Since Message Channels may or may not buffer Messages (as discussed in the overview), there are two sub-interfaces defining the buffering (pollable) and non-buffering (subscribable) channel behavior. Here is the definition of PollableChannel.

public interface PollableChannel extends MessageChannel {

    Message<?> receive();

    Message<?> receive(long timeout);

}

Similar to the send methods, when receiving a message, the return value will be null in the case of a timeout or interrupt.

The SubscribableChannel base interface is implemented by channels that send Messages directly to their subscribed MessageHandlers. Therefore, they do not provide receive methods for polling, but instead define methods for managing those subscribers:

public interface SubscribableChannel extends MessageChannel {

    boolean subscribe(MessageHandler handler);

    boolean unsubscribe(MessageHandler handler);

}

The PublishSubscribeChannel implementation broadcasts any Message sent to it to all of its subscribed handlers. This is most often used for sending Event Messages whose primary role is notification as opposed to Document Messages which are generally intended to be processed by a single handler. Note that the PublishSubscribeChannel is intended for sending only. Since it broadcasts to its subscribers directly when its send(Message) method is invoked, consumers cannot poll for Messages (it does not implement PollableChannel and therefore has no receive() method). Instead, any subscriber must be a MessageHandler itself, and the subscriber's handleMessage(Message) method will be invoked in turn.

Prior to version 3.0, invoking the send method on a PublishSubscribeChannel that had no subscribers returned false. When used in conjunction with a MessagingTemplate, a MessageDeliveryException was thrown. Starting with version 3.0, the behavior has changed such that a send is always considered successful if at least the minimum subscribers are present (and successfully handle the message). This behavior can be modified by setting the minSubscribers property, which defaults to 0.

	Note
	If a TaskExecutor is used, only the presence of the correct number of subscribers is used for this determination, because the actual handling of the message is performed asynchronously.

The QueueChannel implementation wraps a queue. Unlike the PublishSubscribeChannel, the QueueChannel has point-to-point semantics. In other words, even if the channel has multiple consumers, only one of them should receive any Message sent to that channel. It provides a default no-argument constructor (providing an essentially unbounded capacity of Integer.MAX_VALUE) as well as a constructor that accepts the queue capacity:

public QueueChannel(int capacity)

A channel that has not reached its capacity limit will store messages in its internal queue, and the send() method will return immediately even if no receiver is ready to handle the message. If the queue has reached capacity, then the sender will block until room is available. Or, if using the send call that accepts a timeout, it will block until either room is available or the timeout period elapses, whichever occurs first. Likewise, a receive call will return immediately if a message is available on the queue, but if the queue is empty, then a receive call may block until either a message is available or the timeout elapses. In either case, it is possible to force an immediate return regardless of the queue's state by passing a timeout value of 0. Note however, that calls to the no-arg versions of send() and receive() will block indefinitely.

Whereas the QueueChannel enforces first-in/first-out (FIFO) ordering, the PriorityChannel is an alternative implementation that allows for messages to be ordered within the channel based upon a priority. By default the priority is determined by the 'priority' header within each message. However, for custom priority determination logic, a comparator of type Comparator<Message<?>> can be provided to the PriorityChannel's constructor.

The RendezvousChannel enables a "direct-handoff" scenario where a sender will block until another party invokes the channel's receive() method or vice-versa. Internally, this implementation is quite similar to the QueueChannel except that it uses a SynchronousQueue (a zero-capacity implementation of BlockingQueue). This works well in situations where the sender and receiver are operating in different threads but simply dropping the message in a queue asynchronously is not appropriate. In other words, with a RendezvousChannel at least the sender knows that some receiver has accepted the message, whereas with a QueueChannel, the message would have been stored to the internal queue and potentially never received.

Tip

Keep in mind that all of these queue-based channels are storing messages in-memory only by default. When persistence is required, you can either provide a 'message-store' attribute within the 'queue' element to reference a persistent MessageStore implementation, or you can replace the local channel with one that is backed by a persistent broker, such as a JMS-backed channel or Channel Adapter. The latter option allows you to take advantage of any JMS provider's implementation for message persistence, and it will be discussed in Chapter 19, JMS Support. However, when buffering in a queue is not necessary, the simplest approach is to rely upon the DirectChannel discussed next.

The RendezvousChannel is also useful for implementing request-reply operations. The sender can create a temporary, anonymous instance of RendezvousChannel which it then sets as the 'replyChannel' header when building a Message. After sending that Message, the sender can immediately call receive (optionally providing a timeout value) in order to block while waiting for a reply Message. This is very similar to the implementation used internally by many of Spring Integration's request-reply components.

The DirectChannel has point-to-point semantics but otherwise is more similar to the PublishSubscribeChannel than any of the queue-based channel implementations described above. It implements the SubscribableChannel interface instead of the PollableChannel interface, so it dispatches Messages directly to a subscriber. As a point-to-point channel, however, it differs from the PublishSubscribeChannel in that it will only send each Message to a single subscribed MessageHandler.

In addition to being the simplest point-to-point channel option, one of its most important features is that it enables a single thread to perform the operations on "both sides" of the channel. For example, if a handler is subscribed to a DirectChannel, then sending a Message to that channel will trigger invocation of that handler's handleMessage(Message) method directly in the sender's thread, before the send() method invocation can return.

The key motivation for providing a channel implementation with this behavior is to support transactions that must span across the channel while still benefiting from the abstraction and loose coupling that the channel provides. If the send call is invoked within the scope of a transaction, then the outcome of the handler's invocation (e.g. updating a database record) will play a role in determining the ultimate result of that transaction (commit or rollback).

Note

Since the DirectChannel is the simplest option and does not add any additional overhead that would be required for scheduling and managing the threads of a poller, it is the default channel type within Spring Integration. The general idea is to define the channels for an application and then to consider which of those need to provide buffering or to throttle input, and then modify those to be queue-based PollableChannels. Likewise, if a channel needs to broadcast messages, it should not be a DirectChannel but rather a PublishSubscribeChannel. Below you will see how each of these can be configured.

The DirectChannel internally delegates to a Message Dispatcher to invoke its subscribed Message Handlers, and that dispatcher can have a load-balancing strategy exposed via load-balancer or load-balancer-ref attributes (mutually exclusive). The load balancing strategy is used by the Message Dispatcher to help determine how Messages are distributed amongst Message Handlers in the case that there are multiple Message Handlers subscribed to the same channel. As a convinience the load-balancer attribute exposes enumeration of values pointing to pre-existing implementations of LoadBalancingStrategy. The "round-robin" (load-balances across the handlers in rotation) and "none" (for the cases where one wants to explicitely disable load balancing) are the only available values. Other strategy implementations may be added in future versions. However, since version 3.0 you can provide your own implementation of the LoadBalancingStrategy and inject it using load-balancer-ref attribute which should point to a bean that implements LoadBalancingStrategy.

<int:channel id="lbRefChannel">
  <int:dispatcher load-balancer-ref="lb"/>
</int:channel>
  
<bean id="lb" class="foo.bar.SampleLoadBalancingStrategy"/>

Note that load-balancer or load-balancer-ref attributes are mutually exclusive.

The load-balancing also works in combination with a boolean failover property. If the "failover" value is true (the default), then the dispatcher will fall back to any subsequent handlers as necessary when preceding handlers throw Exceptions. The order is determined by an optional order value defined on the handlers themselves or, if no such value exists, the order in which the handlers are subscribed.

If a certain situation requires that the dispatcher always try to invoke the first handler, then fallback in the same fixed order sequence every time an error occurs, no load-balancing strategy should be provided. In other words, the dispatcher still supports the failover boolean property even when no load-balancing is enabled. Without load-balancing, however, the invocation of handlers will always begin with the first according to their order. For example, this approach works well when there is a clear definition of primary, secondary, tertiary, and so on. When using the namespace support, the "order" attribute on any endpoint will determine that order.

	Note
	Keep in mind that load-balancing and failover only apply when a channel has more than one subscribed Message Handler. When using the namespace support, this means that more than one endpoint shares the same channel reference in the "input-channel" attribute.

The ExecutorChannel is a point-to-point channel that supports the same dispatcher configuration as DirectChannel (load-balancing strategy and the failover boolean property). The key difference between these two dispatching channel types is that the ExecutorChannel delegates to an instance of TaskExecutor to perform the dispatch. This means that the send method typically will not block, but it also means that the handler invocation may not occur in the sender's thread. It therefore does not support transactions spanning the sender and receiving handler.

Tip

Note that there are occasions where the sender may block. For example, when using a TaskExecutor with a rejection-policy that throttles back on the client (such as the ThreadPoolExecutor.CallerRunsPolicy), the sender's thread will execute the method directly anytime the thread pool is at its maximum capacity and the executor's work queue is full. Since that situation would only occur in a non-predictable way, that obviously cannot be relied upon for transactions.

Spring Integration 1.0 provided a ThreadLocalChannel implementation, but that has been removed as of 2.0. Now, there is a more general way for handling the same requirement by simply adding a "scope" attribute to a channel. The value of the attribute can be any name of a Scope that is available within the context. For example, in a web environment, certain Scopes are available, and any custom Scope implementations can be registered with the context. Here's an example of a ThreadLocal-based scope being applied to a channel, including the registration of the Scope itself.

<int:channel id="threadScopedChannel" scope="thread">
     <int:queue />
</int:channel>

<bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
    <property name="scopes">
        <map>
            <entry key="thread" value="org.springframework.context.support.SimpleThreadScope" />
        </map>
    </property>
</bean>

The channel above also delegates to a queue internally, but the channel is bound to the current thread, so the contents of the queue are as well. That way the thread that sends to the channel will later be able to receive those same Messages, but no other thread would be able to access them. While thread-scoped channels are rarely needed, they can be useful in situations where DirectChannels are being used to enforce a single thread of operation but any reply Messages should be sent to a "terminal" channel. If that terminal channel is thread-scoped, the original sending thread can collect its replies from it.

Now, since any channel can be scoped, you can define your own scopes in addition to Thread Local.

As mentioned above, DirectChannel is the default type.

<int:channel id="directChannel"/>

A default channel will have a round-robin load-balancer and will also have failover enabled (See the discussion in the section called “DirectChannel” for more detail). To disable one or both of these, add a <dispatcher/> sub-element and configure the attributes:

<int:channel id="failFastChannel">
    <int:dispatcher failover="false"/>
</channel>

<int:channel id="channelWithFixedOrderSequenceFailover">
    <int:dispatcher load-balancer="none"/>
</int:channel>

There are times when a consumer can only process a particular type of payload and you need to therefore ensure the payload type of input Messages. Of course the first thing that comes to mind is Message Filter. However all that Message Filter will do is filter out Messages that are not compliant with the requirements of the consumer. Another way would be to use a Content Based Router and route Messages with non-compliant data-types to specific Transformers to enforce transformation/conversion to the required data-type. This of course would work, but a simpler way of accomplishing the same thing is to apply the Datatype Channel pattern. You can use separate Datatype Channels for each specific payload data-type.

To create a Datatype Channel that only accepts messages containing a certain payload type, provide the fully-qualified class name in the channel element's datatype attribute:

<int:channel id="numberChannel" datatype="java.lang.Number"/>

Note that the type check passes for any type that is assignable to the channel's datatype. In other words, the "numberChannel" above would accept messages whose payload is java.lang.Integer or java.lang.Double. Multiple types can be provided as a comma-delimited list:

<int:channel id="stringOrNumberChannel" datatype="java.lang.String,java.lang.Number"/>

So the 'numberChannel' above will only accept Messages with a data-type of java.lang.Number. But what happens if the payload of the Message is not of the required type? It depends on whether you have defined a bean named "integrationConversionService" that is an instance of Spring's Conversion Service. If not, then an Exception would be thrown immediately, but if you do have an "integrationConversionService" bean defined, it will be used in an attempt to convert the Message's payload to the acceptable type.

You can even register custom converters. For example, let's say you are sending a Message with a String payload to the 'numberChannel' we configured above.

MessageChannel inChannel = context.getBean("numberChannel", MessageChannel.class);
inChannel.send(new GenericMessage<String>("5"));

Typically this would be a perfectly legal operation, however since we are using Datatype Channel the result of such operation would generate an exception:

Exception in thread "main" org.springframework.integration.MessageDeliveryException:
Channel 'numberChannel'
expected one of the following datataypes [class java.lang.Number],
but received [class java.lang.String]
…

And rightfully so since we are requiring the payload type to be a Number while sending a String. So we need something to convert String to a Number. All we need to do is implement a Converter.

public static class StringToIntegerConverter implements Converter<String, Integer> {
	public Integer convert(String source) {
		return Integer.parseInt(source);
	}
}

Then, register it as a Converter with the Integration Conversion Service:

<int:converter ref="strToInt"/>

<bean id="strToInt" class="org.springframework.integration.util.Demo.StringToIntegerConverter"/>

When the 'converter' element is parsed, it will create the "integrationConversionService" bean on-demand if one is not already defined. With that Converter in place, the send operation would now be successful since the Datatype Channel will use that Converter to convert the String payload to an Integer.

	Note
	For more information regarding Payload Type Conversion, please read Section 7.1.6, “Payload Type Conversion”.

Beginning with version 4.0, the integrationConversionService is invoked by the DefaultDatatypeChannelMessageConverter, which looks up the conversion service in the application context. To use a different conversion technique, you can specify the message-converter attribute on the channel. This must be a reference to a MessageConverter implementation. Only the fromMessage method is used, which provides the converter with access to the message headers (for example if the conversion might need information from the headers, such as content-type). The method can return just the converted payload, or a full Message object. If the latter, the converter must be careful to copy all the headers from the inbound message.

Alternatively, declare a <bean/> of type MessageConverter with an id "datatypeChannelMessageConverter" and that converter will be used by all channels with a datatype.

To create a QueueChannel, use the <queue/> sub-element. You may specify the channel's capacity:

<int:channel id="queueChannel">
    <queue capacity="25"/>
</int:channel>

	Note
	If you do not provide a value for the 'capacity' attribute on this <queue/> sub-element, the resulting queue will be unbounded. To avoid issues such as OutOfMemoryErrors, it is highly recommended to set an explicit value for a bounded queue.

Persistent QueueChannel Configuration

Since a QueueChannel provides the capability to buffer Messages, but does so in-memory only by default, it also introduces a possibility that Messages could be lost in the event of a system failure. To mitigate this risk, a QueueChannel may be backed by a persistent implementation of the MessageGroupStore strategy interface. For more details on MessageGroupStore and MessageStore see Section 8.3, “Message Store”.

When a QueueChannel receives a Message, it will add it to the Message Store, and when a Message is polled from a QueueChannel, it is removed from the Message Store.

By default any QueueChannel only stores its Messages in an in-memory Queue and can therefore lead to the lost message scenario mentioned above. However Spring Integration provides a JdbcMessageStore to allow a QueueChannel to be backed by an RDBMS.

You can configure a Message Store for any QueueChannel by adding the message-store attribute as shown in the next example.

<int:channel id="dbBackedChannel">
    <int:queue message-store="messageStore"/>
</int:channel>

<int-jdbc:message-store id="messageStore" data-source="someDataSource"/>

The above example also shows that JdbcMessageStore can be configured with the namespace support provided by the Spring Integration JDBC module. All you need to do is inject any javax.sql.DataSource instance. The Spring Integration JDBC module also provides schema DDL for most popular databases. These schemas are located in the org.springframework.integration.jdbc package of that module (spring-integration-jdbc).

	Important
	One important feature is that with any transactional persistent store (e.g., JdbcChannelMessageStore), as long as the poller has a transaction configured, a Message removed from the store will only be permanently removed if the transaction completes successfully, otherwise the transaction will roll back and the Message will not be lost.

Many other implementations of the Message Store will be available as the growing number of Spring projects related to "NoSQL" data stores provide the underlying support. Of course, you can always provide your own implementation of the MessageGroupStore interface if you cannot find one that meets your particular needs.

Since version 4.0, it is recommended that QueueChannels are configured to use a ChannelMessageStore if possible. These are generally optimized for this use, when compared with a general message store. If the ChannelMessageStore is a ChannelPriorityMessageStore the messages will be received in FIFO within priority order. The notion of priority is determined by the message store implementation.

To create a PublishSubscribeChannel, use the <publish-subscribe-channel/> element. When using this element, you can also specify the task-executor used for publishing Messages (if none is specified it simply publishes in the sender's thread):

<int:publish-subscribe-channel id="pubsubChannel" task-executor="someExecutor"/>

If you are providing a Resequencer or Aggregator downstream from a PublishSubscribeChannel, then you can set the 'apply-sequence' property on the channel to true. That will indicate that the channel should set the sequence-size and sequence-number Message headers as well as the correlation id prior to passing the Messages along. For example, if there are 5 subscribers, the sequence-size would be set to 5, and the Messages would have sequence-number header values ranging from 1 to 5.

<int:publish-subscribe-channel id="pubsubChannel" apply-sequence="true"/>

	Note
	The apply-sequence value is false by default so that a Publish Subscribe Channel can send the exact same Message instances to multiple outbound channels. Since Spring Integration enforces immutability of the payload and header references, the channel creates new Message instances with the same payload reference but different header values when the flag is set to true.

To create an ExecutorChannel, add the <dispatcher> sub-element along with a task-executor attribute. Its value can reference any TaskExecutor within the context. For example, this enables configuration of a thread-pool for dispatching messages to subscribed handlers. As mentioned above, this does break the "single-threaded" execution context between sender and receiver so that any active transaction context will not be shared by the invocation of the handler (i.e. the handler may throw an Exception, but the send invocation has already returned successfully).

<int:channel id="executorChannel">
    <int:dispatcher task-executor="someExecutor"/>
</int:channel>

Note

The load-balancer and failover options are also both available on the <dispatcher/> sub-element as described above in the section called “DirectChannel Configuration”. The same defaults apply as well. So, the channel will have a round-robin load-balancing strategy with failover enabled unless explicit configuration is provided for one or both of those attributes. <int:channel id="executorChannelWithoutFailover"> <int:dispatcher task-executor="someExecutor" failover="false"/> </int:channel>

To create a PriorityChannel, use the <priority-queue/> sub-element:

<int:channel id="priorityChannel">
    <int:priority-queue capacity="20"/>
</int:channel>

By default, the channel will consult the priority header of the message. However, a custom Comparator reference may be provided instead. Also, note that the PriorityChannel (like the other types) does support the datatype attribute. As with the QueueChannel, it also supports a capacity attribute. The following example demonstrates all of these:

<int:channel id="priorityChannel" datatype="example.Widget">
    <int:priority-queue comparator="widgetComparator"
                    capacity="10"/>
</int:channel>

Since version 4.0, the priority-channel child element supports the message-store option (comparator is not allowed in that case). The message store must be a ChannelPriorityMessageStore and, in this case, the namespace parser will declare a QueueChannel instead of a PriorityChannel. See the section called “QueueChannel Configuration”.

A RendezvousChannel is created when the queue sub-element is a <rendezvous-queue>. It does not provide any additional configuration options to those described above, and its queue does not accept any capacity value since it is a 0-capacity direct handoff queue.

<int:channel id="rendezvousChannel"/>
    <int:rendezvous-queue/>
</int:channel>

Any channel can be configured with a "scope" attribute.

<int:channel id="threadLocalChannel" scope="thread"/>

Message channels may also have interceptors as described in Section 3.1.3, “Channel Interceptors”. The <interceptors/> sub-element can be added within <channel/> (or the more specific element types). Provide the ref attribute to reference any Spring-managed object that implements the ChannelInterceptor interface:

<int:channel id="exampleChannel">
    <int:interceptors>
        <ref bean="trafficMonitoringInterceptor"/>
    </int:interceptors>
</int:channel>

In general, it is a good idea to define the interceptor implementations in a separate location since they usually provide common behavior that can be reused across multiple channels.

Channel Interceptors provide a clean and concise way of applying cross-cutting behavior per individual channel. If the same behavior should be applied on multiple channels, configuring the same set of interceptors for each channel would not be the most efficient way. To avoid repeated configuration while also enabling interceptors to apply to multiple channels, Spring Integration provides Global Interceptors. Look at the example below:

<int:channel-interceptor pattern="input*, bar*, foo" order="3">
    <bean class="foo.barSampleInterceptor"/>
</int:channel-interceptor>

or

<int:channel-interceptor ref="myInterceptor" pattern="input*, bar*, foo" order="3"/>

<bean id="myInterceptor" class="foo.barSampleInterceptor"/>

Each <channel-interceptor/> element allows you to define a global interceptor which will be applied on all channels that match any patterns defined via the pattern attribute. In the above case the global interceptor will be applied on the 'foo' channel and all other channels that begin with 'bar' or 'input'. The order attribute allows you to manage where this interceptor will be injected if there are multiple interceptors on a given channel. For example, channel 'inputChannel' could have individual interceptors configured locally (see below):

<int:channel id="inputChannel"> 
  <int:interceptors>
    <int:wire-tap channel="logger"/> 
  </int:interceptors>
</int:channel>

A reasonable question is how will a global interceptor be injected in relation to other interceptors configured locally or through other global interceptor definitions? The current implementation provides a very simple mechanism for defining the order of interceptor execution. A positive number in the order attribute will ensure interceptor injection after any existing interceptors and a negative number will ensure that the interceptor is injected before existing interceptors. This means that in the above example, the global interceptor will be injected AFTER (since its order is greater than 0) the 'wire-tap' interceptor configured locally. If there were another global interceptor with a matching pattern, its order would be determined by comparing the values of the order attribute. To inject a global interceptor BEFORE the existing interceptors, use a negative value for the order attribute.

	Note
	Note that both the order and pattern attributes are optional. The default value for order will be 0 and for pattern, the default is '*' (to match all channels).

As mentioned above, Spring Integration provides a simple Wire Tap interceptor out of the box. You can configure a Wire Tap on any channel within an <interceptors/> element. This is especially useful for debugging, and can be used in conjunction with Spring Integration's logging Channel Adapter as follows:

<int:channel id="in">
    <int:interceptors>
        <int:wire-tap channel="logger"/>
    </int:interceptors>
</int:channel>

<int:logging-channel-adapter id="logger" level="DEBUG"/>

Tip

The 'logging-channel-adapter' also accepts an 'expression' attribute so that you can evaluate a SpEL expression against 'payload' and/or 'headers' variables. Alternatively, to simply log the full Message toString() result, provide a value of "true" for the 'log-full-message' attribute. That is false by default so that only the payload is logged. Setting that to true enables logging of all headers in addition to the payload. The 'expression' option does provide the most flexibility, however (e.g. expression="payload.user.name").

A little more on Wire Tap

One of the common misconceptions about the wire tap and other similar components (Section B.1, “Message Publishing Configuration”) is that they are automatically asynchronous in nature. Wire-tap as a component is not invoked asynchronously be default. Instead, Spring Integration focuses on a single unified approach to configuring asynchronous behavior: the Message Channel. What makes certain parts of the message flow sync or async is the type of Message Channel that has been configured within that flow. That is one of the primary benefits of the Message Channel abstraction. From the inception of the framework, we have always emphasized the need and the value of the Message Channel as a first-class citizen of the framework. It is not just an internal, implicit realization of the EIP pattern, it is fully exposed as a configurable component to the end user. So, the Wire-tap component is ONLY responsible for performing the following 3 tasks:

It is essentially a variation of the Bridge, but it is encapsulated within a channel definition (and hence easier to enable and disable without disrupting a flow). Also, unlike the bridge, it basically forks another message flow. Is that flow synchronous or asynchronous? The answer simply depends on the type of Message Channel that 'channelB' is. And, now you know that we have: Direct Channel, Pollable Channel, and Executor Channel as options. The last two do break the thread boundary making communication via such channels asynchronous simply because the dispatching of the message from that channel to its subscribed handlers happens on a different thread than the one used to send the message to that channel. That is what is going to make your wire-tap flow sync or async. It is consistent with other components within the framework (e.g., Message Publisher) and actually brings a level of consistency and simplicity by sparing you from worrying in advance (other than writing thread safe code) whether a particular piece of code should be implemented as sync or async. The actual wiring of two pieces of code (component A and component B) via Message Channel is what makes their collaboration sync or async. You may even want to change from sync to async in the future and Message Channel is what's going to allow you to do it swiftly without ever touching the code.

One final point regarding the Wire Tap is that, despite the rationale provided above for not being async be default, one should keep in mind it is usually desirable to hand off the Message as soon as possible. Therefore, it would be quite common to use an asynchronous channel option as the wire-tap's outbound channel. Nonetheless, another reason that we do not enforce asynchronous behavior by default is that you might not want to break a transactional boundary. Perhaps you are using the Wire Tap for auditing purposes, and you DO want the audit Messages to be sent within the original transaction. As an example, you might connect the wire-tap to a JMS outbound-channel-adapter. That way, you get the best of both worlds: 1) the sending of a JMS Message can occur within the transaction while 2) it is still a "fire-and-forget" action thereby preventing any noticeable delay in the main message flow.

Important

Starting with version 4.0, it is important to avoid circular references when an interceptor (such as WireTap) references a channel itself. You need to exclude such channels from those being intercepted by the current interceptor. This can be done with appropriate patterns or programmatically. If you have a custom ChannelInterceptor that references a channel, consider implementing VetoCapableInterceptor. That way, the framework will ask the interceptor if it's OK to intercept each channel that is a candidate based on the pattern. You can also add runtime protection in the interceptor methods that ensures that the channel is not one that is referenced by the interceptor. The WireTap uses both of these techniques.

It is possible to configure a global wire tap as a special case of the Global Channel Interceptor. Simply configure a top level wire-tap element. Now, in addition to the normal wire-tap namespace support, the pattern and order attributes are supported and work in exactly the same way as with the channel-interceptor

<int:wire-tap pattern="input*, bar*, foo" order="3" channel="wiretapChannel"/>

	Tip
	A global wire tap provides a convenient way to configure a single channel wire tap externally without modifying the existing channel configuration. Simply set the pattern attribute to the target channel name. For example, This technique may be used to configure a test case to verify messages on a channel.

When a message transitions through an application, each time it is mutated (e.g. by a transformer) a new message id is assigned. The message id is a UUID. Beginning with Spring Integration 3.0, the default strategy used for id generation is more efficient than the previous java.util.UUID.randomUUID() implementation. It uses simple random numbers based on a secure random seed, instead of creating a secure random number each time.

A different UUID generation strategy can be selected by declaring a bean that implements MessageHeaders.IdGenerator in the application context.

Important

Only one UUID generation strategy can be used in a classloader. This means that if two or more application contexts are running in the same classloader, they will share the same strategy. If one of the contexts changes the strategy, it will be used by all contexts. If two or more contexts in the same classloader declare a bean of type MessageHeaders.IdGenerator, they must all be an instance of the same class, otherwise the context attempting to replace a custom strategy will fail to initialize. If the strategy is the same, but parameterized, the strategy in the first context to initialize will be used.

In addition to the default strategy, two additional IdGenerators are provided; MessageHeaders.JdkIdGenerator uses the previous UUID.randomUUID() mechanism; MessageHeaders.SimpleIncrementingIdGenerator can be used in cases where a UUID is not really needed and a simple incrementing value is sufficient.

The following parameters are valid for all routers inside and outside of chains.

The following parameters are valid only across all top-level routers that are ourside of chains.

A PayloadTypeRouter will send Messages to the channel as defined by payload-type mappings.

<bean id="payloadTypeRouter"
      class="org.springframework.integration.router.PayloadTypeRouter">
    <property name="channelIdentifierMap">
        <map>
            <entry key="java.lang.String" value-ref="stringChannel"/>
            <entry key="java.lang.Integer" value-ref="integerChannel"/>
        </map>
    </property>
</bean>

Configuration of the PayloadTypeRouter is also supported via the namespace provided by Spring Integration (see Section F.2, “Namespace Support”), which essentially simplifies configuration by combining the <router/> configuration and its corresponding implementation defined using a <bean/> element into a single and more concise configuration element. The example below demonstrates a PayloadTypeRouter configuration which is equivalent to the one above using the namespace support:

<int:payload-type-router input-channel="routingChannel">
    <int:mapping type="java.lang.String" channel="stringChannel" />
    <int:mapping type="java.lang.Integer" channel="integerChannel" />
</int:payload-type-router>

A HeaderValueRouter will send Messages to the channel based on the individual header value mappings. When a HeaderValueRouter is created it is initialized with the name of the header to be evaluated. The value of the header could be one of two things:

1. Arbitrary value

2. Channel name

If arbitrary then additional mappings for these header values to channel names is required, otherwise no additional configuration is needed.

Spring Integration provides a simple namespace-based XML configuration to configure a HeaderValueRouter. The example below demonstrates two types of namespace-based configuration for the HeaderValueRouter.

1. Configuration where mapping of header values to channels is required

<int:header-value-router input-channel="routingChannel" header-name="testHeader">
    <int:mapping value="someHeaderValue" channel="channelA" />
    <int:mapping value="someOtherHeaderValue" channel="channelB" />
</int:header-value-router>

During the resolution process this router may encounter channel resolution failures, causing an exception. If you want to suppress such exceptions and send unresolved messages to the default output channel (identified with the default-output-channel attribute) set resolution-required to false.

Normally, messages for which the header value is not explicitly mapped to a channel will be sent to the default-output-channel. However, in cases where the header value is mapped to a channel name but the channel cannot be resolved, setting the resolution-required attribute to false will result in routing such messages to the default-output-channel.

	Important
	With Spring Integration 2.1 the attribute was changed from ignore-channel-name-resolution-failures to resolution-required. Attribute resolution-required will default to true.

2. Configuration where mapping of header values to channel names is not required since header values themselves represent channel names

<int:header-value-router input-channel="routingChannel" header-name="testHeader"/>

Note

Since Spring Integration 2.1 the behavior of resolving channels is more explicit. For example, if you ommit the default-output-channel attribute and the Router was unable to resolve at least one valid channel, and any channel name resolution failures were ignored by setting resolution-required to false, then a MessageDeliveryException is thrown. Basically, by default the Router must be able to route messages successfully to at least one channel. If you really want to drop messages, you must also have default-output-channel set to nullChannel.

A RecipientListRouter will send each received Message to a statically defined list of Message Channels:

<bean id="recipientListRouter"
      class="org.springframework.integration.router.RecipientListRouter">
    <property name="channels">
        <list>
            <ref bean="channel1"/>
            <ref bean="channel2"/>
            <ref bean="channel3"/>
        </list>
    </property>
</bean>

Spring Integration also provides namespace support for the RecipientListRouter configuration (see Section F.2, “Namespace Support”) as the example below demonstrates.

<int:recipient-list-router id="customRouter" input-channel="routingChannel"
        timeout="1234"
        ignore-send-failures="true"
        apply-sequence="true">
  <int:recipient channel="channel1"/>
  <int:recipient channel="channel2"/>
</int:recipient-list-router>

	Note
	The 'apply-sequence' flag here has the same effect as it does for a publish-subscribe-channel, and like a publish-subscribe-channel, it is disabled by default on the recipient-list-router. Refer to the section called “PublishSubscribeChannel Configuration” for more information.

Another convenient option when configuring a RecipientListRouter is to use Spring Expression Language (SpEL) support as selectors for individual recipient channels. This is similar to using a Filter at the beginning of 'chain' to act as a "Selective Consumer". However, in this case, it's all combined rather concisely into the router's configuration.

<int:recipient-list-router id="customRouter" input-channel="routingChannel">
    <int:recipient channel="channel1" selector-expression="payload.equals('foo')"/>
    <int:recipient channel="channel2" selector-expression="headers.containsKey('bar')"/>
</int:recipient-list-router>

In the above configuration a SpEL expression identified by the selector-expression attribute will be evaluated to determine if this recipient should be included in the recipient list for a given input Message. The evaluation result of the expression must be a boolean. If this attribute is not defined, the channel will always be among the list of recipients.

The XPath Router is part of the XML Module. As such, please read chapter Routing XML Messages Using XPath

Spring Integration also provides a special type-based router called ErrorMessageExceptionTypeRouter for routing Error Messages (Messages whose payload is a Throwable instance). ErrorMessageExceptionTypeRouter is very similar to the PayloadTypeRouter. In fact they are almost identical. The only difference is that while PayloadTypeRouter navigates the instance hierarchy of a payload instance (e.g., payload.getClass().getSuperclass()) to find the most specific type/channel mappings, the ErrorMessageExceptionTypeRouter navigates the hierarchy of 'exception causes' (e.g., payload.getCause()) to find the most specific Throwable type/channel mappings.

Below is a sample configuration for ErrorMessageExceptionTypeRouter.

<int:exception-type-router input-channel="inputChannel"
                           default-output-channel="defaultChannel">
    <int:mapping exception-type="java.lang.IllegalArgumentException"
                 channel="illegalChannel"/>
    <int:mapping exception-type="java.lang.NullPointerException"
                 channel="npeChannel"/>
</int:exception-type-router>

<int:channel id="illegalChannel" />
<int:channel id="npeChannel" />

The "router" element provides a simple way to connect a router to an input channel and also accepts the optional default-output-channel attribute. The ref attribute references the bean name of a custom Router implementation (extending AbstractMessageRouter):

<int:router ref="payloadTypeRouter" input-channel="input1"
            default-output-channel="defaultOutput1"/>

<int:router ref="recipientListRouter" input-channel="input2"
            default-output-channel="defaultOutput2"/>

<int:router ref="customRouter" input-channel="input3"
            default-output-channel="defaultOutput3"/>

<beans:bean id="customRouterBean class="org.foo.MyCustomRouter"/>

Alternatively, ref may point to a simple POJO that contains the @Router annotation (see below), or the ref may be combined with an explicit method name. Specifying a method applies the same behavior described in the @Router annotation section below.

<int:router input-channel="input" ref="somePojo" method="someMethod"/>

Using a ref attribute is generally recommended if the custom router implementation is referenced in other <router> definitions. However if the custom router implementation should be scoped to a single definition of the <router>, you may provide an inner bean definition:

<int:router method="someMethod" input-channel="input3"
            default-output-channel="defaultOutput3">
    <beans:bean class="org.foo.MyCustomRouter"/>
</int:router>

	Note
	Using both the ref attribute and an inner handler definition in the same <router> configuration is not allowed, as it creates an ambiguous condition, and an Exception will be thrown.

Routers and the Spring Expression Language (SpEL)

Sometimes the routing logic may be simple and writing a separate class for it and configuring it as a bean may seem like overkill. As of Spring Integration 2.0 we offer an alternative where you can now use SpEL to implement simple computations that previously required a custom POJO router.

	Note
	For more information about the Spring Expression Language, please refer to the respective chapter in the Spring Framework Reference Documentation at: http://static.springsource.org/spring/docs/current/spring-framework-reference/html/expressions.html

Generally a SpEL expression is evaluated and the result is mapped to a channel:

<int:router input-channel="inChannel" expression="payload.paymentType">
    <int:mapping value="CASH" channel="cashPaymentChannel"/>
    <int:mapping value="CREDIT" channel="authorizePaymentChannel"/>
    <int:mapping value="DEBIT" channel="authorizePaymentChannel"/>
</int:router>

To simplify things even more, the SpEL expression may evaluate to a channel name:

<int:router input-channel="inChannel" expression="payload + 'Channel'"/>

In the above configuration the result channel will be computed by the SpEL expression which simply concatenates the value of the payload with the literal String 'Channel'.

Another value of SpEL for configuring routers is that an expression can actually return a Collection, effectively making every <router> a Recipient List Router. Whenever the expression returns multiple channel values the Message will be forwarded to each channel.

<int:router input-channel="inChannel" expression="headers.channels"/>

In the above configuration, if the Message includes a header with the name 'channels' the value of which is a List of channel names then the Message will be sent to each channel in the list. You may also find Collection Projection and Collection Selection expressions useful to select multiple channels. For further information, please see:

When using @Router to annotate a method, the method may return either a MessageChannel or String type. In the latter case, the endpoint will resolve the channel name as it does for the default output channel. Additionally, the method may return either a single value or a collection. If a collection is returned, the reply message will be sent to multiple channels. To summarize, the following method signatures are all valid.

@Router
public MessageChannel route(Message message) {...}

@Router
public List<MessageChannel> route(Message message) {...}

@Router
public String route(Foo payload) {...}

@Router
public List<String> route(Foo payload) {...}

In addition to payload-based routing, a Message may be routed based on metadata available within the message header as either a property or attribute. In this case, a method annotated with @Router may include a parameter annotated with @Header which is mapped to a header value as illustrated below and documented in Section F.5, “Annotation Support”.

@Router
public List<String> route(@Header("orderStatus") OrderStatus status)

One way to manage the router mappings is through the Control Bus pattern which exposes a Control Channel where you can send control messages to manage and monitor Spring Integration components, including routers.

	Note
	For more information about the Control Bus, please see chapter Section 8.5, “Control Bus”.

Typically you would send a control message asking to invoke a particular operation on a particular managed component (e.g. router). The two managed operations (methods) that are specific to changing the router resolution process are:

You can also expose a router instance with Spring's JMX support, and then use your favorite JMX client (e.g., JConsole) to manage those operations (methods) for changing the router's configuration.

	Note
	For more information about Spring Integration's JMX suppor, please see chapter JMX Support.

The <filter> element is used to create a Message-selecting endpoint. In addition to "input-channel and output-channel attributes, it requires a ref. The ref may point to a MessageSelector implementation:

<int:filter input-channel="input" ref="selector" output-channel="output"/>

<bean id="selector" class="example.MessageSelectorImpl"/>

Alternatively, the method attribute can be added at which point the ref may refer to any object. The referenced method may expect either the Message type or the payload type of inbound Messages. The method must return a boolean value. If the method returns 'true', the Message will be sent to the output-channel.

<int:filter input-channel="input" output-channel="output"
    ref="exampleObject" method="someBooleanReturningMethod"/>

<bean id="exampleObject" class="example.SomeObject"/>

If the selector or adapted POJO method returns false, there are a few settings that control the handling of the rejected Message. By default (if configured like the example above), rejected Messages will be silently dropped. If rejection should instead result in an error condition, then set the throw-exception-on-rejection attribute to true:

<int:filter input-channel="input" ref="selector"
    output-channel="output" throw-exception-on-rejection="true"/> 

If you want rejected messages to be routed to a specific channel, provide that reference as the discard-channel:

<int:filter input-channel="input" ref="selector"
    output-channel="output" discard-channel="rejectedMessages"/> 

Note

Message Filters are commonly used in conjunction with a Publish Subscribe Channel. Many filter endpoints may be subscribed to the same channel, and they decide whether or not to pass the Message to the next endpoint which could be any of the supported types (e.g. Service Activator). This provides a reactive alternative to the more proactive approach of using a Message Router with a single Point-to-Point input channel and multiple output channels.

Using a ref attribute is generally recommended if the custom filter implementation is referenced in other <filter> definitions. However if the custom filter implementation is scoped to a single <filter> element, provide an inner bean definition:

<int:filter method="someMethod" input-channel="inChannel" output-channel="outChannel">
  <beans:bean class="org.foo.MyCustomFilter"/>
</filter>

	Note
	Using both the ref attribute and an inner handler definition in the same <filter> configuration is not allowed, as it creates an ambiguous condition, and an Exception will be thrown.

With the introduction of SpEL support, Spring Integration added the expression attribute to the filter element. It can be used to avoid Java entirely for simple filters.

<int:filter input-channel="input" expression="payload.equals('nonsense')"/>

The string passed as the expression attribute will be evaluated as a SpEL expression with the Message available in the evaluation context. If it is necessary to include the result of an expression in the scope of the application context you can use the #{} notation as defined in the SpEL reference documentation .

<int:filter input-channel="input"
            expression="payload.matches(#{filterPatterns.nonsensePattern})"/>

If the Expression itself needs to be dynamic, then an 'expression' sub-element may be used. That provides a level of indirection for resolving the Expression by its key from an ExpressionSource. That is a strategy interface that you can implement directly, or you can rely upon a version available in Spring Integration that loads Expressions from a "resource bundle" and can check for modifications after a given number of seconds. All of this is demonstrated in the following configuration sample where the Expression could be reloaded within one minute if the underlying file had been modified. If the ExpressionSource bean is named "expressionSource", then it is not necessary to provide the source attribute on the <expression> element, but in this case it's shown for completeness.

<int:filter input-channel="input" output-channel="output">
    <int:expression key="filterPatterns.example" source="myExpressions"/>
</int:filter>

<beans:bean id="myExpressions" id="myExpressions"
    class="o.s.i.expression.ReloadableResourceBundleExpressionSource">
    <beans:property name="basename" value="config/integration/expressions"/>
    <beans:property name="cacheSeconds" value="60"/>
</beans:bean>

Then, the 'config/integration/expressions.properties' file (or any more specific version with a locale extension to be resolved in the typical way that resource-bundles are loaded) would contain a key/value pair:

filterPatterns.example=payload > 100

Note

All of these examples that use expression as an attribute or sub-element can also be applied within transformer, router, splitter, service-activator, and header-enricher elements. Of course, the semantics/role of the given component type would affect the interpretation of the evaluation result in the same way that the return value of a method-invocation would be interpreted. For example, an expression can return Strings that are to be treated as Message Channel names by a router component. However, the underlying functionality of evaluating the expression against the Message as the root object, and resolving bean names if prefixed with '@' is consistent across all of the core EIP components within Spring Integration.

A filter configured using annotations would look like this.

public class PetFilter {
    ...
    @Filter 
    public boolean dogsOnly(String input) {
        ...
    }
}

All of the configuration options provided by the xml element are also available for the @Filter annotation.

The filter can be either referenced explicitly from XML or, if the @MessageEndpoint annotation is defined on the class, detected automatically through classpath scanning.

Also see Section 7.7.5, “Advising Endpoints Using Annotations”.

A splitter can be configured through XML as follows:

<int:channel id="inputChannel"/>

<int:splitter id="splitter" 
  ref="splitterBean" 
  method="split" 
  input-channel="inputChannel" 
  output-channel="outputChannel" />

<int:channel id="outputChannel"/>

<beans:bean id="splitterBean" class="sample.PojoSplitter"/>

Using a ref attribute is generally recommended if the custom splitter implementation may be referenced in other <splitter> definitions. However if the custom splitter handler implementation should be scoped to a single definition of the <splitter>, configure an inner bean definition:

<int:splitter id="testSplitter" input-channel="inChannel" method="split"
                output-channel="outChannel">
  <beans:bean class="org.foo.TestSplitter"/>
</int:spliter>

	Note
	Using both a ref attribute and an inner handler definition in the same <int:splitter> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

The @Splitter annotation is applicable to methods that expect either the Message type or the message payload type, and the return values of the method should be a Collection of any type. If the returned values are not actual Message objects, then each item will be wrapped in a Message as its payload. Each message will be sent to the designated output channel for the endpoint on which the @Splitter is defined.

@Splitter
List<LineItem> extractItems(Order order) {
    return order.getItems()
}

Also see Section 7.7.5, “Advising Endpoints Using Annotations”.

The AggregatingMessageHandler (subclass of AbstractCorrelatingMessageHandler) is a MessageHandler implementation, encapsulating the common functionalities of an Aggregator (and other correlating use cases), which are:

The responsibility of deciding how the messages should be grouped together is delegated to a CorrelationStrategy instance. The responsibility of deciding whether the message group can be released is delegated to a ReleaseStrategy instance.

Here is a brief highlight of the base AbstractAggregatingMessageGroupProcessor (the responsibility of implementing the aggregatePayloads method is left to the developer):

public abstract class AbstractAggregatingMessageGroupProcessor
              implements MessageGroupProcessor {

    protected Map<String, Object> aggregateHeaders(MessageGroup group) {
        // default implementation exists
    }

    protected abstract Object aggregatePayloads(MessageGroup group, Map<String, Object> defaultHeaders);

}

public AbstractCorrelatingMessageHandler(MessageGroupProcessor processor, MessageGroupStore store,
        CorrelationStrategy correlationStrategy, ReleaseStrategy releaseStrategy) {
    ...
    this.correlationStrategy = correlationStrategy == null ?
        new HeaderAttributeCorrelationStrategy(MessageHeaders.CORRELATION_ID) : correlationStrategy;
    this.releaseStrategy = releaseStrategy == null ? new SequenceSizeReleaseStrategy() : releaseStrategy;
    ...
}

As for actual processing of the message group, the default implementation is the DefaultAggregatingMessageGroupProcessor. It creates a single Message whose payload is a List of the payloads received for a given group. This works well for simple Scatter Gather implementations with either a Splitter, Publish Subscribe Channel, or Recipient List Router upstream.

Note

When using a Publish Subscribe Channel or Recipient List Router in this type of scenario, be sure to enable the flag to apply-sequence. That will add the necessary headers (CORRELATION_ID, SEQUENCE_NUMBER and SEQUENCE_SIZE). That behavior is enabled by default for Splitters in Spring Integration, but it is not enabled for the Publish Subscribe Channel or Recipient List Router because those components may be used in a variety of contexts in which these headers are not necessary.

When implementing a specific aggregator strategy for an application, a developer can extend AbstractAggregatingMessageGroupProcessor and implement the aggregatePayloads method. However, there are better solutions, less coupled to the API, for implementing the aggregation logic which can be configured easily either through XML or through annotations.

In general, any POJO can implement the aggregation algorithm if it provides a method that accepts a single java.util.List as an argument (parameterized lists are supported as well). This method will be invoked for aggregating messages as follows:

	Note
	In the interest of code simplicity, and promoting best practices such as low coupling, testability, etc., the preferred way of implementing the aggregation logic is through a POJO, and using the XML or annotation support for configuring it in the application.

The ReleaseStrategy interface is defined as follows:

public interface ReleaseStrategy {

  boolean canRelease(MessageGroup group);

}

In general, any POJO can implement the completion decision logic if it provides a method that accepts a single java.util.List as an argument (parameterized lists are supported as well), and returns a boolean value. This method will be invoked after the arrival of each new message, to decide whether the group is complete or not, as follows:

public class MyReleaseStrategy {

    @ReleaseStrategy
    public boolean canMessagesBeReleased(List<Message<?>>) {...}
}

public class MyReleaseStrategy {

    @ReleaseStrategy
    public boolean canMessagesBeReleased(List<String>) {...}
}

As you can see based on the above signatures, the POJO-based Release Strategy will be passed a Collection of not-yet-released Messages (if you need access to the whole Message) or a Collection of payload objects (if the type parameter is anything other than Message). Typically this would satisfy the majority of use cases. However if, for some reason, you need to access the full MessageGroup then you should simply provide an implementation of the ReleaseStrategy interface.

Caution

When handling potentially large groups, it is important to understand how these methods are invoked because the release strategy may be invoked multiple times before the group is released. The most efficient is an implementation of ReleaseStrategy because the aggregator can invoke it directly. The second most efficient is a POJO method with a Collection<Message<?>> parameter type. The least efficient is a POJO method with a Collection<Foo> type - the framework has to copy the payloads from the messages in the group into a new collection (and possibly attempt conversion on the payloads to Foo) every time the release strategy is called. Collection<?> avoids the conversion but still requires creating the new Collection. For these reasons, for large groups, it is recommended that you implement ReleaseStrategy.

When the group is released for aggregation, all its not-yet-released messages are processed and removed from the group. If the group is also complete (i.e. if all messages from a sequence have arrived or if there is no sequence defined), then the group is marked as complete. Any new messages for this group will be sent to the discard channel (if defined). Setting expire-groups-upon-completion to true (default is false) removes the entire group and any new messages, with the same correlation id as the removed group, will form a new group. Partial sequences can be released by using a MessageGroupStoreReaper together with send-partial-result-on-expiry being set to true.

Important

To facilitate discarding of late-arriving messages, the aggregator must maintain state about the group after it has been released. This can eventually cause out of memory conditions. To avoid such situations, you should consider configuring a MessageGroupStoreReaper to remove the group metadata; the expiry parameters should be set to expire groups after it is not expected that late messages will arrive. For information about configuring a reaper, see Section 5.4.5, “Managing State in an Aggregator: MessageGroupStore”.

Spring Integration provides an out-of-the box implementation for ReleaseStrategy, the SequenceSizeReleaseStrategy. This implementation consults the SEQUENCE_NUMBER and SEQUENCE_SIZE headers of each arriving message to decide when a message group is complete and ready to be aggregated. As shown above, it is also the default strategy.

The CorrelationStrategy interface is defined as follows:

public interface CorrelationStrategy {

  Object getCorrelationKey(Message<?> message);

}

The method returns an Object which represents the correlation key used for associating the message with a message group. The key must satisfy the criteria used for a key in a Map with respect to the implementation of equals() and hashCode().

In general, any POJO can implement the correlation logic, and the rules for mapping a message to a method's argument (or arguments) are the same as for a ServiceActivator (including support for @Header annotations). The method must return a value, and the value must not be null.

Spring Integration provides an out-of-the box implementation for CorrelationStrategy, the HeaderAttributeCorrelationStrategy. This implementation returns the value of one of the message headers (whose name is specified by a constructor argument) as the correlation key. By default, the correlation strategy is a HeaderAttributeCorrelationStrategy returning the value of the CORRELATION_ID header attribute. If you have a custom header name you would like to use for correlation, then simply configure that on an instance of HeaderAttributeCorrelationStrategy and provide that as a reference for the Aggregator's correlation-strategy.

Spring Integration supports the configuration of an aggregator via XML through the <aggregator/> element. Below you can see an example of an aggregator.

<channel id="inputChannel"/>

<int:aggregator id="myAggregator" 
		auto-startup="true" 
		input-channel="inputChannel" 
		output-channel="outputChannel" 
		discard-channel="throwAwayChannel" 
		message-store="persistentMessageStore" 
		order="1" 
		send-partial-result-on-expiry="false" 
		send-timeout="1000" 

		correlation-strategy="correlationStrategyBean" 
		correlation-strategy-method="correlate" 
		correlation-strategy-expression="headers['foo']" 

		ref="aggregatorBean" 
		method="aggregate" 

		release-strategy="releaseStrategyBean" 
		release-strategy-method="release" (16)
		release-strategy-expression="size() == 5" (17)

		expire-groups-upon-completion="false" (18)
		empty-group-min-timeout="60000" /> (19)

<int:channel id="outputChannel"/>

<int:channel id="throwAwayChannel"/>

<bean id="persistentMessageStore" class="org.springframework.integration.jdbc.JdbcMessageStore">
	<constructor-arg ref="dataSource"/>
</bean>

<bean id="aggregatorBean" class="sample.PojoAggregator"/>

<bean id="releaseStrategyBean" class="sample.PojoReleaseStrategy"/>

<bean id="correlationStrategyBean" class="sample.PojoCorrelationStrategy"/>

Using a ref attribute is generally recommended if a custom aggregator handler implementation may be referenced in other <aggregator> definitions. However if a custom aggregator implementation is only being used by a single definition of the <aggregator>, you can use an inner bean definition (starting with version 1.0.3) to configure the aggregation POJO within the <aggregator> element:

<aggregator input-channel="input" method="sum" output-channel="output">
    <beans:bean class="org.foo.PojoAggregator"/>
</aggregator>

	Note
	Using both a ref attribute and an inner bean definition in the same <aggregator> configuration is not allowed, as it creates an ambiguous condition. In such cases, an Exception will be thrown.

An example implementation of the aggregator bean looks as follows:

public class PojoAggregator {

  public Long add(List<Long> results) {
    long total = 0l;
    for (long partialResult: results) {
      total += partialResult;
    }
    return total;
  }

}

An implementation of the completion strategy bean for the example above may be as follows:

public class PojoReleaseStrategy {
...
  public boolean canRelease(List<Long> numbers) {
    int sum = 0;
    for (long number: numbers) {
      sum += number;
    }
    return sum >= maxValue;
  }
}

	Note
	Wherever it makes sense, the release strategy method and the aggregator method can be combined in a single bean.

An implementation of the correlation strategy bean for the example above may be as follows:

public class PojoCorrelationStrategy {
...
  public Long groupNumbersByLastDigit(Long number) {
    return number % 10;
  }
}

For example, this aggregator would group numbers by some criterion (in our case the remainder after dividing by 10) and will hold the group until the sum of the numbers provided by the payloads exceeds a certain value.

	Note
	Wherever it makes sense, the release strategy method, correlation strategy method and the aggregator method can be combined in a single bean (all of them or any two).

Aggregators and Spring Expression Language (SpEL)

Since Spring Integration 2.0, the various strategies (correlation, release, and aggregation) may be handled with SpEL which is recommended if the logic behind such release strategy is relatively simple. Let's say you have a legacy component that was designed to receive an array of objects. We know that the default release strategy will assemble all aggregated messages in the List. So now we have two problems. First we need to extract individual messages from the list, and then we need to extract the payload of each message and assemble the array of objects (see code below).

public String[] processRelease(List<Message<String>> messages){
    List<String> stringList = new ArrayList<String>();
    for (Message<String> message : messages) {
        stringList.add(message.getPayload());
    }
    return stringList.toArray(new String[]{});
}

However, with SpEL such a requirement could actually be handled relatively easily with a one-line expression, thus sparing you from writing a custom class and configuring it as a bean.

<int:aggregator input-channel="aggChannel"
    output-channel="replyChannel"
    expression="#this.![payload].toArray()"/>

In the above configuration we are using a Collection Projection expression to assemble a new collection from the payloads of all messages in the list and then transforming it to an Array, thus achieving the same result as the java code above.

The same expression-based approach can be applied when dealing with custom Release and Correlation strategies.

Instead of defining a bean for a custom CorrelationStrategy via the correlation-strategy attribute, you can implement your simple correlation logic via a SpEL expression and configure it via the correlation-strategy-expression attribute.

For example:

correlation-strategy-expression="payload.person.id"

In the above example it is assumed that the payload has an attribute person with an id which is going to be used to correlate messages.

Likewise, for the ReleaseStrategy you can implement your release logic as a SpEL expression and configure it via the release-strategy-expression attribute. The only difference is that since ReleaseStrategy is passed the List of Messages, the root object in the SpEL evaluation context is the List itself. That List can be referenced as #this within the expression.

For example:

release-strategy-expression="#this.size() gt 5"

In this example the root object of the SpEL Evaluation Context is the MessageGroup itself, and you are simply stating that as soon as there are more than 5 messages in this group, it should be released.

An aggregator configured using annotations would look like this.

public class Waiter {
  ...

  @Aggregator 
  public Delivery aggregatingMethod(List<OrderItem> items) {
    ...
  }

  @ReleaseStrategy 
  public boolean releaseChecker(List<Message<?>> messages) {
    ...
  }

  @CorrelationStrategy 
  public String correlateBy(OrderItem item) {
    ...
  }

}

All of the configuration options provided by the xml element are also available for the @Aggregator annotation.

The aggregator can be either referenced explicitly from XML or, if the @MessageEndpoint is defined on the class, detected automatically through classpath scanning.

The <transformer> element is used to create a Message-transforming endpoint. In addition to "input-channel" and "output-channel" attributes, it requires a "ref". The "ref" may either point to an Object that contains the @Transformer annotation on a single method (see below) or it may be combined with an explicit method name value provided via the "method" attribute.

<int:transformer id="testTransformer" ref="testTransformerBean" input-channel="inChannel"
             method="transform" output-channel="outChannel"/>
<beans:bean id="testTransformerBean" class="org.foo.TestTransformer" />

Using a "ref" attribute is generally recommended if the custom transformer handler implementation can be reused in other <transformer> definitions. However if the custom transformer handler implementation should be scoped to a single definition of the <transformer>, you can define an inner bean definition:

<int:transformer id="testTransformer" input-channel="inChannel" method="transform"
                output-channel="outChannel">
  <beans:bean class="org.foo.TestTransformer"/>
</transformer>

	Note
	Using both the "ref" attribute and an inner handler definition in the same <transformer> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

The method that is used for transformation may expect either the Message type or the payload type of inbound Messages. It may also accept Message header values either individually or as a full map by using the @Header and @Headers parameter annotations respectively. The return value of the method can be any type. If the return value is itself a Message, that will be passed along to the transformer's output channel.

As of Spring Integration 2.0, a Message Transformer's transformation method can no longer return null. Returning null will result in an exception since a Message Transformer should always be expected to transform each source Message into a valid target Message. In other words, a Message Transformer should not be used as a Message Filter since there is a dedicated <filter> option for that. However, if you do need this type of behavior (where a component might return NULL and that should not be considered an error), a service-activator could be used. Its requires-reply value is FALSE by default, but that can be set to TRUE in order to have Exceptions thrown for NULL return values as with the transformer.

Transformers and Spring Expression Language (SpEL)

Just like Routers, Aggregators and other components, as of Spring Integration 2.0 Transformers can also benefit from SpEL support (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html) whenever transformation logic is relatively simple.

<int:transformer input-channel="inChannel"
	output-channel="outChannel"
	expression="payload.toUpperCase() + '- [' + T(java.lang.System).currentTimeMillis() + ']'"/>

In the above configuration we are achieving a simple transformation of the payload with a simple SpEL expression and without writing a custom transformer. Our payload (assuming String) will be upper-cased and concatenated with the current timestamp with some simple formatting.

Common Transformers