key, String value), removeHeader(String key), removeHeaders(), and copyProperties(MessageProperties properties). Each property setting method has a set*IfAbsent() variant. In the cases where a default initial value exists, the method is named set*IfAbsentOrDefault().

提供了5个静态方法来创建初始的消息构建器:

public static MessageBuilder withBody(byte[] body) ![1](./Spring AMQP_files/1.png)

public static MessageBuilder withClonedBody(byte[] body) ![2](./Spring AMQP_files/2.png)

public static MessageBuilder withBody(byte[] body, int from, int to) ![3](./Spring AMQP_files/3.png)

public static MessageBuilder fromMessage(Message message) ![4](./Spring AMQP_files/4.png)

public static MessageBuilder fromClonedMessage(Message message) ![5](./Spring AMQP_files/5.png)

	直接把参数内容作为消息体的内容..
	The message created by the builder will have a body that is a new array containing a copy of bytes in the argument.
	The message created by the builder will have a body that is a new array containing the range of bytes from the argument. See Arrays.copyOfRange() for more details.
	The message created by the builder will have a body that is a direct reference to the body of the argument. The argument’s properties are copied to a new MessageProperties object.
	The message created by the builder will have a body that is a new array containing a copy of the argument’s body. The argument’s properties are copied to a new MessageProperties object.

public static MessagePropertiesBuilder newInstance() ![1](./Spring AMQP_files/1.png)

public static MessagePropertiesBuilder fromProperties(MessageProperties properties) ![2](./Spring AMQP_files/2.png)

public static MessagePropertiesBuilder fromClonedProperties(MessageProperties properties) ![3](./Spring AMQP_files/3.png)

	A new message properties object is initialized with default values.
	The builder is initialized with, and build() will return, the provided properties object.,
	The argument’s properties are copied to a new MessageProperties object.

With the RabbitTemplate implementation of AmqpTemplate, each of the send() methods has an overloaded version that takes an additional CorrelationData object. When publisher confirms are enabled, this object is returned in the callback described in Section 3.1.3, “AmqpTemplate”. This allows the sender to correlate a confirm (ack or nack) with the sent message.

Publisher Returns

When the template’s mandatory property is true returned messages are provided by the callback described in Section 3.1.3, “AmqpTemplate”.

Starting with version 1.4 the RabbitTemplate supports the SpEL mandatoryExpression property, which is evaluated against each request message, as the root evaluation object, resolving to a boolean value. Bean references, such as "@myBean.isMandatory(#root)" can be used in the expression.

Publisher returns can also be used internally by the RabbitTemplate in send and receive operations. See the section called “Reply Timeout” for more information.

批量

从 version 1.4.2 开始, 引入了 BatchingRabbitTemplate . 它是 RabbitTemplate 的子类，with an overridden send method that batches messages according to the BatchingStrategy; only when a batch is complete is the message sent to RabbitMQ.

public interface BatchingStrategy {

    MessageBatch addToBatch(String exchange, String routingKey, Message message);

    Date nextRelease();

    Collection<MessageBatch> releaseBatches();

}

	Caution
Batched data is held in memory; unsent messages can be lost in the event of a system failure.

A SimpleBatchingStrategy is provided. It supports sending messages to a single exchange/routing key. It has properties:

• batchSize - the number of messages in a batch before it is sent
• bufferLimit - the maximum size of the batched message; this will preempt the batchSize if exceeded, and cause a partial batch to be sent
• timeout - a time after which a partial batch will be sent when there is no new activity adding messages to the batch

The SimpleBatchingStrategy formats the batch by preceding each embedded message with a 4 byte binary length. This is communicated to the receiving system by setting the springBatchFormat message property to lengthHeader4.

	Important
Batched messages are automatically de-batched by listener containers (using the springBatchFormat message header). Rejecting any message from a batch will cause the entire batch to be rejected.

3.1.5 Receiving messages

Introduction

Message reception is always a little more complicated than sending. There are two ways to receive a Message. The simpler option is to poll for a single Message at a time with a polling method call. The more complicated yet more common approach is to register a listener that will receive Messages on-demand, asynchronously. We will look at an example of each approach in the next two sub-sections.

Polling Consumer

The AmqpTemplate itself can be used for polled Message reception. By default, if no message is available, null is returned immediately; there is no blocking. Starting with version 1.5, you can now set a receiveTimeout, in milliseconds, and the receive methods will block for up to that long, waiting for a message. A value less than zero means block indefinitely (or at least until the connection to the broker is lost). Version 1.6 introduced variants of the receive methods allowing the timeout to be passed in on each call.

	Caution
Since the receive operation creates a new QueueingConsumer for each message, this technique is not really appropriate for high-volume environments; consider using an asynchronous consumer, or a receiveTimeout of zero for those use cases.

There are four simple receive methods available. As with the Exchange on the sending side, there is a method that requires a default queue property having been set directly on the template itself, and there is a method that accepts a queue parameter at runtime. Version 1.6 introduced variants to accept timeoutMillis to override receiveTimeout on a per-request basis.

Message receive() throws AmqpException;

Message receive(String queueName) throws AmqpException;

Message receive(long timeoutMillis) throws AmqpException;

Message receive(String queueName, long timeoutMillis) throws AmqpException;

Just like in the case of sending messages, the AmqpTemplate has some convenience methods for receiving POJOs instead of Message instances, and implementations will provide a way to customize the MessageConverter used to create the Object returned:

Object receiveAndConvert() throws AmqpException;

Object receiveAndConvert(String queueName) throws AmqpException;

Message receiveAndConvert(long timeoutMillis) throws AmqpException;

Message receiveAndConvert(String queueName, long timeoutMillis) throws AmqpException;

Similar to sendAndReceive methods, beginning with version 1.3, the AmqpTemplate has several convenience receiveAndReply methods for synchronously receiving, processing and replying to messages:

 boolean receiveAndReply(ReceiveAndReplyCallback callback)
       throws AmqpException;

 boolean receiveAndReply(String queueName, ReceiveAndReplyCallback callback)
     throws AmqpException;

 boolean receiveAndReply(ReceiveAndReplyCallback callback,
    String replyExchange, String replyRoutingKey) throws AmqpException;

 boolean receiveAndReply(String queueName, ReceiveAndReplyCallback callback,
    String replyExchange, String replyRoutingKey) throws AmqpException;

 boolean receiveAndReply(ReceiveAndReplyCallback callback,
     ReplyToAddressCallback<S> replyToAddressCallback) throws AmqpException;

 boolean receiveAndReply(String queueName, ReceiveAndReplyCallback callback,
            ReplyToAddressCallback<S> replyToAddressCallback) throws AmqpException;

The AmqpTemplate implementation takes care of the receive and reply phases. In most cases you should provide only an implementation of ReceiveAndReplyCallback to perform some business logic for the received message and build a reply object or message, if needed. Note, a ReceiveAndReplyCallback may return null. In this case no reply is sent and receiveAndReply works like the receive method. This allows the same queue to be used for a mixture of messages, some of which may not need a reply.

Automatic message (request and reply) conversion is applied only if the provided callback is not an instance of ReceiveAndReplyMessageCallback - which provides a raw message exchange contract.

The ReplyToAddressCallback is useful for cases requiring custom logic to determine the replyTo address at runtime against the received message and reply from the ReceiveAndReplyCallback. By default, replyTo information in the request message is used to route the reply.

The following is an example of POJO-based receive and reply…​

boolean received =
        this.template.receiveAndReply(ROUTE, new ReceiveAndReplyCallback() {

                public Invoice handle(Order order) {
                        return processOrder(order);
                }
        });
if (received) {
        log.info("We received an order!");
}

Asynchronous Consumer

	Important
Spring AMQP also supports annotated-listener endpoints through the use of the @RabbitListener annotation and provides an open infrastructure to register endpoints programmatically. This is by far the most convenient way to setup an asynchronous consumer, see the section called “Annotation-driven Listener Endpoints” for more details.

Message Listener

For asynchronous Message reception, a dedicated component (not the AmqpTemplate) is involved. That component is a container for a Message consuming callback. We will look at the container and its properties in just a moment, but first we should look at the callback since that is where your application code will be integrated with the messaging system. There are a few options for the callback starting with an implementation of the MessageListener interface:

public interface MessageListener {
    void onMessage(Message message);
}

If your callback logic depends upon the AMQP Channel instance for any reason, you may instead use the ChannelAwareMessageListener. It looks similar but with an extra parameter:

public interface ChannelAwareMessageListener {
    void onMessage(Message message, Channel channel) throws Exception;
}

MessageListenerAdapter

If you prefer to maintain a stricter separation between your application logic and the messaging API, you can rely upon an adapter implementation that is provided by the framework. This is often referred to as "Message-driven POJO" support. When using the adapter, you only need to provide a reference to the instance that the adapter itself should invoke.

MessageListenerAdapter listener = new MessageListenerAdapter(somePojo);
    listener.setDefaultListenerMethod("myMethod");

You can subclass the adapter and provide an implementation of getListenerMethodName() to dynamically select different methods based on the message. This method has two parameters, the originalMessage and extractedMessage, the latter being the result of any conversion. By default, a SimpleMessageConverter is configured; see the section called “SimpleMessageConverter” for more information and information about other converters available.

Starting with version 1.4.2, the original message has properties consumerQueue and consumerTag which can be used to determine which queue a message was received from.

Starting with version 1.5, you can configure a map of consumer queue/tag to method name, to dynamically select the method to call. If no entry is in the map, we fall back to the default listener method.

Container

Now that you’ve seen the various options for the Message-listening callback, we can turn our attention to the container. Basically, the container handles the "active" responsibilities so that the listener callback can remain passive. The container is an example of a "lifecycle" component. It provides methods for starting and stopping. When configuring the container, you are essentially bridging the gap between an AMQP Queue and the MessageListener instance. You must provide a reference to the ConnectionFactory and the queue name or Queue instance(s) from which that listener should consume Messages. Here is the most basic example using the default implementation, SimpleMessageListenerContainer :

SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
container.setConnectionFactory(rabbitConnectionFactory);
container.setQueueNames("some.queue");
container.setMessageListener(new MessageListenerAdapter(somePojo));

As an "active" component, it’s most common to create the listener container with a bean definition so that it can simply run in the background. This can be done via XML:

Or, you may prefer to use the @Configuration style which will look very similar to the actual code snippet above:

_@Configuration_
public class ExampleAmqpConfiguration {

    _@Bean_
    public SimpleMessageListenerContainer messageListenerContainer() {
        SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
        container.setConnectionFactory(rabbitConnectionFactory());
        container.setQueueName("some.queue");
        container.setMessageListener(exampleListener());
        return container;
    }

    _@Bean_
    public ConnectionFactory rabbitConnectionFactory() {
        CachingConnectionFactory connectionFactory =
            new CachingConnectionFactory("localhost");
        connectionFactory.setUsername("guest");
        connectionFactory.setPassword("guest");
        return connectionFactory;
    }

    _@Bean_
    public MessageListener exampleListener() {
        return new MessageListener() {
            public void onMessage(Message message) {
                System.out.println("received: " + message);
            }
        };
    }
}

Starting with RabbitMQ Version 3.2, the broker now supports consumer priority (see Using Consumer Priorities with RabbitMQ). This is enabled by setting the x-priority argument on the consumer. The SimpleMessageListenerContainer now supports setting consumer arguments:

container.setConsumerArguments(Collections.
 singletonMap("x-priority", Integer.valueOf(10)));

For convenience, the namespace provides the priority attribute on the listener element:

Starting with version 1.3 the queue(s) on which the container is listening can be modified at runtime; see Section 3.1.17, “Listener Container Queues”.

auto-delete Queues

When a container is configured to listen to auto-delete queue(s), or the queue has an x-expires option or the Time-To-Live policy is configured on the Broker, the queue is removed by the broker when the container is stopped (last consumer is cancelled). Before version 1.3, the container could not be restarted because the queue was missing; the RabbitAdmin only automatically redeclares queues etc, when the connection is closed/opens, which does not happen when the container is stopped/started.

Starting with version 1.3, the container will now use a RabbitAdmin to redeclare any missing queues during startup.

You can also use conditional declaration (the section called “Conditional Declaration”) together with an auto-startup="false" admin to defer queue declaration until the container is started.

    <rabbit:bindings>
        
    </rabbit:bindings>



    

In this case, the queue and exchange are declared by containerAdmin which has auto-startup="false" so the elements are not declared during context initialization. Also, the container is not started for the same reason. When the container is later started, it uses it’s reference to containerAdmin to declare the elements.

Batched Messages

Batched messages are automatically de-batched by listener containers (using the springBatchFormat message header). Rejecting any message from a batch will cause the entire batch to be rejected. See the section called “Batching” for more information about batching.

Consumer Failure Events

Starting with version 1.5, the SimpleMessageListenerContainer publishes application events whenever a listener (consumer) experiences a failure of some kind. The event ListenerContainerConsumerFailedEvent has the following properties:

• container - the listener container where the consumer experienced the problem.
• reason - a textual reason for the failure.
• fatal - a boolean indicating whether the failure was fatal; with non-fatal exceptions, the container will attempt to restart the consumer, according to the retryInterval.
• throwable - the Throwable that was caught.

These events can be consumed by implementing ApplicationListener<ListenerContainerConsumerFailedEvent>.

	Note
System-wide events (such as connection failures) will be published by all consumers when concurrentConsumers is greater than 1.

If a consumer fails because one if its queues is being used exclusively, by default, as well as publishing the event, a WARN log is issued. To change this logging behavior, provide a custom ConditionalExceptionLogger in the SimpleMessageListenerContainer 's exclusiveConsumerExceptionLogger property. See also the section called “Logging Channel Close Events”.

Fatal errors are always logged at ERROR level; this it not modifiable.

Consumer Tags

Starting with version 1.4.5, you can now provide a strategy to generate consumer tags. By default, the consumer tag will be generated by the broker.

public interface ConsumerTagStrategy {

    String createConsumerTag(String queue);

}

The queue is made available so it can (optionally) be used in the tag.

See Section 3.1.14, “Message Listener Container Configuration”.

Annotation-driven Listener Endpoints

Introduction

Starting with version 1.4, the easiest way to receive a message asynchronously is to use the annotated listener endpoint infrastructure. In a nutshell, it allows you to expose a method of a managed bean as a Rabbit listener endpoint.

_@Component_
public class MyService {

    _@RabbitListener(queues = "myQueue")_
    public void processOrder(String data) {
        ...
    }

}

The idea of the example above is that, whenever a message is available on the org.springframework.amqp.core.Queue "myQueue", the processOrder method is invoked accordingly (in this case, with the payload of the message).

The annotated endpoint infrastructure creates a message listener container behind the scenes for each annotated method, using a RabbitListenerContainerFactory.

In the example above, myQueue must already exist and be bound to some exchange. Starting with version 1.5.0, the queue can be declared and bound automatically, as long as a RabbitAdmin exists in the application context.

_@Component_
public class MyService {

  _@RabbitListener(bindings = @QueueBinding(
        value = @Queue(value = "myQueue", durable = "true"),
        exchange = @Exchange(value = "auto.exch"),
        key = "orderRoutingKey")
  )_
  public void processOrder(String data) {
    ...
  }

  _@RabbitListener(bindings = @QueueBinding(
        value = @Queue(),
        exchange = @Exchange(value = "auto.exch"),
        key = "invoiceRoutingKey")
  )_
  public void processInvoice(String data) {
    ...
  }

}

In the first example, a queue myQueue will be declared automatically (durable) together with the exchange, if needed, and bound to the exchange with the routing key. In the second example, an anonymous (exclusive, auto-delete) queue will be declared and bound. Multiple QueueBinding entries can be provided, allowing the listener to listen to multiple queues.

Enable listener endpoint annotations

To enable support for @RabbitListener annotations add @EnableRabbit to one of your @Configuration classes.

_@Configuration_
_@EnableRabbit_
public class AppConfig {

    _@Bean_
    public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory() {
        SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();
        factory.setConnectionFactory(connectionFactory());
        factory.setConcurrentConsumers(3);
        factory.setMaxConcurrentConsumers(10);
        return factory;
    }
}

By default, the infrastructure looks for a bean named rabbitListenerContainerFactory as the source for the factory to use to create message listener containers. In this case, and ignoring the RabbitMQ infrastructure setup, the processOrder method can be invoked with a core poll size of 3 threads and a maximum pool size of 10 threads.

It is possible to customize the listener container factory to use per annotation or an explicit default can be configured by implementing the RabbitListenerConfigurer interface. The default is only required if at least one endpoint is registered without a specific container factory. See the javadoc for full details and examples.

If you prefer XML configuration, use the <rabbit:annotation-driven> element.

    
    
    
</bean>

Message Conversion for Annotated Methods

There are two conversion steps in the pipeline before invoking the listener. The first uses a MessageConverter to convert the incoming Spring AMQP Message to a spring-messaging Message. When the target method is invoked, the message payload is converted, if necessary, to the method parameter type.

The default MessageConverter for the first step is a Spring AMQP SimpleMessageConverter that handles conversion to String and java.io.Serializable objects; all others remain as a byte[]. In the following discussion, we call this the message converter.

The default converter for the second step is a GenericMessageConverter which delegates to a conversion service (an instance of DefaultFormattingConversionService). In the following discussion, we call this the method argument converter.

To change the message converter, simply add it as a property to the container factory bean:

_@Bean_
public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory() {
    SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();
    ...
    factory.setMessageConverter(new Jackson2JsonMessageConverter());
    ...
    return factory;
}

This configures a Jackson2 converter that expects header information to be present to guide the conversion.

You can also consider a ContentTypeDelegatingMessageConverter which can handle conversion of different content types.

In most cases, it is not necessary to customize the method argument converter unless, for example, you want to use a custom ConversionService.

If you wish to customize the method argument converter, you can do so as follows:

_@Configuration_
_@EnableRabbit_
public class AppConfig implements RabbitListenerConfigurer {

    ...

    _@Bean_
    public DefaultMessageHandlerMethodFactory myHandlerMethodFactory() {
        DefaultMessageHandlerMethodFactory factory = new DefaultMessageHandlerMethodFactory();
        factory.setMessageConverter(new GenericMessageConverter(myConversionService()));
        return factory;
    }

    _@Bean_
    public ConversionService myConversionService() {
        DefaultConversionService conv = new DefaultConversionService();
        conv.addConverter(mySpecialConverter());
        return conv;
    }

    _@Override_
    public void configureRabbitListeners(RabbitListenerEndpointRegistrar registrar) {
        registrar.setMessageHandlerMethodFactory(myHandlerMethodFactory());
    }

    ...

}

	Important
for multi-method listeners (see the section called “Multi-Method Listeners”), the method selection is based on the payload of the message after the message conversion; the method argument converter is only called after the method has been selected.

Programmatic Endpoint Registration

RabbitListenerEndpoint provides a model of a Rabbit endpoint and is responsible for configuring the container for that model. The infrastructure allows you to configure endpoints programmatically in addition to the ones that are detected by the RabbitListener annotation.

_@Configuration_
_@EnableRabbit_
public class AppConfig implements RabbitListenerConfigurer {

    _@Override_
    public void configureRabbitListeners(RabbitListenerEndpointRegistrar registrar) {
        SimpleRabbitListenerEndpoint endpoint = new SimpleRabbitListenerEndpoint();
        endpoint.setQueueNames("anotherQueue");
        endpoint.setMessageListener(message -> {
            // processing
        });
        registrar.registerEndpoint(endpoint);
    }
}

In the example above, we used SimpleRabbitListenerEndpoint which provides the actual MessageListener to invoke but you could just as well build your own endpoint variant describing a custom invocation mechanism.

It should be noted that you could just as well skip the use of @RabbitListener altogether and only register your endpoints programmatically through RabbitListenerConfigurer.

Annotated Endpoint Method Signature

So far, we have been injecting a simple String in our endpoint but it can actually have a very flexible method signature. Let’s rewrite it to inject the Order with a custom header:

_@Component_
public class MyService {

    _@RabbitListener(queues = "myQueue")_
    public void processOrder(Order order, _@Header("order_type")_ String orderType) {
        ...
    }
}

These are the main elements you can inject in listener endpoints:

The raw org.springframework.amqp.core.Message.

The com.rabbitmq.client.Channel on which the message was received

The org.springframework.messaging.Message representing the incoming AMQP message. Note that this message holds both the custom and the standard headers (as defined by AmqpHeaders).

@Header-annotated method arguments to extract a specific header value, including standard AMQP headers.

@Headers-annotated argument that must also be assignable to java.util.Map for getting access to all headers.

A non-annotated element that is not one of the supported types (i.e. Message and Channel) is considered to be the payload. You can make that explicit by annotating the parameter with @Payload. You can also turn on validation by adding an extra @Valid.

The ability to inject Spring’s Message abstraction is particularly useful to benefit from all the information stored in the transport-specific message without relying on transport-specific API.

_@RabbitListener(queues = "myQueue")_
public void processOrder(Message<Order> order) { ...
}

Handling of method arguments is provided by DefaultMessageHandlerMethodFactory which can be further customized to support additional method arguments. The conversion and validation support can be customized there as well.

For instance, if we want to make sure our Order is valid before processing it, we can annotate the payload with @Valid and configure the necessary validator as follows:

_@Configuration_
_@EnableRabbit_
public class AppConfig implements RabbitListenerConfigurer {

    _@Override_
    public void configureRabbitListeners(RabbitListenerEndpointRegistrar registrar) {
        registrar.setMessageHandlerMethodFactory(myHandlerMethodFactory());
    }

    _@Bean_
    public DefaultMessageHandlerMethodFactory myHandlerMethodFactory() {
        DefaultMessageHandlerMethodFactory factory = new DefaultMessageHandlerMethodFactory();
        factory.setValidator(myValidator());
        return factory;
    }
}

Listening to Multiple Queues

When using the queues attribute, you can specify that the associated container can listen to multiple queues. You can use a @Header annotation to make the queue name from which a message was received available to the POJO method:

_@Component_
public class MyService {

    _@RabbitListener(queues = { "queue1", "queue2" } )_
    public void processOrder(String data, _@Header(AmqpHeaders.CONSUMER_QUEUE)_ String queue) {
        ...
    }

}

Starting with version 1.5, you can externalize the queue names using property placeholders, and SpEL:

_@Component_
public class MyService {

    _@RabbitListener(queues = "#{'${property.with.comma.delimited.queue.names}'.split(',')}" )_
    public void processOrder(String data, _@Header(AmqpHeaders.CONSUMER_QUEUE)_ String queue) {
        ...
    }

}

Prior to version 1.5, only a single queue could be specified this way; each queue needed a separate property.

Reply Management

The existing support in MessageListenerAdapter already allows your method to have a non-void return type. When that’s the case, the result of the invocation is encapsulated in a message sent either in the address specified in the ReplyToAddress header of the original message or in the default address configured on the listener. That default address can now be set using the @SendTo annotation of the messaging abstraction.

Assuming our processOrder method should now return an OrderStatus, it is possible to write it as follow to automatically send a reply:

_@RabbitListener(destination = "myQueue")_
_@SendTo("status")_
public OrderStatus processOrder(Order order) {
    // order processing
    return status;
}

If you need to set additional headers in a transport-independent manner, you could return a Message instead, something like:

_@RabbitListener(destination = "myQueue")_
_@SendTo("status")_
public Message<OrderStatus> processOrder(Order order) {
    // order processing
    return MessageBuilder
        .withPayload(status)
        .setHeader("code", 1234)
        .build();
}

The @SendTo value is assumed as a reply exchange and routingKey pair following the pattern exchange/routingKey, where one of those parts can be omitted. The valid values are:

foo/bar - the replyTo exchange and routingKey.

foo/ - the replyTo exchange and default (empty) routingKey.

bar or /bar - the replyTo routingKey and default (empty) exchange.

/ or empty - the replyTo default exchange and default routingKey.

Also @SendTo can be used without a value attribute. This case is equal to an empty sendTo pattern. @SendTo is only used if the inbound message does not have a replyToAddress property.

Starting with version 1.5, the @SendTo value can be a SpEL Expression, for example…​

_@RabbitListener(queues = "test.sendTo.spel")_
_@SendTo("#{spelReplyTo}")_
public String capitalizeWithSendToSpel(String foo) {
    return foo.toUpperCase();
}
...
_@Bean_
public String spelReplyTo() {
    return "test.sendTo.reply.spel";
}

The expression must evaluate to a String, which can be a simple queue name (sent to the default exchange) or with the form exchange/routingKey as discussed above. The expression is evaluated once, during context initialization. For dynamic reply routing, the message sender should include a reply_to message property.

Multi-Method Listeners

Starting with version 1.5.0, the @RabbitListener annotation can now be specified at the class level. Together with the new @RabbitHandler annotation, this allows a single listener to invoke different methods, based on the payload type of the incoming message. This is best described using an example:

_@RabbitListener(id="multi", queues = "someQueue")_
public class MultiListenerBean {

    _@RabbitHandler_
    _@SendTo("my.reply.queue")_
    public String bar(Bar bar) {
        ...
    }

    _@RabbitHandler_
    public String baz(Baz baz) {
        ...
    }

    _@RabbitHandler_
    public String qux(_@Header("amqp_receivedRoutingKey")_ String rk, _@Payload_ Qux qux) {
        ...
    }

}

In this case, the individual @RabbitHandler methods are invoked if the converted payload is a Bar, Baz or Qux. It is important to understand that the system must be able to identify a unique method based on the payload type. The type is checked for assignability to a single parameter that has no annotations, or is annotated with the @Payload annotation. Notice that the same method signatures apply as discussed in the method-level @RabbitListener described above.

Notice that the @SendTo must be specified on each method (if needed); it is not supported at the class level.

@Repeatable @RabbitListener

Starting with version 1.6, the @RabbitListener annotation is marked with @Repeatable. This means that the annotation can appear on the same annotated element (method or class) multiple times. In this case, a separate listener container is created for each annotation, each of which invokes the same listener @Bean. Repeatable annotations can be used with Java 8 or above; when using Java 7 or earlier, the same effect can be achieved by using the @RabbitListeners "container" annotation, with an array of @RabbitListener annotations.

Container Management

Containers created for annotations are not registered with the application context. You can obtain a collection of all containers by invoking getListenerContainers() on the RabbitListenerEndpointRegistry bean. You can then iterate over this collection, for example, to stop/start all containers or invoke the Lifecycle methods on the registry itself which will invoke the operations on each container.

You can also get a reference to an individual container using its id, using getListenerContainer(String id); for example registry.getListenerContainer("multi") for the container created by the snippet above.

Starting with version 1.5.2, you can obtain the id s of the registered containers with getListenerContainerIds().

Starting with version 1.5, you can now assign a group to the container on the RabbitListener endpoint. This provides a mechanism to get a reference to a subset of containers; adding a group attribute causes a bean of type Collection<MessageListenerContainer> to be registered with the context with the group name.

Threading and Asynchronous Consumers

A number of different threads are involved with asynchronous consumers.

Threads from the TaskExecutor configured in the SimpleMessageListener are used to invoke the MessageListener when a new message is delivered by RabbitMQ Client. If not configured, a SimpleAsyncTaskExecutor is used. If a pooled executor is used, ensure the pool size is sufficient to handle the configured concurrency.

The Executor configured in the CachingConnectionFactory is passed into the RabbitMQ Client when creating the connection, and its threads are used to deliver new messages to the listener container. At the time of writing, if this is not configured, the client uses an internal thread pool executor with a pool size of 5.

The RabbitMQ client uses a ThreadFactory to create threads for low-level I/O (socket) operations. To modify this factory, you need to configure the underlying RabbitMQ ConnectionFactory, as discussed in the section called “Configuring the Underlying Client Connection Factory”.

Detecting Idle Asynchronous Consumers

While efficient, one problem with asynchronous consumers is detecting when they are idle - users might want to take some action if no messages arrive for some period of time.

Starting with version 1.6, it is now possible to configure the listener container to publish a ListenerContainerIdleEvent when some time passes with no message delivery. While the container is idle, an event will be published every idleEventInterval milliseconds.

To configure this feature, set the idleEventInterval on the container:

xml

Java

_@Bean_
public SimpleMessageListenerContainer(ConnectionFactory connectionFactory) {
    SimpleMessageListenerContainer container = new SimpleMessageListenerContainer(connectionFactory);
    ...
    container.setIdleEventInterval(60000L);
    ...
    return container;
}

@RabbitListener

_@Bean_
public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory() {
    SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();
    factory.setConnectionFactory(rabbitConnectionFactory());
    factory.setIdleEventInterval(60000L);
    ...
    return factory;
}

In each of these cases, an event will be published once per minute while the container is idle.

Event Consumption

You can capture these events by implementing ApplicationListener - either a general listener, or one narrowed to only receive this specific event. You can also use @EventListener, introduced in Spring Framework 4.2.

The following example combines the @RabbitListener and @EventListener into a single class. It’s important to understand that the application listener will get events for all containers so you may need to check the listener id if you want to take specific action based on which container is idle. You can also use the @EventListener condition for this purpose.

The events have 4 properties:

• source - the listener container instance
• id - the listener id (or container bean name)
• idleTime - the time the container had been idle when the event was published
• queueNames - the names of the queue(s) that the container listens to

public class Listener {

    private final CountDownLatch latch = new CountDownLatch(2);

    private volatile ListenerContainerIdleEvent event;

    @RabbitListener(id="foo", queues="#{queue.name}")
    public String listen(String foo) {
        return foo.toUpperCase();
    }

    @EventListener(condition = "event.listenerId == 'foo'")
    public void onApplicationEvent(ListenerContainerIdleEvent event) {
        ...
    }

}

	Important
Event listeners will see events for all containers; so, in the example above, we narrow the events received based on the listener ID.

3.1.6 Message Converters

Introduction

The AmqpTemplate also defines several methods for sending and receiving Messages that will delegate to a MessageConverter. The MessageConverter itself is quite straightforward. It provides a single method for each direction: one for converting to a Message and another for converting from a Message. Notice that when converting to a Message, you may also provide properties in addition to the object. The "object" parameter typically corresponds to the Message body.

public interface MessageConverter {

    Message toMessage(Object object, MessageProperties messageProperties)
            throws MessageConversionException;

    Object fromMessage(Message message) throws MessageConversionException;

}

The relevant Message-sending methods on the AmqpTemplate are listed below. They are simpler than the methods we discussed previously because they do not require the Message instance. Instead, the MessageConverter is responsible for "creating" each Message by converting the provided object to the byte array for the Message body and then adding any provided MessageProperties.

void convertAndSend(Object message) throws AmqpException;

void convertAndSend(String routingKey, Object message) throws AmqpException;

void convertAndSend(String exchange, String routingKey, Object message)
    throws AmqpException;

void convertAndSend(Object message, MessagePostProcessor messagePostProcessor)
    throws AmqpException;

void convertAndSend(String routingKey, Object message,
    MessagePostProcessor messagePostProcessor) throws AmqpException;

void convertAndSend(String exchange, String routingKey, Object message,
    MessagePostProcessor messagePostProcessor) throws AmqpException;

On the receiving side, there are only two methods: one that accepts the queue name and one that relies on the template’s "queue" property having been set.

Object receiveAndConvert() throws AmqpException;

Object receiveAndConvert(String queueName) throws AmqpException;

	Note
The MessageListenerAdapter mentioned in the section called “Asynchronous Consumer” also uses a MessageConverter.

SimpleMessageConverter

The default implementation of the MessageConverter strategy is called SimpleMessageConverter. This is the converter that will be used by an instance of RabbitTemplate if you do not explicitly configure an alternative. It handles text-based content, serialized Java objects, and simple byte arrays.

Converting From a Message

If the content type of the input Message begins with "text" (e.g. "text/plain"), it will also check for the content-encoding property to determine the charset to be used when converting the Message body byte array to a Java String. If no content-encoding property had been set on the input Message, it will use the "UTF-8" charset by default. If you need to override that default setting, you can configure an instance of SimpleMessageConverter, set its "defaultCharset" property and then inject that into a RabbitTemplate instance.

If the content-type property value of the input Message is set to "application/x-java-serialized-object", the SimpleMessageConverter will attempt to deserialize (rehydrate) the byte array into a Java object. While that might be useful for simple prototyping, it’s generally not recommended to rely on Java serialization since it leads to tight coupling between the producer and consumer. Of course, it also rules out usage of non-Java systems on either side. With AMQP being a wire-level protocol, it would be unfortunate to lose much of that advantage with such restrictions. In the next two sections, we’ll explore some alternatives for passing rich domain object content without relying on Java serialization.

For all other content-types, the SimpleMessageConverter will return the Message body content directly as a byte array.

Converting To a Message

When converting to a Message from an arbitrary Java Object, the SimpleMessageConverter likewise deals with byte arrays, Strings, and Serializable instances. It will convert each of these to bytes (in the case of byte arrays, there is nothing to convert), and it will set the content-type property accordingly. If the Object to be converted does not match one of those types, the Message body will be null.

JsonMessageConverter and Jackson2JsonMessageConverter

As mentioned in the previous section, relying on Java serialization is generally not recommended. One rather common alternative that is more flexible and portable across different languages and platforms is JSON (JavaScript Object Notation). Two implementations are available and can be configured on any RabbitTemplate instance to override its usage of the SimpleMessageConverter default. The JsonMessageConverter which uses the org.codehaus.jackson 1.x library and Jackson2JsonMessageConverter which uses the com.fasterxml.jackson 2.x library.

    
    
        
            
            
        </bean>
    </property>
</bean>

    
    
        
            
            
        </bean>
    </property>
</bean>

As shown above, the JsonMessageConverter and Jackson2JsonMessageConverter uses a DefaultClassMapper by default. Type information is added to (and retrieved from) the MessageProperties. If an inbound message does not contain type information in the MessageProperties, but you know the expected type, you can configure a static type using the defaultType property

    
        
            
        </bean>
    </property>
</bean>

    
        
            
        </bean>
    </property>
</bean>

MarshallingMessageConverter

Yet another option is the MarshallingMessageConverter. It delegates to the Spring OXM library’s implementations of the Marshaller and Unmarshaller strategy interfaces. You can read more about that library here. In terms of configuration, it’s most common to provide the constructor argument only since most implementations of Marshaller will also implement Unmarshaller.

    
    
        
            
        </bean>
    </property>
</bean>

ContentTypeDelegatingMessageConverter

This class was introduced in version 1.4.2 and allows delegation to a specific MessageConverter based on the content type property in the MessageProperties. By default, it will delegate to a SimpleMessageConverter if there is no contentType property, or a value that matches none of the configured converters.

    
        <map>
            
            
        </map>
    </property>
</bean>

Message Properties Converters

The MessagePropertiesConverter strategy interface is used to convert between the Rabbit Client BasicProperties and Spring AMQP MessageProperties. The default implementation (DefaultMessagePropertiesConverter) is usually sufficient for most purposes but you can implement your own if needed. The default properties converter will convert BasicProperties elements of type LongString to String s when the size is not greater than 1024 bytes. Larger LongString s are not converted (see below). This limit can be overridden with a constructor argument.

Starting with version 1.6, headers longer than the long string limit (default 1024) are now left as LongString s by default by the DefaultMessagePropertiesConverter. You can access the contents via the getBytes[], toString(), or getStream() methods.

Previously, the DefaultMessagePropertiesConverter "converted" such headers to a DataInputStream (actually it just referenced the LongString's DataInputStream). On output, this header was not converted (except to a String, e.g. java.io.DataInputStream@1d057a39 by calling toString() on the stream).

Large incoming LongString headers are now correctly "converted" on output too (by default).

A new constructor is provided to allow you to configure the converter to work as before:

**/**
 * Construct an instance where LongStrings will be returned
 * unconverted or as a java.io.DataInputStream when longer than this limit.
 * Use this constructor with 'true' to restore pre-1.6 behavior.
 * @param longStringLimit the limit.
 * @param convertLongLongStrings LongString when false,
 * DataInputStream when true.
 * @since 1.6
 */**
public DefaultMessagePropertiesConverter(int longStringLimit, boolean convertLongLongStrings) { ... }

Also starting with version 1.6, a new property correlationIdString has been added to MessageProperties. Previously, when converting to/from BasicProperties used by the RabbitMQ client, an unnecessary byte[] <-> String conversion was performed because MessageProperties.correlationId is a byte[] but BasicProperties uses a String. (Ultimately, the RabbitMQ client uses UTF-8 to convert the String to bytes to put in the protocol message).

To provide maximum backwards compatibility, a new property correlationIdPolicy has been added to the DefaultMessagePropertiesConverter. This takes an DefaultMessagePropertiesConverter.CorrelationIdPolicy enum argument. By default it is set to BYTES which replicates the previous behavior.

For inbound messages:

• STRING - just the correlationIdString property is mapped
• BYTES - just the correlationId property is mapped
• BOTH - both properties are mapped

For outbound messages:

• STRING - just the correlationIdString property is mapped
• BYTES - just the correlationId property is mapped
• BOTH - Both properties will be considered, with the String property taking precedence

3.1.7 Modifying Messages - Compression and More

A number of extension points exist where you can perform some processing on a message, either before it is sent to RabbitMQ, or immediately after it is received.

As can be seen in Section 3.1.6, “Message Converters”, one such extension point is in the AmqpTemplate convertAndReceive operations, where you can provide a MessagePostProcessor. For example, after your POJO has been converted, the MessagePostProcessor enables you to set custom headers or properties on the Message.

Starting with version 1.4.2, additional extension points have been added to the RabbitTemplate - setBeforePublishPostProcessors() and setAfterReceivePostProcessors(). The first enables a post processor to run immediately before sending to RabbitMQ. When using batching (see the section called “Batching”), this is invoked after the batch is assembled and before the batch is sent. The second is invoked immediately after a message is received.

These extension points are used for such features as compression and, for this purpose, several MessagePostProcessor s are provided:

• GZipPostProcessor
• ZipPostProcessor

for compressing messages before sending, and

• GUnzipPostProcessor
• UnzipPostProcessor

for decompressing received messages.

Similarly, the SimpleMessageListenerContainer also has a setAfterReceivePostProcessors() method, allowing the decompression to be performed after messages are received by the container.

3.1.8 Request/Reply Messaging

Introduction

The AmqpTemplate also provides a variety of sendAndReceive methods that accept the same argument options that you have seen above for the one-way send operations (exchange, routingKey, and Message). Those methods are quite useful for request/reply scenarios since they handle the configuration of the necessary "reply-to" property before sending and can listen for the reply message on an exclusive Queue that is created internally for that purpose.

Similar request/reply methods are also available where the MessageConverter is applied to both the request and reply. Those methods are named convertSendAndReceive. See the Javadoc of AmqpTemplate for more detail.

Starting with version 1.5.0, each of the sendAndReceive method variants has an overloaded version that takes CorrelationData. Together with a properly configured connection factory, this enables the receipt of publisher confirms for the send side of the operation. See the section called “Publisher Confirms and Returns” for more information.

Reply Timeout

By default, the send and receive methods will timeout after 5 seconds and return null. This can be modified by setting the replyTimeout property. Starting with version 1.5, if you set the mandatory property to true (or the mandatory-expression evaluates to true for a particular message), if the message cannot be delivered to a queue an AmqpMessageReturnedException will be thrown. This exception has returnedMessage, replyCode, replyText properties, as well as the exchange and routingKey used for the send.

	Note
This feature uses publisher returns and is enabled by setting publisherReturns to true on the CachingConnectionFactory (see the section called “Publisher Confirms and Returns”). Also, you must not have registered your own ReturnCallback with the RabbitTemplate.

RabbitMQ Direct reply-to

Important

Starting with version 3.4.0, the RabbitMQ server now supports Direct reply-to; this eliminates the main reason for a fixed reply queue (to avoid the need to create a temporary queue for each request). Starting with Spring AMQP version 1.4.1 Direct reply-to will be used by default (if supported by the server) instead of creating temporary reply queues. When no replyQueue is provided (or it is set with the name amq.rabbitmq.reply-to), the RabbitTemplate will automatically detect whether Direct reply-to is supported and either use it or fall back to using a temporary reply queue. When using Direct reply-to, a reply-listener is not required and should not be configured.

Reply listeners are still supported with named queues (other than amq.rabbitmq.reply-to), allowing control of reply concurrency etc.

Starting with version 1.6 if, for some reason, you wish to use a temporary, exclusive, auto-delete queue for each reply, set the useTemporaryReplyQueues property to true. This property is ignored if you you set a replyAddress.

The decision whether or not to use direct reply-to can be changed to use different criteria by subclassing RabbitTemplate and overriding useDirectReplyTo(). The method is called once only; when the first request is sent.

Message Correlation With A Reply Queue

When using a fixed reply queue (other than amq.rabbitmq.reply-to), it is necessary to provide correlation data so that replies can be correlated to requests. See RabbitMQ Remote Procedure Call (RPC). By default, the standard correlationId property will be used to hold the correlation data. However, if you wish to use a custom property to hold correlation data, you can set the correlation-key attribute on the . Explicitly setting the attribute to correlationId is the same as omitting the attribute. Of course, the client and server must use the same header for correlation data.

	Note
Spring AMQP version 1.1 used a custom property spring_reply_correlation for this data. If you wish to revert to this behavior with the current version, perhaps to maintain compatibility with another application using 1.1, you must set the attribute to spring_reply_correlation.

Reply Listener Container

When using RabbitMQ versions prior to 3.4.0, a new temporary queue is used for each reply. However, a single reply queue can be configured on the template, which can be more efficient, and also allows you to set arguments on that queue. In this case, however, you must also provide a sub element. This element provides a listener container for the reply queue, with the template being the listener. All of the Section 3.1.14, “Message Listener Container Configuration” attributes allowed on a are allowed on the element, except for connection-factory and message-converter, which are inherited from the template’s configuration.

    
</rabbit:template>

While the container and template share a connection factory, they do not share a channel and therefore requests and replies are not performed within the same transaction (if transactional).

Note

Prior to version 1.5.0, the reply-address attribute was not available, replies were always routed using the default exchange and the reply-queue name as the routing key. This is still the default but you can now specify the new reply-address attribute. The reply-address can contain an address with the form &lt;exchange&gt;/&lt;routingKey&gt; and the reply will be routed to the specified exchange and routed to a queue bound with the routing key. The reply-address has precedence over reply-queue. The <reply-listener> must be configured as a separate <listener-container> component, when only reply-address is in use, anyway reply-address and reply-queue (or queues attribute on the <listener-container>) must refer to the same queue logically.

With this configuration, a SimpleListenerContainer is used to receive the replies; with the RabbitTemplate being the MessageListener. When defining a template with the <rabbit:template/> namespace element, as shown above, the parser defines the container and wires in the template as the listener.

	Note
When the template does not use a fixed replyQueue (or is using Direct reply-to - see the section called “RabbitMQ Direct reply-to”) a listener container is not needed. Direct reply-to is the preferred mechanism when using RabbitMQ 3.4.0 or later.

If you define your RabbitTemplate as a <bean/>, or using an @Configuration class to define it as an @Bean, or when creating the template programmatically, you will need to define and wire up the reply listener container yourself. If you fail to do this, the template will never receive the replies and will eventually time out and return null as the reply to a call to a sendAndReceive method.

Starting with version 1.5, the RabbitTemplate will detect if it has been configured as a MessageListener to receive replies. If not, attempts to send and receive messages with a reply address will fail with an IllegalStateException (because the replies will never be received).

Further, if a simple replyAddress (queue name) is used, the reply listener container will verify that it is listening to a queue with the same name. This check cannot be performed if the reply address is an exchange and routing key and a debug log message will be written.

	Important
When wiring the reply listener and template yourself, it is important to ensure that the template’s replyQueue and the container’s queues (or queueNames) properties refer to the same queue. The template inserts the reply queue into the outbound message replyTo property.

The following are examples of how to manually wire up the beans.

    
    
    
    
    
</bean>


    
    
    
</bean>

    _@Bean_
    public RabbitTemplate amqpTemplate() {
        RabbitTemplate rabbitTemplate = new RabbitTemplate(connectionFactory());
        rabbitTemplate.setMessageConverter(msgConv());
        rabbitTemplate.setReplyQueue(replyQueue());
        rabbitTemplate.setReplyTimeout(60000);
        return rabbitTemplate;
    }

    _@Bean_
    public SimpleMessageListenerContainer replyListenerContainer() {
        SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
        container.setConnectionFactory(connectionFactory());
        container.setQueues(replyQueue());
        container.setMessageListener(amqpTemplate());
        return container;
    }

    _@Bean_
    public Queue replyQueue() {
        return new Queue("my.reply.queue");
    }

A complete example of a RabbitTemplate wired with a fixed reply queue, together with a "remote" listener container that handles the request and returns the reply is shown in this test case.

	Important
When the reply times out (replyTimeout), the sendAndReceive() methods return null.

Prior to version 1.3.6, late replies for timed out messages were simply logged. Now, if a late reply is received, it is rejected (the template throws an AmqpRejectAndDontRequeueException). If the reply queue is configured to send rejected messages to a dead letter exchange, the reply can be retrieved for later analysis. Simply bind a queue to the configured dead letter exchange with a routing key equal to the reply queue’s name.

Refer to the RabbitMQ Dead Letter Documentation for more information about configuring dead lettering. You can also take a look at the FixedReplyQueueDeadLetterTests test case for an example.

AsyncRabbitTemplate

Version 1.6 introduced the AsyncRabbitTemplate. This has similar sendAndReceive (and convertSendAndReceive) methods to those on the AmqpTemplate but instead of blocking, they return a ListenableFuture.

The sendAndReceive methods return a RabbitMessageFuture; the convertSendAndReceive methods return a RabbitConverterFuture.

You can either synchronously retrieve the result later, by invoking get() on the future, or you can register a callback which will be called asynchronously with the result.

_@Autowired_
private AsyncRabbitTemplate template;

...

public void doSomeWorkAndGetResultLater() {

    ...

    ListenableFuture<String> future = this.template.convertSendAndReceive("foo");

    // do some more work

    String reply = null;
    try {
        reply = future.get();
    }
    catch (ExecutionException e) {
        ...
    }

    ...

}

public void doSomeWorkAndGetResultAsync() {

    ...

    ListenableFuture<String> future = this.template.convertSendAndReceive("foo");
    future.addCallback(new ListenableFutureCallback<String>() {

        _@Override_
        public void onSuccess(String result) {
            ...
        }

        _@Override_
        public void onFailure(Throwable ex) {
            ...
        }

    });

    ...

}

If mandatory is set, and the message can’t be delivered, the future will throw an ExecutionException with a cause of AmqpMessageReturnedException which encapsulates the returned message and information about the return.

If enableConfirms is set, the future will have a property confirm which is itself a ListenableFuture<Boolean> with true indicating a successful publish. If the confirm future is false, the RabbitFuture will have a further property nackCause - the reason for the failure, if available.

	Important
The publisher confirm is discarded if it is received after the reply - since the reply implies a successful publish.

Spring Remoting with AMQP

The Spring Framework has a general remoting capability, allowing Remote Procedure Calls (RPC) using various transports. Spring-AMQP supports a similar mechanism with a AmqpProxyFactoryBean on the client and a AmqpInvokerServiceExporter on the server. This provides RPC over AMQP. On the client side, a RabbitTemplate is used as described above; on the server side, the invoker (configured as a MessageListener) receives the message, invokes the configured service, and returns the reply using the inbound message’s replyTo information.

The client factory bean can be injected into any bean (using its serviceInterface); the client can then invoke methods on the proxy, resulting in remote execution over AMQP.

	Note
With the default MessageConverter s, the method parameters and returned value must be instances of Serializable.

On the server side, the AmqpInvokerServiceExporter has both AmqpTemplate and MessageConverter properties. Currently, the template’s MessageConverter is not used. If you need to supply a custom message converter, then you should provide it using the messageConverter property. On the client side, a custom message converter can be added to the AmqpTemplate which is provided to the AmqpProxyFactoryBean using its amqpTemplate property.

Sample client and server configurations are shown below.

    
    
</bean>










    <rabbit:bindings>
        
    </rabbit:bindings>

    
    
    
</bean>










    

	Important
The AmqpInvokerServiceExporter can only process properly formed messages, such as those sent from the AmqpProxyFactoryBean. If it receives a message that it cannot interpret, a serialized RuntimeException will be sent as a reply. If the message has no replyToAddress property, the message will be rejected and permanently lost if no Dead Letter Exchange has been configured.

Note

By default, if the request message cannot be delivered, the calling thread will eventually timeout and a RemoteProxyFailureException will be thrown. The timeout is 5 seconds by default, and can be modified by setting the replyTimeout property on the RabbbitTemplate. Starting with version 1.5, setting the mandatory property to true, and enabling returns on the connection factory (see the section called “Publisher Confirms and Returns”), the calling thread will throw an AmqpMessageReturnedException. See the section called “Reply Timeout” for more information.

3.1.9 Configuring the broker

Introduction

The AMQP specification describes how the protocol can be used to configure Queues, Exchanges and Bindings on the broker. These operations which are portable from the 0.8 specification and higher are present in the AmqpAdmin interface in the org.springframework.amqp.core package. The RabbitMQ implementation of that class is RabbitAdmin located in the org.springframework.amqp.rabbit.core package.

The AmqpAdmin interface is based on using the Spring AMQP domain abstractions and is shown below:

public interface AmqpAdmin {

    // Exchange Operations

    void declareExchange(Exchange exchange);

    void deleteExchange(String exchangeName);

    // Queue Operations

    Queue declareQueue();

    String declareQueue(Queue queue);

    void deleteQueue(String queueName);

    void deleteQueue(String queueName, boolean unused, boolean empty);

    void purgeQueue(String queueName, boolean noWait);

    // Binding Operations

    void declareBinding(Binding binding);

    void removeBinding(Binding binding);

    Properties getQueueProperties(String queueName);

}

The getQueueProperties() method returns some limited information about the queue (message count and consumer count). The keys for the properties returned are available as constants in the RabbitTemplate (QUEUE_NAME, QUEUE_MESSAGE_COUNT, QUEUE_CONSUMER_COUNT). The RabbitMQ REST API provides much more information in the QueueInfo object.

The no-arg declareQueue() method defines a queue on the broker with a name that is automatically generated. The additional properties of this auto-generated queue are exclusive=true, autoDelete=true, and durable=false.

The declareQueue(Queue queue) method takes a Queue object and returns the name of the declared queue. If the provided Queue's name property is an empty String, the broker declares the queue with a generated name and that name is returned to the caller. The Queue object itself is not changed. This functionality can only be used programmatically by invoking the RabbitAdmin directly. It is not supported for auto-declaration by the admin by defining a queue declaratively in the application context.

This is in contrast to an AnonymousQueue where the framework generates a unique (UUID) name and sets durable to false and exclusive, autoDelete to true. A <rabbit:queue/> with an empty, or missing, name attribute will always create an AnonymousQueue.

See the section called “AnonymousQueue” to understand why AnonymousQueue is preferred over broker-generated queue names, as well as how to control the format of the name. Declarative queues must have fixed names because they might be referenced elsewhere in the context, for example, in a listener:

See the section called “Automatic Declaration of Exchanges, Queues and Bindings”.

The RabbitMQ implementation of this interface is RabbitAdmin which when configured using Spring XML would look like this:

When the CachingConnectionFactory cache mode is CHANNEL (the default), the RabbitAdmin implementation does automatic lazy declaration of Queues, Exchanges and Bindings declared in the same ApplicationContext. These components will be declared as son as a Connection is opened to the broker. There are some namespace features that make this very convenient, e.g. in the Stocks sample application we have:

    <bindings>
        
    </bindings>



    <bindings>
        
    </bindings>

In the example above we are using anonymous Queues (actually internally just Queues with names generated by the framework, not by the broker) and refer to them by ID. We can also declare Queues with explicit names, which also serve as identifiers for their bean definitions in the context. E.g.

	Tip
You can provide both an id and a name attribute. This allows you to refer to the queue (for example in a binding) by an id that is independent of the queue name. It also allows standard Spring features such as property placeholders, and SpEL expressions for the queue name; these features are not available when using the name as the bean identifier.

Queues can be configured with additional arguments, for example, x-message-ttl or x-ha-policy. Using the namespace support, they are provided in the form of a Map of argument name/argument value pairs, using the element.

    
        
    
</rabbit:queue>

By default, the arguments are assumed to be strings. For arguments of other types, the type needs to be provided.

    
        
    
</rabbit:queue>

When providing arguments of mixed types, the type is provided for each entry element:

    
        
            100</value>
        </entry>
        
    
</rabbit:queue>

With Spring Framework 3.2 and later, this can be declared a little more succinctly:

    
        
        
    
</rabbit:queue>

	Important
The RabbitMQ broker will not allow declaration of a queue with mismatched arguments. For example, if a queue already exists with no time to live argument, and you attempt to declare it with, say, key="x-message-ttl" value="100", an exception will be thrown.

By default, the RabbitAdmin will immediately stop processing all declarations when any exception occurs; this could cause downstream issues - such as a listener container failing to initialize because another queue (defined after the one in error) is not declared.

This behavior can be modified by setting the ignore-declaration-failures attribute to true on the RabbitAdmin. This option instructs the RabbitAdmin to log the exception, and continue declaring other elements. When configuring the RabbitAdmin using java, this property is ignoreDeclarationFailures.

Prior to version 1.6, this property only took effect if an IOException occurred on the channel - such as when there is a mismatch between current and desired properties. Now, this property takes effect on any exception, including TimeoutException etc.

In addition, any declaration exceptions result in the publishing of a DeclarationExceptionEvent, which is an ApplicationEvent that can be consumed by any ApplicationListener in the context. The event contains a reference to the admin, the element that was being declared, and the Throwable.

Starting with version 1.3 the HeadersExchange can be configured to match on multiple headers; you can also specify whether any or all headers must match:

    <rabbit:bindings>
        
            
                
                
                
            
        </rabbit:binding>
    </rabbit:bindings>

To see how to use Java to configure the AMQP infrastructure, look at the Stock sample application, where there is the @Configuration class AbstractStockRabbitConfiguration which in turn has RabbitClientConfiguration and RabbitServerConfiguration subclasses. The code for AbstractStockRabbitConfiguration is shown below

_@Configuration_
public abstract class AbstractStockAppRabbitConfiguration {

    _@Bean_
    public ConnectionFactory connectionFactory() {
        CachingConnectionFactory connectionFactory =
            new CachingConnectionFactory("localhost");
        connectionFactory.setUsername("guest");
        connectionFactory.setPassword("guest");
        return connectionFactory;
    }

    _@Bean_
    public RabbitTemplate rabbitTemplate() {
        RabbitTemplate template = new RabbitTemplate(connectionFactory());
        template.setMessageConverter(jsonMessageConverter());
        configureRabbitTemplate(template);
        return template;
    }

    _@Bean_
    public MessageConverter jsonMessageConverter() {
        return new JsonMessageConverter();
    }

    _@Bean_
    public TopicExchange marketDataExchange() {
        return new TopicExchange("app.stock.marketdata");
    }

    // additional code omitted for brevity

}

In the Stock application, the server is configured using the following @Configuration class:

_@Configuration_
public class RabbitServerConfiguration extends AbstractStockAppRabbitConfiguration  {

    _@Bean_
    public Queue stockRequestQueue() {
        return new Queue("app.stock.request");
    }
}

This is the end of the whole inheritance chain of @Configuration classes. The end result is the the TopicExchange and Queue will be declared to the broker upon application startup. There is no binding of the TopicExchange to a queue in the server configuration, as that is done in the client application. The stock request queue however is automatically bound to the AMQP default exchange - this behavior is defined by the specification.

The client @Configuration class is a little more interesting and is shown below.

_@Configuration_
public class RabbitClientConfiguration extends AbstractStockAppRabbitConfiguration {

    _@Value("${stocks.quote.pattern}")_
    private String marketDataRoutingKey;

    _@Bean_
    public Queue marketDataQueue() {
        return amqpAdmin().declareQueue();
    }

    **/**
     * Binds to the market data exchange.
     * Interested in any stock quotes
     * that match its routing key.
     */**
    _@Bean_
    public Binding marketDataBinding() {
        return BindingBuilder.bind(
                marketDataQueue()).to(marketDataExchange()).with(marketDataRoutingKey);
    }

    // additional code omitted for brevity

}

The client is declaring another queue via the declareQueue() method on the AmqpAdmin, and it binds that queue to the market data exchange with a routing pattern that is externalized in a properties file.

Declaring Collections of Exchanges, Queues, Bindings

Starting with version 1.5, it is now possible to declare multiple entities with one @Bean, by returing a collection.

Only collections where the first element is a Declarable are considered, and only Declarable elements from such collections are processed.

_@Configuration_
public static class Config {

    _@Bean_
    public ConnectionFactory cf() {
        return new CachingConnectionFactory("localhost");
    }

    _@Bean_
    public RabbitAdmin admin(ConnectionFactory cf) {
        return new RabbitAdmin(cf);
    }

    _@Bean_
    public DirectExchange e1() {
        return new DirectExchange("e1", false, true);
    }

    _@Bean_
    public Queue q1() {
        return new Queue("q1", false, false, true);
    }

    _@Bean_
    public Binding b1() {
        return BindingBuilder.bind(q1()).to(e1()).with("k1");
    }

    _@Bean_
    public List<Exchange> es() {
        return Arrays.<Exchange>asList(
                new DirectExchange("e2", false, true),
                new DirectExchange("e3", false, true)
        );
    }

    _@Bean_
    public List<Queue> qs() {
        return Arrays.asList(
                new Queue("q2", false, false, true),
                new Queue("q3", false, false, true)
        );
    }

    _@Bean_
    public List<Binding> bs() {
        return Arrays.asList(
                new Binding("q2", DestinationType.QUEUE, "e2", "k2", null),
                new Binding("q3", DestinationType.QUEUE, "e3", "k3", null)
        );
    }

    _@Bean_
    public List<Declarable> ds() {
        return Arrays.<Declarable>asList(
                new DirectExchange("e4", false, true),
                new Queue("q4", false, false, true),
                new Binding("q4", DestinationType.QUEUE, "e4", "k4", null)
        );
    }

}

Conditional Declaration

By default, all queues, exchanges, and bindings are declared by all RabbitAdmin instances (that have auto-startup="true") in the application context.

	Note
Starting with the 1.2 release, it is possible to conditionally declare these elements. This is particularly useful when an application connects to multiple brokers and needs to specify with which broker(s) a particular element should be declared.

The classes representing these elements implement Declarable which has two methods: shouldDeclare() and getDeclaringAdmins(). The RabbitAdmin uses these methods to determine whether a particular instance should actually process the declarations on its Connection.

The properties are available as attributes in the namespace, as shown in the following examples.

    <rabbit:bindings>
        
    </rabbit:bindings>

	Note
The auto-declare attribute is true by default and if the declared-by is not supplied (or is empty) then all RabbitAdmin s will declare the object (as long as the admin’s auto-startup attribute is true; the default).

Similarly, you can use Java-based @Configuration to achieve the same effect. In this example, the components will be declared by admin1 but not admin2:

_@Bean_
public RabbitAdmin admin() {
    RabbitAdmin rabbitAdmin = new RabbitAdmin(cf1());
    rabbitAdmin.afterPropertiesSet();
    return rabbitAdmin;
}

_@Bean_
public RabbitAdmin admin2() {
    RabbitAdmin rabbitAdmin = new RabbitAdmin(cf2());
    rabbitAdmin.afterPropertiesSet();
    return rabbitAdmin;
}

_@Bean_
public Queue queue() {
    Queue queue = new Queue("foo");
    queue.setAdminsThatShouldDeclare(admin());
    return queue;
}

_@Bean_
public Exchange exchange() {
    DirectExchange exchange = new DirectExchange("bar");
    exchange.setAdminsThatShouldDeclare(admin());
    return exchange;
}

_@Bean_
public Binding binding() {
    Binding binding = new Binding("foo", DestinationType.QUEUE, exchange().getName(), "foo", null);
    binding.setAdminsThatShouldDeclare(admin());
    return binding;
}

AnonymousQueue

In general, when needing a uniquely-named, exclusive, auto-delete queue, it is recommended that the AnonymousQueue is used instead of broker-defined queue names (using "" as a Queue name will cause the broker to generate the queue name).

This is because:

1. The queues are actually declared when the connection to the broker is established; this is long after the beans are created and wired together; beans using the queue need to know its name. In fact, the broker might not even be running when the app is started.
2. If the connection to the broker is lost for some reason, the admin will re-declare the AnonymousQueue with the same name. If we used broker-declared queues, the queue name would change.

Starting with version 1.5.3, you can control the format of the queue name used by AnonymousQueue s.

By default, the queue name is the String representation of a UUID; for example: 07afcfe9-fe77-4983-8645-0061ec61a47a.

You can now provide an AnonymousQueue.NamingStrategy implementation in a constructor argument:

_@Bean_
public Queue anon1() {
    return new AnonymousQueue(new AnonymousQueue.Base64UrlNamingStrategy());
}

_@Bean_
public Queue anon2() {
    return new AnonymousQueue(new AnonymousQueue.Base64UrlNamingStrategy("foo-"));
}

The first will generate a queue name prefixed by spring.gen- followed by a base64 representation of the UUID, for example: spring.gen-MRBv9sqISkuCiPfOYfpo4g. The second will generate a queue name prefixed by foo- followed by a base64 representation of the UUID.

The base64 encoding uses the "URL and Filename Safe Alphabet" from RFC 4648; trailing padding characters (=) are removed.

You can provide your own naming strategy, whereby you can include other information (e.g. application, client host) in the queue name.

Starting with version 1.6, the naming strategy can be specified when using XML configuration; the naming-strategy attribute is present on the <rabbit:queue> element for a bean reference that implements AnonymousQueue.NamingStrategy.

    
</bean>

The first creates names with a String representation of a UUID. The second creates names like spring.gen-MRBv9sqISkuCiPfOYfpo4g. The third creates names like custom.gen-MRBv9sqISkuCiPfOYfpo4g.

Of course, you can provide your own naming strategy bean.

3.1.10 Delayed Message Exchange

Version 1.6 introduces support for the Delayed Message Exchange Plugin

	Note
The plugin is currently marked as experimental but has been available for over a year (at the time of writing). If changes to the plugin make it necessary, we will add support for such changes as soon as practical. For that reason, this support in Spring AMQP should be considered experimental, too. This functionality was tested with RabbitMQ 3.6.0 and version 0.0.1 of the plugin.

To use a RabbitAdmin to declare an exchange as delayed, simply set the delayed property on the exchange bean to true. The RabbitAdmin will use the exchange type (Direct, Fanout etc) to set the x-delayed-type argument and declare the exchange with type x-delayed-message.

The delayed property (default false) is also available when configuring exchange beans using XML.

To send a delayed message, it’s simply a matter of setting the x-delay header, via the MessageProperties:

MessageProperties properties = new MessageProperties();
properties.setXDelay(15000);
template.send(exchange, routingKey,
        MessageBuilder.withBody("foo".getBytes()).andProperties(properties).build());

or

rabbitTemplate.convertAndSend(exchange, routingKey, "foo", new MessagePostProcessor() {

    _@Override_
    public Message postProcessMessage(Message message) throws AmqpException {
        message.getMessageProperties().setXDelay(15000);
        return message;
    }

});

To check if a message was delayed, use the getReceivedDelay() method on the MessageProperties. It is a separate property to avoid unintended propagation to an output message generated from an input messasge.

3.1.11 RabbitMQ REST API

When the management plugin is enabled, the RabbitMQ server exposes a REST API to monitor and configure the broker. A Java Binding for the API is now provided. In general, you can use that API directly, but a convenience wrapper is provided to use the familiar Spring AMQP Queue, Exchange, and Binding domain objects with the API. Much more information is available for these objects when using the com.rabbitmq.http.client.Client API directly (QueueInfo, ExchangeInfo, and BindingInfo respectively). The following operations are available on the RabbitManagementTemplate:

public interface AmqpManagementOperations {

    void addExchange(Exchange exchange);

    void addExchange(String vhost, Exchange exchange);

    void purgeQueue(Queue queue);

    void purgeQueue(String vhost, Queue queue);

    void deleteQueue(Queue queue);

    void deleteQueue(String vhost, Queue queue);

    Queue getQueue(String name);

    Queue getQueue(String vhost, String name);

    List<Queue> getQueues();

    List<Queue> getQueues(String vhost);

    void addQueue(Queue queue);

    void addQueue(String vhost, Queue queue);

    void deleteExchange(Exchange exchange);

    void deleteExchange(String vhost, Exchange exchange);

    Exchange getExchange(String name);

    Exchange getExchange(String vhost, String name);

    List<Exchange> getExchanges();

    List<Exchange> getExchanges(String vhost);

    List<Binding> getBindings();

    List<Binding> getBindings(String vhost);

    List<Binding> getBindingsForExchange(String vhost, String exchange);

}

Refer to the javadocs for more information.

3.1.12 Exception Handling

Many operations with the RabbitMQ Java client can throw checked Exceptions. For example, there are a lot of cases where IOExceptions may be thrown. The RabbitTemplate, SimpleMessageListenerContainer, and other Spring AMQP components will catch those Exceptions and convert into one of the Exceptions within our runtime hierarchy. Those are defined in the org.springframework.amqp package, and AmqpException is the base of the hierarchy.

When a listener throws an exception, it is wrapped in a ListenerExecutionFailedException and, normally the message is rejected and requeued by the broker. Setting defaultRequeueRejected to false will cause messages to be discarded (or routed to a dead letter exchange). As discussed in the section called “Message Listeners and the Asynchronous Case”, the listener can throw an AmqpRejectAndDontRequeueException to conditionally control this behavior.

However, there is a class of errors where the listener cannot control the behavior. When a message that cannot be converted is encountered (for example an invalid content_encoding header), the MessageConversionException is thrown before the message reaches user code. With defaultRequeueRejected set to true (default), such messages would be redelivered over and over. Before version 1.3.2, users needed to write a custom ErrorHandler, as discussed in Section 3.1.12, “Exception Handling” to avoid this situation.

Starting with version 1.3.2, the default ErrorHandler is now a ConditionalRejectingErrorHandler which will reject (and not requeue) messages that fail with a MessageConversionException. An instance of this error handler can be configured with a FatalExceptionStrategy so users can provide their own rules for conditional message rejection, e.g. a delegate implementation to the BinaryExceptionClassifier from Spring Retry (the section called “Message Listeners and the Asynchronous Case”). In addition, the ListenerExecutionFailedException now has a failedMessage property which can be used in the decision. If the FatalExceptionStrategy.isFatal() method returns true, the error handler throws an AmqpRejectAndDontRequeueException. The default FatalExceptionStrategy logs a warning message.

3.1.13 Transactions

Introduction

The Spring Rabbit framework has support for automatic transaction management in the synchronous and asynchronous use cases with a number of different semantics that can be selected declaratively, as is familiar to existing users of Spring transactions. This makes many if not most common messaging patterns very easy to implement.

There are two ways to signal the desired transaction semantics to the framework. In both the RabbitTemplate and SimpleMessageListenerContainer there is a flag channelTransacted which, if true, tells the framework to use a transactional channel and to end all operations (send or receive) with a commit or rollback depending on the outcome, with an exception signaling a rollback. Another signal is to provide an external transaction with one of Spring’s PlatformTransactionManager implementations as a context for the ongoing operation. If there is already a transaction in progress when the framework is sending or receiving a message, and the channelTransacted flag is true, then the commit or rollback of the messaging transaction will be deferred until the end of the current transaction. If the channelTransacted flag is false, then no transaction semantics apply to the messaging operation (it is auto-acked).

The channelTransacted flag is a configuration time setting: it is declared and processed once when the AMQP components are created, usually at application startup. The external transaction is more dynamic in principle because the system responds to the current Thread state at runtime, but in practice is often also a configuration setting, when the transactions are layered onto an application declaratively.

For synchronous use cases with RabbitTemplate the external transaction is provided by the caller, either declaratively or imperatively according to taste (the usual Spring transaction model). An example of a declarative approach (usually preferred because it is non-invasive), where the template has been configured with channelTransacted=true:

_@Transactional_
public void doSomething() {
    String incoming = rabbitTemplate.receiveAndConvert();
    // do some more database processing...
    String outgoing = processInDatabaseAndExtractReply(incoming);
    rabbitTemplate.convertAndSend(outgoing);
}

A String payload is received, converted and sent as a message body inside a method marked as @Transactional, so if the database processing fails with an exception, the incoming message will be returned to the broker, and the outgoing message will not be sent. This applies to any operations with the RabbitTemplate inside a chain of transactional methods (unless the Channel is directly manipulated to commit the transaction early for instance).

For asynchronous use cases with SimpleMessageListenerContainer if an external transaction is needed it has to be requested by the container when it sets up the listener. To signal that an external transaction is required the user provides an implementation of PlatformTransactionManager to the container when it is configured. For example:

_@Configuration_
public class ExampleExternalTransactionAmqpConfiguration {

    _@Bean_
    public SimpleMessageListenerContainer messageListenerContainer() {
        SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
        container.setConnectionFactory(rabbitConnectionFactory());
        container.setTransactionManager(transactionManager());
        container.setChannelTransacted(true);
        container.setQueueName("some.queue");
        container.setMessageListener(exampleListener());
        return container;
    }

}

In the example above, the transaction manager is added as a dependency injected from another bean definition (not shown), and the channelTransacted flag is also set to true. The effect is that if the listener fails with an exception the transaction will be rolled back, and the message will also be returned to the broker. Significantly, if the transaction fails to commit (e.g. a database constraint error, or connectivity problem), then the AMQP transaction will also be rolled back, and the message will be returned to the broker. This is sometimes known as a Best Efforts 1 Phase Commit, and is a very powerful pattern for reliable messaging. If the channelTransacted flag was set to false in the example above, which is the default, then the external transaction would still be provided for the listener, but all messaging operations would be auto-acked, so the effect is to commit the messaging operations even on a rollback of the business operation.

A note on Rollback of Received Messages

AMQP transactions only apply to messages and acks sent to the broker, so when there is a rollback of a Spring transaction and a message has been received, what Spring AMQP has to do is not just rollback the transaction, but also manually reject the message (sort of a nack, but that’s not what the specification calls it). The action taken on message rejection is independent of transactions and depends on the defaultRequeueRejected property (default true). For more information about rejecting failed messages, see the section called “Message Listeners and the Asynchronous Case”.

For more information about RabbitMQ transactions, and their limitations, refer to RabbitMQ Broker Semantics.

	Note
Prior to RabbitMQ 2.7.0, such messages (and any that are unacked when a channel is closed or aborts) went to the back of the queue on a Rabbit broker, since 2.7.0, rejected messages go to the front of the queue, in a similar manner to JMS rolled back messages.

Using the RabbitTransactionManager

The RabbitTransactionManager is an alternative to executing Rabbit operations within, and synchronized with, external transactions. This Transaction Manager is an implementation of the PlatformTransactionManager interface and should be used with a single Rabbit ConnectionFactory.

	Important
This strategy is not able to provide XA transactions, for example in order to share transactions between messaging and database access.

Application code is required to retrieve the transactional Rabbit resources via ConnectionFactoryUtils.getTransactionalResourceHolder(ConnectionFactory, boolean) instead of a standard Connection.createChannel() call with subsequent Channel creation. When using Spring AMQP’s RabbitTemplate, it will autodetect a thread-bound Channel and automatically participate in its transaction.

With Java Configuration you can setup a new RabbitTransactionManager using:

_@Bean_
public RabbitTransactionManager rabbitTransactionManager() {
    return new RabbitTransactionManager(connectionFactory);
}

If you prefer using XML configuration, declare the following bean in your XML Application Context file:

    
</bean>

3.1.14 Message Listener Container Configuration

There are quite a few options for configuring a SimpleMessageListenerContainer related to transactions and quality of service, and some of them interact with each other.

The table below shows the container property names and their equivalent attribute names (in parentheses) when using the namespace to configure a <rabbit:listener-container/>.

Some properties are not exposed by the namespace; indicated by N/Afor the attribute.

Table 3.1. Configuration options for a message listener container

(group)

| This is only available when using the namespace. When specified, a bean of type Collection<MessageListenerContainer> is registered with this name, and the container for each <listener/> element is added to the collection. This allows, for example, starting/stopping the group of containers by iterating over the collection. If multiple <listener-container/> elements have the same group value, the containers in the collection is an aggregate of all containers so designated. | |

channelTransacted
(channel-transacted)

| Boolean flag to signal that all messages should be acknowledged in a transaction (either manually or automatically) | |

acknowledgeMode
(acknowledge)

|

• NONE = no acks will be sent (incompatible with channelTransacted=true). RabbitMQ calls this "autoack" because the broker assumes all messages are acked without any action from the consumer.
• MANUAL = the listener must acknowledge all messages by calling Channel.basicAck().

• AUTO = the container will acknowledge the message automatically, unless the MessageListener throws an exception. Note that acknowledgeMode is complementary to channelTransacted - if the channel is transacted then the broker requires a commit notification in addition to the ack. This is the default mode. See also txSize.

  | |

transactionManager
(transaction-manager)

| External transaction manager for the operation of the listener. Also complementary to channelTransacted - if the Channel is transacted then its transaction will be synchronized with the external transaction. | |

prefetchCount
(prefetch)

| The number of messages to accept from the broker in one socket frame. The higher this is the faster the messages can be delivered, but the higher the risk of non-sequential processing. Ignored if the acknowledgeMode is NONE. This will be increased, if necessary, to match the txSize. | |

shutdownTimeout
(N/A)

| When a container shuts down (e.g. if its enclosing ApplicationContext is closed) it waits for in-flight messages to be processed up to this limit. Defaults to 5 seconds. After the limit is reached, if the channel is not transacted messages will be discarded. | |

txSize
(transaction-size)

| When used with acknowledgeMode AUTO, the container will attempt to process up to this number of messages before sending an ack (waiting for each one up to the receive timeout setting). This is also when a transactional channel is committed. If the prefetchCount is less than the txSize, it will be increased to match the txSize. | |

receiveTimeout
(receive-timeout)

| The maximum time to wait for each message. If acknowledgeMode=NONE this has very little effect - the container just spins round and asks for another message. It has the biggest effect for a transactional Channel with txSize > 1, since it can cause messages already consumed not to be acknowledged until the timeout expires. | |

autoStartup
(auto-startup)

| Flag to indicate that the container should start when the ApplicationContext does (as part of the SmartLifecycle callbacks which happen after all beans are initialized). Defaults to true, but set it to false if your broker might not be available on startup, and then call start() later manually when you know the broker is ready. | |

phase
(phase)

| When autoStartup is true, the lifecycle phase within which this container should start and stop. The lower the value the earlier this container will start and the later it will stop. The default is Integer.MAX_VALUE meaning the container will start as late as possible and stop as soon as possible. | |

adviceChain
(advice-chain)

| An array of AOP Advice to apply to the listener execution. This can be used to apply additional cross cutting concerns such as automatic retry in the event of broker death. Note that simple re-connection after an AMQP error is handled by the CachingConnectionFactory, as long as the broker is still alive. | |

taskExecutor
(task-executor)

| A reference to a Spring TaskExecutor (or standard JDK 1.5+ Executor) for executing listener invokers. Default is a SimpleAsyncTaskExecutor, using internally managed threads. | |

errorHandler
(error-handler)

| A reference to an ErrorHandler strategy for handling any uncaught Exceptions that may occur during the execution of the MessageListener. Default: ConditionalRejectingErrorHandler | |

concurrentConsumers
(concurrency)

| The number of concurrent consumers to initially start for each listener. See Section 3.1.15, “Listener Concurrency”. | |

maxConcurrentConsumers
(max-concurrency)

| The maximum number of concurrent consumers to start, if needed, on demand. Must be greater than or equal to concurrentConsumers. See Section 3.1.15, “Listener Concurrency”. | |

startConsumerMinInterval
(min-start-interval)

| The time in milliseconds which must elapse before each new consumer is started on demand. See Section 3.1.15, “Listener Concurrency”. Default 10000 (10 seconds). | |

stopConsumerMinInterval
(min-stop-interval)

| The time in milliseconds which must elapse before a consumer is stopped, since the last consumer was stopped, when an idle consumer is detected. See Section 3.1.15, “Listener Concurrency”. Default 60000 (1 minute). | |

consecutiveActiveTrigger
(min-consecutive-active)

| The minimum number of consecutive messages received by a consumer, without a receive timeout occurring, when considering starting a new consumer. Also impacted by txSize. See Section 3.1.15, “Listener Concurrency”. Default 10. | |

consecutiveIdleTrigger
(min-consecutive-idle)

| The minimum number of receive timeouts a consumer must experience before considering stopping a consumer. Also impacted by txSize. See Section 3.1.15, “Listener Concurrency”. Default 10. | |

connectionFactory
(connection-factory)

| A reference to the connectionFactory; when configuring using the XML namespace, the default referenced bean name is "rabbitConnectionFactory". | |

defaultRequeueRejected
(requeue-rejected)

| Determines whether messages that are rejected because the listener threw an exception should be requeued or not. Default true. | |

recoveryInterval
(recovery-interval)

| Determines the time in milliseconds between attempts to start a consumer if it fails to start for non-fatal reasons. Default 5000. Mutually exclusive with recoveryBackOff. | |

recoveryBackOff
(recovery-back-off)

| Specifies the BackOff for intervals between attempts to start a consumer if it fails to start for non-fatal reasons. Default is FixedBackOff with unlimited retries every 5 seconds. Mutually exclusive with recoveryInterval. | |

exclusive
(exclusive)

| Determines whether the single consumer in this container has exclusive access to the queue(s). The concurrency of the container must be 1 when this is true. If another consumer has exclusive access, the container will attempt to recover the consumer, according to the recovery-interval or recovery-back-off. When using the namespace, this attribute appears on the <rabbit:listener/> element along with the queue names. Default false. | |

rabbitAdmin
(admin)

| When a listener container listens to at least one auto-delete queue and it is found to be missing during startup, the container uses a RabbitAdmin to declare the queue and any related bindings and exchanges. If such elements are configured to use conditional declaration (see the section called “Conditional Declaration”), the container must use the admin that was configured to declare those elements. Specify that admin here; only required when using auto-delete queues with conditional declaration. If you do not wish the auto-delete queue(s) to be declared until the container is started, set auto-startup to false on the admin. Defaults to a RabbitAdmin that will declare all non-conditional elements. | |

missingQueuesFatal
(missing-queues-fatal)

| Starting with version 1.3.5, SimpleMessageListenerContainer has this new property. | |

mismatchedQueuesFatal
(mismatched-queues-fatal)

| This was added in version 1.6. When the container starts, if this property is true (default: false), the container checks that all queues declared in the context are compatible with queues already on the broker. If mismatched properties (e.g. auto-delete) or arguments (e.g. x-message-ttl) exist, the container (and application context) will fail to start with a fatal exception. | |

autoDeclare
(auto-declare)

| Starting with version 1.4, SimpleMessageListenerContainer has this new property. | |

declarationRetries
(declaration-retries)

| Starting with versions 1.4.3, 1.3.9, SimpleMessageListenerContainer has this new property. The namespace attribute is available in version 1.5.x | |

failedDeclarationRetryInterval
(failed-declaration-retry-
interval)

| Starting with versions 1.4.3, 1.3.9, SimpleMessageListenerContainer has this new property. The namespace attribute is available in version 1.5.x | |

retryDeclarationInterval
(missing-queue-retry-
interval)

| Starting with versions 1.4.3, 1.3.9, SimpleMessageListenerContainer has this new property. The namespace attribute is available in version 1.5.x | |

consumerTagStrategy
(consumer-tag-strategy)

| Starting with version 1.4.5, SimpleMessageListenerContainer has this new property. The namespace attribute is available in version 1.5.x | |

idleEventInterval
(idle-event-integer)

| Starting with version 1.6, SimpleMessageListenerContainer has this new property. See the section called “Detecting Idle Asynchronous Consumers”. |

3.1.15 Listener Concurrency

By default, the listener container will start a single consumer which will receive messages from the queue(s).

When examining the table in the previous section, you will see a number of properties/attributes that control concurrency. The simplest is concurrentConsumers, which simply creates that (fixed) number of consumers which will concurrently process messages.

Prior to version 1.3.0, this was the only setting available and the container had to be stopped and started again to change the setting.

Since version 1.3.0, you can now dynamically adjust the concurrentConsumers property. If it is changed while the container is running, consumers will be added or removed as necessary to adjust to the new setting.

In addition, a new property maxConcurrentConsumers has been added and the container will dynamically adjust the concurrency based on workload. This works in conjunction with four additional properties: consecutiveActiveTrigger, startConsumerMinInterval, consecutiveIdleTrigger, stopConsumerMinInterval. With the default settings, the algorithm to increase consumers works as follows:

If the maxConcurrentConsumers has not been reached and an existing consumer is active for 10 consecutive cycles AND at least 10 seconds has elapsed since the last consumer was started, a new consumer is started. A consumer is considered active if it received at least one message in txSize * receiveTimeout milliseconds.

With the default settings, the algorithm to decrease consumers works as follows:

If there are more than concurrentConsumers running and a consumer detects 10 consecutive timeouts (idle) AND the last consumer was stopped at least 60 seconds ago, a consumer will be stopped. The timeout depends on the receiveTimeout and the txSize properties. A consumer is considered idle if it receives no messages in txSize * receiveTimeout milliseconds. So, with the default timeout (1 second) and a txSize of 4, stopping a consumer will be considered after 40 seconds of idle time (4 timeouts correspond to 1 idle detection).

	Note
Practically, consumers will only be stopped if the whole container is idle for some time. This is because the broker will share its work across all the active consumers.

3.1.16 Exclusive Consumer

Also starting with version 1.3, the listener container can be configured with a single exclusive consumer; this prevents other containers from consuming from the queue(s) until the current consumer is cancelled. The concurrency of such a container must be 1.

When using exclusive consumers, other containers will attempt to consume from the queue(s) according to the recoveryInterval property, and log a WARNing if the attempt fails.

3.1.17 Listener Container Queues

version 1.3 introduced a number of improvements for handling multiple queues in a listener container.

The container must be configured to listen on at least one queue; this was the case previously too, but now queues can be added and removed at runtime. The container will recycle (cancel and re-create) the consumers when any pre-fetched messages have been processed. See methods addQueues, addQueueNames, removeQueues and removeQueueNames. When removing queues, at least one queue must remain.

A consumer will now start if any of its queues are available - previously the container would stop if any queues were unavailable. Now, this is only the case if none of the queues are available. If not all queues are available, the container will attempt to passively declare (and consume from) the missing queue(s) every 60 seconds.

Also, if a consumer receives a cancel from the broker (for example if a queue is deleted) the consumer will attempt to recover and the recovered consumer will continue to process messages from any other configured queues. Previously a cancel on one queue cancelled the entire consumer and eventually the container would stop due to the missing queue.

If you wish to permanently remove a queue, you should update the container before or after deleting to queue, to avoid future attempts to consume from it.

3.1.18 Resilience: Recovering from Errors and Broker Failures

Introduction

Some of the key (and most popular) high-level features that Spring AMQP provides are to do with recovery and automatic re-connection in the event of a protocol error or broker failure. We have seen all the relevant components already in this guide, but it should help to bring them all together here and call out the features and recovery scenarios individually.

The primary reconnection features are enabled by the CachingConnectionFactory itself. It is also often beneficial to use the RabbitAdmin auto-declaration features. In addition, if you care about guaranteed delivery, you probably also need to use the channelTransacted flag in RabbitTemplate and SimpleMessageListenerContainer and also the AcknowledgeMode.AUTO (or manual if you do the acks yourself) in the SimpleMessageListenerContainer.

Automatic Declaration of Exchanges, Queues and Bindings

The RabbitAdmin component can declare exchanges, queues and bindings on startup. It does this lazily, through a ConnectionListener, so if the broker is not present on startup it doesn’t matter. The first time a Connection is used (e.g. by sending a message) the listener will fire and the admin features will be applied. A further benefit of doing the auto declarations in a listener is that if the connection is dropped for any reason (e.g. broker death, network glitch, etc.) they will be applied again the next time they are needed.

	Note
Queues declared this way must have fixed names; either explicitly declared, or generated by the framework for AnonymousQueue s. Anonymous queues are non-durable, exclusive, and auto-delete.

	Important
Automatic declaration is only performed when the CachingConnectionFactory cache mode is CHANNEL (the default). This limitation exists because exlusive and auto-delete queues are bound to the connection.

Failures in Synchronous Operations and Options for Retry

If you lose your connection to the broker in a synchronous sequence using RabbitTemplate (for instance), then Spring AMQP will throw an AmqpException (usually but not always AmqpIOException). We don’t try to hide the fact that there was a problem, so you have to be able to catch and respond to the exception. The easiest thing to do if you suspect that the connection was lost, and it wasn’t your fault, is to simply try the operation again. You can do this manually, or you could look at using Spring Retry to handle the retry (imperatively or declaratively).

Spring Retry provides a couple of AOP interceptors and a great deal of flexibility to specify the parameters of the retry (number of attempts, exception types, backoff algorithm etc.). Spring AMQP also provides some convenience factory beans for creating Spring Retry interceptors in a convenient form for AMQP use cases, with strongly typed callback interfaces for you to implement custom recovery logic. See the Javadocs and properties of StatefulRetryOperationsInterceptor and StatelessRetryOperationsInterceptor for more detail. Stateless retry is appropriate if there is no transaction or if a transaction is started inside the retry callback. Note that stateless retry is simpler to configure and analyse than stateful retry, but it is not usually appropriate if there is an ongoing transaction which must be rolled back or definitely is going to roll back. A dropped connection in the middle of a transaction should have the same effect as a rollback, so for reconnection where the transaction is started higher up the stack, stateful retry is usually the best choice.

Starting with version 1.3, a builder API is provided to aid in assembling these interceptors using Java (or in @Configuration classes), for example:

_@Bean_
public StatefulRetryOperationsInterceptor interceptor() {
    return RetryInterceptorBuilder.stateful()
            .maxAttempts(5)
            .backOffOptions(1000, 2.0, 10000) // initialInterval, multiplier, maxInterval
            .build();
}

Only a subset of retry capabilities can be configured this way; more advanced features would need the configuration of a RetryTemplate as a Spring bean. See the Spring Retry Javadocs for complete information about available policies and their configuration.

Message Listeners and the Asynchronous Case

If a MessageListener fails because of a business exception, the exception is handled by the message listener container and then it goes back to listening for another message. If the failure is caused by a dropped connection (not a business exception), then the consumer that is collecting messages for the listener has to be cancelled and restarted. The SimpleMessageListenerContainer handles this seamlessly, and it leaves a log to say that the listener is being restarted. In fact it loops endlessly trying to restart the consumer, and only if the consumer is very badly behaved indeed will it give up. One side effect is that if the broker is down when the container starts, it will just keep trying until a connection can be established. 如果MessageListener由于业务异常而失败， message listener container就会捕获这个异常然后继续监听其他消息。如果因为连接失败（不是业务异常），consumer必须先取消再重启。SimpleMessageListenerContainer可以马上处理，并且打出listener重启的日志。事实上它会死循环地去尝试重consumer并且当且仅当consumer实在是表现的太差才不得不放弃。副作用是当container启动的时候，broker却奔溃了，container将会持续尝试连接直到connection被创建。 Business exception handling, as opposed to protocol errors and dropped connections, might need more thought and some custom configuration, especially if transactions and/or container acks are in use. Prior to 2.8.x, RabbitMQ had no definition of dead letter behaviour, so by default a message that is rejected or rolled back because of a business exception can be redelivered ad infinitum. To put a limit in the client on the number of re-deliveries, one choice is a StatefulRetryOperationsInterceptor in the advice chain of the listener. The interceptor can have a recovery callback that implements a custom dead letter action: whatever is appropriate for your particular environment. 相对于协议错误和连接中断，业务异常的处理可能需要更多的思考和一些用户配置，特别是当transactions和container ack至少一个正在使用。在2.8.x之前，RabiitMQ对没有死信行为作出定义，所以在默认情况下因业务错误导致被拒绝或者回滚的消息可能会再一次投递和持久化。为了在客户端限制重投递的数目，一个选择是利用StatefulRetryOperationsInterceptor，它是listener建议链中的一个。这个拦截器有一个实现自定义死信行为的恢复回调，适用任何特定环境。 Another alternative is to set the container’s rejectRequeued property to false. This causes all failed messages to be discarded. When using RabbitMQ 2.8.x or higher, this also facilitates delivering the message to a Dead Letter Exchange. 另一个选择是将container中的rejectRequeued属性设置成false。这会导致所有失败的消息将会被抛弃。当用到RabbitMQ 2.8.或者更高版本，这种配置后，死信将会被投递到Dead Letter Exchange中。 Or, you can throw a AmqpRejectAndDontRequeueException; this prevents message requeuing, regardless of the setting of the defaultRequeueRejected property.

Often, a combination of both techniques will be used. Use a StatefulRetryOperationsInterceptor in the advice chain, where it’s MessageRecover throws an AmqpRejectAndDontRequeueException. The MessageRecover is called when all retries have been exhausted. The default MessageRecoverer simply consumes the errant message and emits a WARN message. In which case, the message is ACK’d and won’t be sent to the Dead Letter Exchange, if any.

Starting with version 1.3, a new RepublishMessageRecoverer is provided, to allow publishing of failed messages after retries are exhausted:

_@Bean_
RetryOperationsInterceptor interceptor() {
    return RetryInterceptorBuilder.stateless()
            .maxAttempts(5)
            .recoverer(new RepublishMessageRecoverer(amqpTemplate(), "bar", "baz"))
            .build();
}

The RepublishMessageRecoverer publishes the message with additional information in message headers, such as the exception message, stack trace, original exchange and routing key. Additional headers can be added by creating a subclass and overriding additionalHeaders().

Exception Classification for Retry

Spring Retry has a great deal of flexibility for determining which exceptions can invoke retry. The default configuration will retry for all exceptions. Given that user exceptions will be wrapped in a ListenerExecutionFailedException we need to ensure that the classification examines the exception causes. The default classifier just looks at the top level exception. Spring Retry 为哪些异常需要调用 retry功能提供了很大的灵活性。默认配置下，任何的异常都可以触发retry。鉴于自定义的异常将会被ListenerExecutionFailedException 封装，我们需要确定classification可以识别出异常的原因。默认的classifier只关注于最高级别的异常。

Since Spring Retry 1.0.3, the BinaryExceptionClassifier has a property traverseCauses (default false). When true it will traverse exception causes until it finds a match or there is no cause. 从Spring Retry 1.0.3开始，BinaryExceptionClassifier定义了traverseCauses 属性（默认为false）。当traverseCauses属性为true时，如果有异常原因，BinaryExceptionClassifier就会遍历所有异常原因直到找到与异常匹配的原因。

To use this classifier for retry, use a SimpleRetryPolicy created with the constructor that takes the max attempts, the Map of Exception s and the boolean (traverseCauses), and inject this policy into the RetryTemplate. 在retry中使用classifier的时候，用SimpleRetryPolicy类中带有max attemps,key为Exceptions,value为boolen的map构造方法创建对象然后将它注入到RetryTemplate中。

3.1.19 Debugging

Spring AMQP提供了可扩展的日志功能，特别是DEBUG级别。

如果你希望监控应用和broker之间的AMQP protocol，可以使用例如WireShark之类的工具，这些工具有解析协议的插件。或者RabbitMQ JAVA client 提供一个非常有好用的类：Tracer。运行Tracer的main方法，程序默认监听5673端口，通过端口5672连接localhost。简单地修改connection factory配置以便连接localhost的5673端口，然后运行Tracer，Tracer将会在控制台打印处解析好的协议。更多有关Tracer的资源，请阅读Tracer文档。