3. Reference

Spring AMQP consists of a handful of modules, each represented by a JAR in the distribution. These modules are: spring-amqp, and spring-rabbit. 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. There is currently 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. If you are not, then have a look at the resources listed in Chapter 5, Other Resources

The 0-9-1 AMQP specification does not define a Message class or interface. Instead, when performing an operation such as basicPublish(), the content is passed as a byte-array argument and additional properties are passed in as separate arguments. Spring AMQP defines a Message class as part of a more general AMQP domain model representation. The purpose of the Message class is to simply encapsulate the body and properties within a single instance so that the API can in turn be simpler. The Message class definition is quite straightforward.

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;
    }
}

The MessageProperties interface defines several common properties such as messageId, timestamp, contentType, and several more. Those properties can also be extended with user-defined headers by calling the setHeader(String key, Object value) method.

Starting with versions `1.5.7, 1.6.11, 1.7.4, 2.0.0`, if a message body is a serialized `Serializable` java object, it is no longer deserialized (by default) when performing `toString()` operations (such as in log messages).
This is to prevent unsafe deserialization.
By default, only `java.util` and `java.lang` classes are deserialized.
To revert to the previous behavior, you can add allowable class/package patterns by invoking `Message.addWhiteListPatterns(...)`.
A simple `*` wildcard is supported, for example `com.foo.*, *.MyClass`.
Bodies that cannot be deserialized will be represented by `byte[<size>]` in log messages.

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 Chapter 5, 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 3.1.4, “AmqpTemplate”.

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 3.1.10, “Configuring the broker” for information about declaring queues using namespace support, including queue arguments.

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 3.1.10, “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 3.1.4, “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 25), 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. Set the cacheMode to CacheMode.CONNECTION.

	Note
	This does not limit the number of connections, it specifies how many idle open connections are allowed.

Starting with version 1.5.5, a new property connectionLimit is provided. When this is set, it limits the total number of connections allowed. When set, if the limit is reached, the channelCheckoutTimeLimit is used to wait for a connection to become idle. If the time is exceeded, an AmqpTimeoutException is thrown.

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. 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.

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 1.6, the default channel cache size has been increased from 1 to 25. In high volume, multi-threaded, environments, a small cache means that channels are created and closed at a high rate. Increasing the default cache size will avoid this overhead. You should monitor the channels in use via the RabbitMQ Admin UI and consider increasing the cache size further if you see many channels being created and closed. The cache will only grow on-demand (to suit the concurrency requirements of the application) so this change will not impact existing low-volume applications.

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();

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 25. 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="50"/>
</bean>

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

<rabbit:connection-factory
    id="connectionFactory" channel-cache-size="50"/>

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"/>

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

<rabbit:connection-factory id="multiHost" virtual-host="/bar" addresses="host1:1234,host2,host3:4567"
    thread-factory="tf"
    channel-cache-size="10" username="user" password="password" />

<bean id="tf" class="org.springframework.scheduling.concurrent.CustomizableThreadFactory">
    <constructor-arg value="rabbitmq-" />
</bean>

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.

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

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.

<rabbit:connection-factory id="rabbitConnectionFactory"
    connection-factory="clientConnectionFactory"
    host="${host}"
    port="${port}"
    virtual-host="${vhost}"
    username="${username}" password="${password}" />

<bean id="clientConnectionFactory"
        class="org.springframework.xd.dirt.integration.rabbit.RabbitConnectionFactoryBean">
    <property name="useSSL" value="true" />
    <property name="sslPropertiesLocation" value="file:/secrets/rabbitSSL.properties"/>
</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.

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:

<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());
    }

}

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 listener container. 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).

Starting with version 1.6.9 you can add a qualifier to the lookup key using setLookupKeyQualifier on the listener container. This would enable, for example, listening to queues with the same name, but in different virtual host (where you would have a connection factory for each).

For example, with lookup key qualifier foo and a container listening to queue bar, the lookup key you would register the target connection factory with would be foo[bar].

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.

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.

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:

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

Also see the section called “Consumer Failure Events”.

Staring with version 1.6, the CachingConnectionFactory now provides cache statistics via the getCacheProperties() method. These statistics can be used to tune the cache to optimize it in production. For example, the high water marks can be used to determine whether the cache size should be increased. If it equals the cache size, you might want to consider increasing further.

channelCacheSize

localPort

idleChannelsTx

idleChannelsNotTx

idleChannelsTxHighWater

idleChannelsNotTxHighWater

openConnections

channelCacheSize

connectionCacheSize

idleConnections

idleConnectionsHighWater

idleChannelsTx:<localPort>

idleChannelsNotTx:<localPort>

idleChannelsTxHighWater:
<localPort>

idleChannelsNotTxHighWater:
<localPort>

The cacheMode property (CHANNEL or CONNECTION is also included).

Figure 3.1. JVisualVM Example

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”.

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:

<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>

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<T>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<Object, Exception>() {

        @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.

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.

Only one ConfirmCallback is supported by a RabbitTemplate.

Note

When a rabbit template send operation completes, the channel is closed; this would preclude the reception of confirms or returns in the case when the connection factory cache is full (when there is space in the cache, the channel is not physically closed and the returns/confirms will proceed as normal). When the cache is full, the framework defers the close for up to 5 seconds, in order to allow time for the confirms/returns to be received. When using confirms, the channel will be closed when the last confirm is received. When using only returns, the channel will remain open for the full 5 seconds. It is generally recommended to set the connection factory’s channelCacheSize to a large enough value so that the channel on which a message is published is returned to the cache instead of being closed. You can monitor channel usage using the RabbitMQ management plugin; if you see channels being opened/closed rapidly you should consider increasing the cache size to reduce overhead on the server.

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 send and receive messages using the spring-messaging Message<?> abstraction. This abstraction is used by other Spring projects such as Spring Integration and Spring’s STOMP support. There are two message converters involved; one to convert between a spring-messaging Message<?> and Spring AMQP’s Message abstraction, and one to convert between Spring AMQP’s Message abstraction and the format required by the underlying RabbitMQ client library. By default, the message payload is converted by the provided RabbitTemplate 's message converter. Alternatively, you can inject a custom MessagingMessageConverter with some other payload converter:

MessagingMessageConverter amqpMessageConverter = new MessagingMessageConverter();
amqpMessageConverter.setPayloadConverter(myPayloadConverter);
rabbitMessagingTempalte.setAmqpMessageConverter(amqpMessageConverter);

Starting with version 1.6, the template now supports a user-id-expression (userIdExpression when using Java configuration). If a message is sent, the user id property is set (if not already set) after evaluating this expression. The root object for the evaluation is the message to be sent.

Examples:

<rabbit:template ... user-id-expression="'guest'" />

<rabbit:template ... user-id-expression="@myConnectionFactory.username" />

The first example is a literal expression; the second obtains the username property from a connection factory bean in the application context.

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));

Starting with version 1.3, a message builder API is provided by the MessageBuilder and MessagePropertiesBuilder; they provides a convenient "fluent" means of creating a message or message properties:

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

or

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();

Each of the properties defined on the MessageProperties can be set. Other methods include 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().

Five static methods are provided to create an initial message builder:

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) 

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 3.1.4, “AmqpTemplate”. This allows the sender to correlate a confirm (ack or nack) with the sent message.

Starting with version 1.6.7, the CorrelationAwareMessagePostProcessor interface was introduced, allowing the correlation data to be modified after the message has been converted:

Message postProcessMessage(Message message, Correlation correlation);

Also starting with version 1.6.7 a new callback interface is provided CorrelationDataPostProcessor; this is invoked after all MessagePostProcessor s (provided in the send() method as well as those provided in setBeforePublishPostProcessors()). Implementations can update or replace the correlation data supplied in the send() method (if any). The Message and original CorrelationData (if any) are provided as arguments.

CorrelationData postProcess(Message message, CorrelationData correlationData);

When the template’s mandatory property is true returned messages are provided by the callback described in Section 3.1.4, “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.

Starting with version 1.4.2, the BatchingRabbitTemplate has been introduced. This is a subclass of RabbitTemplate 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:

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.

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.

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:

<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;

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<Order, Invoice>() {

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

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.

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:

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.

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 3.1.15, “Message Listener Container Configuration”.

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.

Note

When using the default SimpleAsyncTaskExecutor, for the threads the listener is invoked on, the listener container beanName is used as the threadNamePrefix. This is useful for log analysis; it’s generally recommended to always include the thread name in the logging appender configuration. When a TaskExecutor is specifically provided via the taskExecutor property on the SimpleMessageListenerContainer, it is used as is, without modification. It is recommended that you use a similar technique to name the threads created by a custom TaskExecutor bean definition, to aid with thread identification in log messages.

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”.

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:

<rabbit:listener-container connection-factory="connectionFactory"
        ...
        idle-event-interval="60000"
        ...
        >
    <rabbit:listener id="container1" queue-names="foo" ref="myListener" method="handle" />
</rabbit:listener-container>

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

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

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 {

    @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.

	Caution
	If you wish to use the idle event to stop the lister container, you should not call container.stop() on the thread that calls the listener - it will cause delays and unnecessary log messages. Instead, you should hand off the event to a different thread that can then stop the container.

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.

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.

This converter is similar to the SimpleMessageConverter except it can be configured with other Spring Framework Serializer and Deserializer implementations for application/x-java-serialized-object conversions.

See the section called “Java Deserialization” for important information.

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 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>

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.

<bean id="contentTypeConverter" class="ContentTypeDelegatingMessageConverter">
    <property name="delegates">
        <map>
            <entry key="application/json" value-ref="jsonMessageConverter" />
            <entry key="application/xml" value-ref="xmlMessageConverter" />
        </map>
    </property>
</bean>

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:

For outbound messages:

Also starting with version 1.6, the inbound deliveryMode property is no longer mapped to MessageProperties.deliveryMode, it is mapped to MessageProperties.receivedDeliveryMode instead. Also, the inbound userId property is no longer mapped to MessageProperties.userId, it is mapped to MessageProperties.receivedUserId instead. These changes are to avoid unexpected propagation of these properties if the same MessageProperties object is used for an outbound message.

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.

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.

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.

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 <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.

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 <reply-listener/> sub element. This element provides a listener container for the reply queue, with the template being the listener. All of the Section 3.1.15, “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.

	Important
	If you run multiple instances of your application or use multiple RabbitTemplate s, you MUST use a unique reply queue for each - RabbitMQ has no capability to select messages from a queue so, if they all use the same queue, each instance would compete for replies and not necessarily receive their own.

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

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 <exchange>/<routingKey> 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 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.

	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.

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() {

    ...

    RabbitConverterFuture<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.

Set the receiveTimeout property on the template to time out replies (it defaults to 30000 - 30 seconds). If a timeout occurs, the future will be completed with an AmqpReplyTimeoutException.

The template implements SmartLifecycle; stopping the template while there are pending replies will cause the pending Future s to be canceled.

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 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.

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 RabbitTemplate. 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.

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:

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

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:

<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 s0on 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-exceptions 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 ignoreDeclarationExceptions. This is a global setting which applies to all elements, queues, exchanges and bindings have a similar property which applies to just those elements.

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: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>

Starting with version 1.6 Exchanges can be configured with an internal flag (defaults to false) and such an Exchange will be properly configured on the Broker via a RabbitAdmin (if one is present in the application context). If the internal flag is true for an exchange, RabbitMQ will not allow clients to use the exchange. This is useful for a dead letter exchange, or exchange-to-exchange binding, where you don’t wish the exchange to be used directly by publishers.

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.

Version 1.6 introduces a convenient fluent API for configuring Queue and Exchange objects when using Java configuration:

@Bean
public Queue queue() {
    return QueueBuilder.nonDurable("foo")
        .autoDelete()
        .exclusive()
        .withArgument("foo", "bar")
        .build();
}

@Bean
public Exchange exchange() {
  return ExchangeBuilder.directExchange("foo")
      .autoDelete()
      .internal()
      .withArgument("foo", "bar")
      .build();
}

See the javadocs for org.springframework.amqp.core.QueueBuilder and org.springframework.amqp.core.ExchangeBuilder for more information.

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)
    	);
    }

}

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 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;
}

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:

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.

<rabbit:queue id="uuidAnon" />

<rabbit:queue id="springAnon" naming-strategy="springNamer" />

<rabbit:queue id="customAnon" naming-strategy="customNamer" />

<bean id="springNamer" class="org.springframework.amqp.core.AnonymousQueue.Base64UrlNamingStrategy" />

<bean id="customNamer" class="org.springframework.amqp.core.AnonymousQueue.Base64UrlNamingStrategy">
    <constructor-arg value="custom.gen-" />
</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.

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.

Prior to version 1.6.6, adding a rollback rule to a container’s transactionAttribute, when using an external transaction manager (e.g. JDBC) had no effect; exceptions always rolled back the transaction.

Also, when using a transaction advice in the container’s advice chain, conditional rollback was not very useful because all listener exceptions are wrapped in a ListenerExecutionFailedException.

The first problem has been corrected and the rules are now applied properly. Further, the ListenerFailedRuleBasedTransactionAttribute is now provided; it is a subclass of RuleBasedTransactionAttribute, with the only difference being that it is aware of the ListenerExecutionFailedException and uses the cause of such exceptions for the rule. This transaction attribute can be used directly in the container, or via a transaction advice.

An example of using this rule follows:

@Bean
public AbstractMessageListenerContainer container() {
    ...
    container.setTransactionManager(transactionManager);
    RuleBasedTransactionAttribute transactionAttribute =
        new ListenerFailedRuleBasedTransactionAttribute();
    transactionAttribute.setRollbackRules(Collections.singletonList(
        new NoRollbackRuleAttribute(DontRollBackException.class)));
    container.setTransactionAttribute(transactionAttribute);
    ...
}

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.

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

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.

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 exclusive and auto-delete queues are bound to the connection.

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.

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.

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.

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.

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().

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 Exception s and the boolean (traverseCauses), and inject this policy into the RetryTemplate.

Each appender supports adding client properties to the RabbitMQ connection.

log4j. 

log4j.appender.amqp.clientConnectionProperties=foo:bar,baz:qux

logback. 

<appender name="AMQP" ...>
    ...
    <clientConnectionProperties>foo:bar,baz:qux</clientConnectionProperties>
    ...
</appender>

log4j2. 

<Appenders>
    ...
    <RabbitMQ name="rabbitmq"
        ...
        clientConnectionProperties="foo:bar,baz:qux"
        ...
    </RabbitMQ>
</Appenders>

The properties are a comma-delimited list of key:value pairs; keys and values cannot contain commas or colons.

These properties appear on the RabbitMQ Admin UI when viewing the connection.

With the log4j and logback appenders, the appenders can be subclassed, allowing you to modify the client connection properties before the connection is established:

Customizing the Client Connection Properties. 

public class MyEnhancedAppender extends AmqpAppender {

    private String foo;

    @Override
    protected void updateConnectionClientProperties(Map<String, Object> clientProperties) {
        clientProperties.put("foo", this.foo);
    }

    public void setFoo(String foo) {
        this.foo = foo;
    }

}

For log4j2, add log4j.appender.amqp.foo=bar to log4j.properties to set the property. For logback, add <foo>bar</foo> to logback.xml.

Of course, for simple String properties like this example, the previous technique can be used; subclasses allow richer properties (such as adding a Map or numeric property).

With log4j2, subclasses are not supported, due to the way log4j2 uses static factory methods.

The Hello World sample demonstrates both synchronous and asynchronous message reception. You can import the spring-rabbit-helloworld sample into the IDE and then follow the discussion below.

Within the src/main/java directory, navigate to the org.springframework.amqp.helloworld package. Open the HelloWorldConfiguration class and notice that it contains the @Configuration annotation at class-level and some @Bean annotations at method-level. This is an example of Spring’s Java-based configuration. You can read more about that here.

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

The configuration also contains an instance of RabbitAdmin, which by default looks for any beans of type Exchange, Queue, or Binding and then declares them on the broker. In fact, the "helloWorldQueue" bean that is generated in HelloWorldConfiguration is an example simply because it is an instance of Queue.

@Bean
public Queue helloWorldQueue() {
    return new Queue(this.helloWorldQueueName);
}

Looking back at the "rabbitTemplate" bean configuration, you will see that it has the helloWorldQueue’s name set as its "queue" property (for receiving Messages) and for its "routingKey" property (for sending Messages).

Now that we’ve explored the configuration, let’s look at the code that actually uses these components. First, open the Producer class from within the same package. It contains a main() method where the Spring ApplicationContext is created.

public static void main(String[] args) {
    ApplicationContext context =
        new AnnotationConfigApplicationContext(RabbitConfiguration.class);
    AmqpTemplate amqpTemplate = context.getBean(AmqpTemplate.class);
    amqpTemplate.convertAndSend("Hello World");
    System.out.println("Sent: Hello World");
}

As you can see in the example above, the AmqpTemplate bean is retrieved and used for sending a Message. Since the client code should rely on interfaces whenever possible, the type is AmqpTemplate rather than RabbitTemplate. Even though the bean created in HelloWorldConfiguration is an instance of RabbitTemplate, relying on the interface means that this code is more portable (the configuration can be changed independently of the code). Since the convertAndSend() method is invoked, the template will be delegating to its MessageConverter instance. In this case, it’s using the default SimpleMessageConverter, but a different implementation could be provided to the "rabbitTemplate" bean as defined in HelloWorldConfiguration.

Now open the Consumer class. It actually shares the same configuration base class which means it will be sharing the "rabbitTemplate" bean. That’s why we configured that template with both a "routingKey" (for sending) and "queue" (for receiving). As you saw in Section 3.1.4, “AmqpTemplate”, you could instead pass the routingKey argument to the send method and the queue argument to the receive method. The Consumer code is basically a mirror image of the Producer, calling receiveAndConvert() rather than convertAndSend().

public static void main(String[] args) {
    ApplicationContext context =
        new AnnotationConfigApplicationContext(RabbitConfiguration.class);
    AmqpTemplate amqpTemplate = context.getBean(AmqpTemplate.class);
    System.out.println("Received: " + amqpTemplate.receiveAndConvert());
}

If you run the Producer, and then run the Consumer, you should see the message "Received: Hello World" in the console output.

Now that we’ve walked through the synchronous Hello World sample, it’s time to move on to a slightly more advanced but significantly more powerful option. With a few modifications, the Hello World sample can provide an example of asynchronous reception, a.k.a. Message-driven POJOs. In fact, there is a sub-package that provides exactly that: org.springframework.amqp.samples.helloworld.async.

Once again, we will start with the sending side. Open the ProducerConfiguration class and notice that it creates a "connectionFactory" and "rabbitTemplate" bean. This time, since the configuration is dedicated to the message sending side, we don’t even need any Queue definitions, and the RabbitTemplate only has the routingKey property set. Recall that messages are sent to an Exchange rather than being sent directly to a Queue. The AMQP default Exchange is a direct Exchange with no name. All Queues are bound to that default Exchange with their name as the routing key. That is why we only need to provide the routing key here.

public RabbitTemplate rabbitTemplate() {
    RabbitTemplate template = new RabbitTemplate(connectionFactory());
    template.setRoutingKey(this.helloWorldQueueName);
    return template;
}

Since this sample will be demonstrating asynchronous message reception, the producing side is designed to continuously send messages (if it were a message-per-execution model like the synchronous version, it would not be quite so obvious that it is in fact a message-driven consumer). The component responsible for sending messages continuously is defined as an inner class within the ProducerConfiguration. It is configured to execute every 3 seconds.

static class ScheduledProducer {

    @Autowired
    private volatile RabbitTemplate rabbitTemplate;

    private final AtomicInteger counter = new AtomicInteger();

    @Scheduled(fixedRate = 3000)
    public void sendMessage() {
        rabbitTemplate.convertAndSend("Hello World " + counter.incrementAndGet());
    }
}

You don’t need to understand all of the details since the real focus should be on the receiving side (which we will cover momentarily). However, if you are not yet familiar with Spring 3.0 task scheduling support, you can learn more here. The short story is that the "postProcessor" bean in the ProducerConfiguration is registering the task with a scheduler.

Now, let’s turn to the receiving side. To emphasize the Message-driven POJO behavior will start with the component that is reacting to the messages. The class is called HelloWorldHandler.

public class HelloWorldHandler {

    public void handleMessage(String text) {
        System.out.println("Received: " + text);
    }

}

Clearly, that is a POJO. It does not extend any base class, it doesn’t implement any interfaces, and it doesn’t even contain any imports. It is being "adapted" to the MessageListener interface by the Spring AMQP MessageListenerAdapter. That adapter can then be configured on a SimpleMessageListenerContainer. For this sample, the container is created in the ConsumerConfiguration class. You can see the POJO wrapped in the adapter there.

@Bean
public SimpleMessageListenerContainer listenerContainer() {
    SimpleMessageListenerContainer container = new SimpleMessageListenerContainer();
    container.setConnectionFactory(connectionFactory());
    container.setQueueName(this.helloWorldQueueName);
    container.setMessageListener(new MessageListenerAdapter(new HelloWorldHandler()));
    return container;
}

The SimpleMessageListenerContainer is a Spring lifecycle component and will start automatically by default. If you look in the Consumer class, you will see that its main() method consists of nothing more than a one-line bootstrap to create the ApplicationContext. The Producer’s main() method is also a one-line bootstrap, since the component whose method is annotated with @Scheduled will also start executing automatically. You can start the Producer and Consumer in any order, and you should see messages being sent and received every 3 seconds.