Spring AMQP consists of a handful of modules, each represented by a JAR in the distribution. These modules are: spring-amqp, spring-rabbit and spring-erlang. The 'spring-amqp' module contains the org.springframework.amqp.core package. Within that package, you will find the classes that represent the core AMQP "model". Our intention is to provide generic abstractions that do not rely on any particular AMQP broker implementation or client library. End user code will be more portable across vendor implementations as it can be developed against the abstraction layer only. These abstractions are then used implemented by broker-specific modules, such as 'spring-rabbit'. For the 1.0 release there is only a RabbitMQ implementation however the abstractions have been validated in .NET using Apache Qpid in addition to RabbitMQ. Since AMQP operates at the protocol level in principle the RabbitMQ client can be used with any broker that supports the same protocol version, but we do not test any other brokers at present.

The overview here assumes that you are already familiar with the basics of the AMQP specification already. If you are not, then have a look at the resources listed in Part IV, “Other Resources”

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;
    }
}

3.1.2. Exchange

The Exchange interface represents an AMQP Exchange, which is what a Message Producer sends to. Each Exchange within a virtual host of a broker will have a unique name as well as a few other properties:

public interface Exchange {

    String getName();

    String getExchangeType();

    boolean isDurable();

    boolean isAutoDelete();

    Map<String, Object> getArguments();

}

As you can see, an Exchange also has a 'type' represented by constants defined in ExchangeTypes. The basic types are: Direct, Topic, Fanout, and Headers. In the core package you will find implementations of the Exchange interface for each of those types. The behavior varies across these Exchange types in terms of how they handle bindings to Queues. For example, a Direct exchange allows for a Queue to be bound by a fixed routing key (often the Queue's name). A Topic exchange supports bindings with routing patterns that may include the '*' and '#' wildcards for 'exactly-one' and 'zero-or-more', respectively. The Fanout exchange publishes to all Queues that are bound to it without taking any routing key into consideration. For much more information about these and the other Exchange types, check out Part IV, “Other Resources”.

	Note
	The AMQP specification also requires that any broker provide a "default" Direct Exchange that has no name. All Queues that are declared will be bound to that default Exchange with their names as routing keys. You will learn more about the default Exchange's usage within Spring AMQP in Section 3.3, “AmqpTemplate”.

3.1.3. Queue

The Queue class represents the component from which a Message Consumer receives Messages. Like the various Exchange classes, our implementation is intended to be an abstract representation of this core AMQP type.

public class Queue  {

    private final String name;

    private volatile boolean durable;

    private volatile boolean exclusive;

    private volatile boolean autoDelete;

    private volatile Map<String, Object> arguments;

    /**
     * The queue is durable, non-exclusive and non auto-delete.
     *
     * @param name the name of the queue.
     */
    public Queue(String name) {
        this(name, true, false, false);
    }

    // Getters and Setters omitted for brevity

Notice that the constructor takes the Queue name. Depending on the implementation, the admin template may provide methods for generating a uniquely named Queue. Such Queues can be useful as a "reply-to" address or other temporary situations. For that reason, the 'exclusive' and 'autoDelete' properties of an auto-generated Queue would both be set to 'true'.

	Note
	See the section on queues in Section 3.8, “Configuring the broker” for information about declaring queues using namespace support, including queue arguments.

3.1.4. Binding

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.

You can bind a Queue to a DirectExchange with a fixed routing key.

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

You can bind a Queue to a TopicExchange with a routing pattern.

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

You can bind a Queue to a FanoutExchange with no routing key.

new Binding(someQueue, someFanoutExchange)

We also provide a BindingBuilder to facilitate a "fluent API" style.

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

	Note
	The BindingBuilder class is shown above for clarity, but this style works well when using a static import for the 'bind()' method.

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 3.8, “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 3.3, “AmqpTemplate”).

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 Section 3.15.1, “Automatic Declaration of Exchanges, Queues and Bindings”) is NOT supported. Also, at the time of writing, the rabbitmq-client library creates a fixed thread pool for each connection (5 threads) by default. When using a large number of connections, you should consider setting a custom executor on the CachingConnectionFactory. Then, the same executor will be used by all connections and its threads can be shared. The executor's thread pool should be unbounded, or set appropriately for the expected utilization (usually, at least one thread per connection). If multiple channels are created on each connection then the pool size will affect the concurrency, so a variable (or simple cached) thread pool executor would be most suitable.

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

Connection connection = connectionFactory.createConnection();

When using XML, the configuration might look like this:

<bean id="connectionFactory"
      class="org.springframework.amqp.rabbit.connection.CachingConnectionFactory">
    <constructor-arg value="somehost"/>
    <property name="username" value="guest"/>
    <property name="password" value="guest"/>
</bean>

Note

There is also a SingleConnectionFactory implementation which is only available in the unit test code of the framework. It is simpler than CachingConnectionFactory since it does not cache channels, but it is not intended for practical usage outside of simple tests due to its lack of performance and resilience. If you find a need to implement your own ConnectionFactory for some reason, the AbstractConnectionFactory base class may provide a nice starting point.

A ConnectionFactory can be created quickly and conveniently using the rabbit namespace:

<rabbit:connection-factory id="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 id="connectionFactory"
      class="org.springframework.amqp.rabbit.connection.CachingConnectionFactory">
    <constructor-arg value="somehost"/>
    <property name="username" value="guest"/>
    <property name="password" value="guest"/>
    <property name="channelCacheSize" value="25"/>
</bean>

And with the namespace you can just add the 'channel-cache-size' attribute:

<rabbit:connection-factory
    id="connectionFactory" channel-cache-size="25"/>

The default cache mode is CHANNEL, but you can configure it to cache connections instead; in this case, we use connection-cache-size:

<rabbit:connection-factory
    id="connectionFactory" cache-mode="CONNECTION" connection-cache-size="25"/>

Host and port attributes can be provided using the namespace

<rabbit:connection-factory
    id="connectionFactory" host="somehost" port="5672"/>

Alternatively, if running in a clustered environment, use the addresses attribute.

<rabbit:connection-factory
    id="connectionFactory" addresses="host1:5672,host2:5672"/>

<rabbit:connection-factory
      id="connectionFactory" connection-factory="rabbitConnectionFactory"/>

<bean id="connectionFactory"
      class="org.springframework.amqp.rabbit.connection.SimpleRoutingConnectionFactory">
	<property name="targetConnectionFactories">
		<map>
			<entry key="#{connectionFactory1.virtualHost}" ref="connectionFactory1"/>
			<entry key="#{connectionFactory2.virtualHost}" ref="connectionFactory2"/>
		</map>
	</property>
</bean>

<rabbit:template id="template" connection-factory="connectionFactory" />

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());
	}

}

3.2.3 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, Channels 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.

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.

<rabbit:template id="template" connection-factory="connectionFactory" retry-template="retryTemplate"/>

<bean id="retryTemplate" class="org.springframework.retry.support.RetryTemplate">
	<property name="backOffPolicy">
		<bean class="org.springframework.retry.backoff.ExponentialBackOffPolicy">
			<property name="initialInterval" value="500" />
			<property name="multiplier" value="10.0" />
			<property name="maxInterval" value="10000" />
		</bean>
	</property>
</bean>

@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;
}

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

void confirm(CorrelationData correlationData, boolean ack);

When sending a Message, one can use any of the following methods:

void send(Message message) throws AmqpException;

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

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

We can begin our discussion with the last method listed above since it is actually the most explicit. It allows an AMQP Exchange name to be provided at runtime along with a routing key. The last parameter is the callback that is responsible for actual creating of the Message instance. An example of using this method to send a Message might look this this:

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

The "exchange" property can be set on the template itself if you plan to use that template instance to send to the same exchange most or all of the time. In such cases, the second method listed above may be used instead. The following example is functionally equivalent to the previous one:

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

If both the "exchange" and "routingKey" properties are set on the template, then the method accepting only the Message may be used:

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

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();

public static MessageBuilder withBody(byte[] body) 

public static MessageBuilder withClonedBody(byte[] body) 

public static MessageBuilder withBody(byte[] body, int from, int to) 

public static MessageBuilder fromMessage(Message message) 

public static MessageBuilder fromClonedMessage(Message message) 

public static MessagePropertiesBuilder newInstance() 

public static MessagePropertiesBuilder fromProperties(MessageProperties properties) 

public static MessagePropertiesBuilder fromClonedProperties(MessageProperties properties) 

Message reception is always a bit more complicated than sending. The reason is that 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.

Message receive() throws AmqpException;

Message receive(String queueName) throws AmqpException;

Object receiveAndConvert() throws AmqpException;

Object receiveAndConvert(String queueName) throws AmqpException;

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

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

<R, S> boolean receiveAndReply(ReceiveAndReplyCallback<R, S> callback,
	String replyExchange, String replyRoutingKey) throws AmqpException;

<R, S> boolean receiveAndReply(String queueName, ReceiveAndReplyCallback<R, S> callback,
	String replyExchange, String replyRoutingKey) throws AmqpException;

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

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

boolean received =
        this.template.receiveAndReply(ROUTE, new ReceiveAndReplyCallback<Order, Invoice>() {

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

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

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

MessageListener listener = new MessageListenerAdapter(somePojo);

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

<rabbit:listener-container connection-factory="rabbitConnectionFactory">
    <rabbit:listener queues="some.queue" ref="somePojo" method="handle"/>
</rabbit:listener-container>

@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);
            }
        };
    }
}

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

<rabbit:listener-container connection-factory="rabbitConnectionFactory">
    <rabbit:listener queues="some.queue" ref="somePojo" method="handle" priority="10" />
</rabbit:listener-container>

<rabbit:queue id="otherAnon" declared-by="containerAdmin" />

<rabbit:direct-exchange name="otherExchange" auto-delete="true" declared-by="containerAdmin">
	<rabbit:bindings>
		<rabbit:binding queue="otherAnon" key="otherAnon" />
	</rabbit:bindings>
</rabbit:direct-exchange>

<rabbit:listener-container id="container2" auto-startup="false">
	<rabbit:listener id="listener2" ref="foo" queues="otherAnon" admin="containerAdmin" />
</rabbit:listener-container>

<rabbit:admin id="containerAdmin" connection-factory="rabbitConnectionFactory"
	auto-startup="false" />

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 Section 3.5.2, “Asynchronous Consumer” also uses a MessageConverter.

<bean class="org.springframework.amqp.rabbit.core.RabbitTemplate">
    <property name="connectionFactory" ref="rabbitConnectionFactory"/>
    <property name="messageConverter">
        <bean class="org.springframework.amqp.support.converter.JsonMessageConverter">
            <!-- if necessary, override the DefaultClassMapper -->
            <property name="classMapper" ref="customClassMapper"/>
        </bean>
    </property>
</bean>

<bean class="org.springframework.amqp.rabbit.core.RabbitTemplate">
    <property name="connectionFactory" ref="rabbitConnectionFactory"/>
    <property name="messageConverter">
        <bean class="org.springframework.amqp.support.converter.Jackson2JsonMessageConverter">
            <!-- if necessary, override the DefaultClassMapper -->
            <property name="classMapper" ref="customClassMapper"/>
        </bean>
    </property>
</bean>

<bean id="jsonConverterWithDefaultType"
      class="o.s.amqp.support.converter.JsonMessageConverter">
    <property name="classMapper">
        <bean class="org.springframework.amqp.support.converter.DefaultClassMapper">
            <property name="defaultType" value="foo.PurchaseOrder"/>
        </bean>
    </property>
</bean>

<bean id="jsonConverterWithDefaultType"
      class="o.s.amqp.support.converter.Jackson2JsonMessageConverter">
    <property name="classMapper">
        <bean class="org.springframework.amqp.support.converter.DefaultClassMapper">
            <property name="defaultType" value="foo.PurchaseOrder"/>
        </bean>
    </property>
</bean>

<bean class="org.springframework.amqp.rabbit.core.RabbitTemplate">
    <property name="connectionFactory" ref="rabbitConnectionFactory"/>
    <property name="messageConverter">
        <bean class="org.springframework.amqp.support.converter.MarshallingMessageConverter">
            <constructor-arg ref="someImplemenationOfMarshallerAndUnmarshaller"/>
        </bean>
    </property>
</bean>

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.

By default, 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 <reply-listener/> sub element. This element provides a listener container for the reply queue, with the template being the listener. All of the Section 3.11, “Message Listener Container Configuration” attributes allowed on a <listener-container/> are allowed on the element, except for connection-factory and message-converter, which are inherited from the template's configuration.

<rabbit:template id="amqpTemplate"
        connection-factory="connectionFactory" reply-queue="replies">
    <rabbit:reply-listener/>
</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).

3.7.1 Message Correlation With A Reply Queue

When using a fixed reply queue, 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 propertry to hold correlation data, you can set the correlation-key attribute on the <rabbit-template/>. 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 a fixed reply queue, 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, a listener container is not needed.

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.

	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 id="amqpTemplate" class="org.springframework.amqp.rabbit.core.RabbitTemplate">
    <constructor-arg ref="connectionFactory" />
    <property name="exchange" value="foo.exchange" />
    <property name="routingKey" value="foo" />
    <property name="replyQueue" ref="replyQ" />
    <property name="replyTimeout" value="600000" />
</bean>

<bean class="org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer">
    <constructor-arg ref="connectionFactory" />
    <property name="queues" ref="replyQ" />
    <property name="messageListener" ref="amqpTemplate" />
</bean>

<rabbit:queue id="replyQ" name="my.reply.queue" />

    @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.

3.7.2 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 MessageConverters, the method paramters 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 id="client"
	class="org.springframework.amqp.remoting.client.AmqpProxyFactoryBean">
	<property name="amqpTemplate" ref="template" />
	<property name="serviceInterface" value="foo.ServiceInterface" />
</bean>

<rabbit:connection-factory id="connectionFactory" />

<rabbit:template id="template" connection-factory="connectionFactory" reply-timeout="2000"
	routing-key="remoting.binding" exchange="remoting.exchange" />

<rabbit:admin connection-factory="connectionFactory" />

<rabbit:queue name="remoting.queue" />

<rabbit:direct-exchange name="remoting.exchange">
	<rabbit:bindings>
		<rabbit:binding queue="remoting.queue" key="remoting.binding" />
	</rabbit:bindings>
</rabbit:direct-exchange>

<bean id="listener"
	class="org.springframework.amqp.remoting.service.AmqpInvokerServiceExporter">
	<property name="serviceInterface" value="foo.ServiceInterface" />
	<property name="service" ref="service" />
	<property name="amqpTemplate" ref="template" />
</bean>

<bean id="service" class="foo.ServiceImpl" />

<rabbit:connection-factory id="connectionFactory" />

<rabbit:template id="template" connection-factory="connectionFactory" />

<rabbit:queue name="remoting.queue" />

<rabbit:listener-container connection-factory="connectionFactory">
	<rabbit:listener ref="listener" queue-names="remoting.queue" />
</rabbit:listener-container>

	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.

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 no-arg declareQueue() method defines a queue on the broker whose name 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. This is useful if you wish the broker to generate the queue's name. This is in contrast to an AnonymousQueue where the framework generates a unique (UUID) name and sets durable to false and exlusive, autoDelete to true. 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. A <rabbit:queue/> with an empty, or missing, name will always create an AnonymousQueue. This is because the name will change if redeclared due to a connection failure. Declarative queues must have fixed names because they might be referenced elsewhere in the context, for example, in a listener:

<rabbit:listener-container>
    <rabbit:listener ref="listener" queue-names="#{someQueue.name}" />
</rabbit:listener-container>

See Section 3.15.1, “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:

<rabbit:connection-factory id="connectionFactory"/>

<rabbit:admin id="amqpAdmin" connection-factory="connectionFactory"/>

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:

<rabbit:queue id="tradeQueue"/>

<rabbit:queue id="marketDataQueue"/>

<fanout-exchange name="broadcast.responses"
                 xmlns="http://www.springframework.org/schema/rabbit">
    <bindings>
        <binding queue="tradeQueue"/>
    </bindings>
</fanout-exchange>

<topic-exchange name="app.stock.marketdata"
                xmlns="http://www.springframework.org/schema/rabbit">
    <bindings>
        <binding queue="marketDataQueue" pattern="${stocks.quote.pattern}"/>
    </bindings>
</topic-exchange>

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.

<rabbit:queue name="stocks.trade.queue"/>

	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 <rabbit:queue-arguments> element.

<rabbit:queue name="withArguments">
    <rabbit:queue-arguments>
        <entry key="x-ha-policy" value="all"/>
    </rabbit:queue-arguments>
</rabbit:queue>

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

<rabbit:queue name="withArguments">
    <rabbit:queue-arguments value-type="java.lang.Long">
        <entry key="x-message-ttl" value="100"/>
    </rabbit:queue-arguments>
</rabbit:queue>

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

<rabbit:queue name="withArguments">
    <rabbit:queue-arguments>
        <entry key="x-message-ttl">
            <value type="java.lang.Long">100</value>
        </entry>
        <entry key="x-ha-policy" value="all"/>
    </rabbit:queue-arguments>
</rabbit:queue>

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

<rabbit:queue name="withArguments">
    <rabbit:queue-arguments>
        <entry key="x-message-ttl" value="100" value-type="java.lang.Long"/>
        <entry key="x-ha-policy" value="all"/>
    </rabbit:queue-arguments>
</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.

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:headers-exchange name="headers-test">
	<rabbit:bindings>
		<rabbit:binding queue="bucket">
			<rabbit:binding-arguments>
				<entry key="foo" value="bar"/>
				<entry key="baz" value="qux"/>
				<entry key="x-match" value="all"/>
			</rabbit:binding-arguments>
		</rabbit:binding>
	</rabbit:bindings>
</rabbit:headers-exchange>

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.

3.8.1 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:admin id="admin1" connection-factory="CF1" />

<rabbit:admin id="admin2" connection-factory="CF2" />

<rabbit:queue id="declaredByBothAdminsImplicitly" />

<rabbit:queue id="declaredByBothAdmins" declared-by="admin1, admin2" />

<rabbit:queue id="declaredByAdmin1Only" declared-by="admin1" />

<rabbit:queue id="notDeclaredByAny" auto-declare="false" />

<rabbit:direct-exchange name="direct" declared-by="admin1, admin2">
	<rabbit:bindings>
		<rabbit:binding key="foo" queue="bar"/>
	</rabbit:bindings>
</rabbit:direct-exchange>

	Note
	The auto-declare attribute is true by default and if the declared-by is not supplied (or is empty) then all RabbitAdmins 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;
}

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 Section 3.15.3, “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 3.9, “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 (Section 3.15.3, “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.

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.

3.10.1 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 Section 3.15.3, “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.

3.10.2 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's RabbitTemplate, it will autodetect a thread-bound Channel and automatically participate in it.

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 id="rabbitTxManager"
      class="org.springframework.amqp.rabbit.transaction.RabbitTransactionManager">
    <property name="connectionFactory" ref="connectionFactory"/>
</bean>

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:message-listener-container/>

<util:properties id="spring.amqp.global.properties">
    <prop key="smlc.missing.queues.fatal">false</prop>
</util:properties>

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.

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.

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.

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.

3.15.1 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 AnonymousQueues. 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.

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

@Bean
RetryOperationsInterceptor interceptor() {
	return RetryInterceptorBuilder.stateless()
			.withMaxAttempts(5)
			.setRecoverer(new RepublishMessageRecoverer(amqpTemplate(), "bar", "baz"))
			.build();
}

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.

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.

To use this classifier for retry, use a SimpleRetryPolicy created with the constructor that takes the max attempts, the Map of Exceptions and the boolean (traverseCauses), and inject this policy into the RetryTemplate.

Spring AMQP provides extensive logging, especially at DEBUG level.

If you wish to monitor the AMQP protocol between the application and broker, you could use a tool such as WireShark, which has a plugin to decode the protocol. Alternatively the RabbitMQ java client comes with a very useful class Tracer. When run as a main, by default, it listens on port 5673 and connects to port 5672 on localhost. Simply run it, and change your connection factory configuration to connect to port 5673 on localhost. It displays the decoded protocol on the console. Refer to the Tracer javadocs for more information.