Since message channels may or may not buffer messages (as discussed in the Chapter 5, Spring Integration Overview), two sub-interfaces define the buffering (pollable) and non-buffering (subscribable) channel behavior. The following listing shows the definition of the PollableChannel interface:

public interface PollableChannel extends MessageChannel {

    Message<?> receive();

    Message<?> receive(long timeout);

}

As with the send methods, when receiving a message, the return value is null in the case of a timeout or interrupt.

The SubscribableChannel base interface is implemented by channels that send messages directly to their subscribed MessageHandler instances. Therefore, they do not provide receive methods for polling. Instead, they define methods for managing those subscribers. The following listing shows the definition of the SubscribableChannel interface:

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 itself be a MessageHandler, and the subscriber’s handleMessage(Message) method is 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 you use a TaskExecutor, 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, as the following listing shows:

public QueueChannel(int capacity)

A channel that has not reached its capacity limit storeS messages in its internal queue, and the send() method returns immediately, even if no receiver is ready to handle the message. If the queue has reached capacity, the sender blocks until room is available. Alternatively, if you use the send call that accepts a timeout, the queue blocks until either room is available or the timeout period elapses, whichever occurs first. Similarly, a receive call returns 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() 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 constructor.

The RendezvousChannel enables a "direct-handoff" scenario, wherein a sender blocks until another party invokes the channel’s receive() method. The other party blocks until the sender sends the message. 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 operate in different threads, but asynchronously dropping the message in a queue is not appropriate. In other words, with a RendezvousChannel, 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 lets you take advantage of any JMS provider’s implementation for message persistence, as discussed in Chapter 23, JMS Support. However, when buffering in a queue is not necessary, the simplest approach is to rely upon the DirectChannel, discussed in the next section.

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 earlier. 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 sends 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 subscribes to a DirectChannel, then sending a Message to that channel triggers 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, the outcome of the handler’s invocation (for example, updating a database record) plays 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, consider which of those need to provide buffering or to throttle input, and 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. Later, we show how each of these channels 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 by 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 when multiple message handlers subscribe to the same channel. As a convenience, the load-balancer attribute exposes an enumeration of values pointing to pre-existing implementations of LoadBalancingStrategy. round-robin (load-balances across the handlers in rotation) and none (for the cases where one wants to explicitly 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 by using the load-balancer-ref attribute, which should point to a bean that implements LoadBalancingStrategy, as the following example shows:

<int:channel id="lbRefChannel">
  <int:dispatcher load-balancer-ref="lb"/>
</int:channel>

<bean id="lb" class="foo.bar.SampleLoadBalancingStrategy"/>

Note that the load-balancer and load-balancer-ref attributes are mutually exclusive.

The load-balancing also works in conjunction with a boolean failover property. If the "failover" value is true (the default), the dispatcher falls 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 subscribed.

If a certain situation requires that the dispatcher always try to invoke the first handler and then fall back 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 always begins 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 determines the order.

	Note
	Keep in mind that load-balancing and failover apply only 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 defined 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 does not block, but it also means that the handler invocation may not occur in the sender’s thread. It therefore does not support transactions that span the sender and receiving handler.

Caution

The sender can sometimes block. For example, when using a TaskExecutor with a rejection policy that throttles the client (such as the ThreadPoolExecutor.CallerRunsPolicy), the sender’s thread can execute the method any time 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, you should not rely upon it for transactions.

Spring Integration 1.0 provided a ThreadLocalChannel implementation, but that has been removed as of 2.0. Now the more general way to handle the same requirement is to add a scope attribute to a channel. The value of the attribute can be the 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. The following example shows a thread-local 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 defined in the previous example also delegates to a queue internally, but the channel is bound to the current thread, so the contents of the queue are similarly bound. That way, the thread that sends to the channel can later 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 DirectChannel instances 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 the terminal channel.

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

As mentioned earlier, DirectChannel is the default type. The following listing shows who to define one in XML:

<int:channel id="directChannel"/>

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

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

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

Sometimes, a consumer can process only a particular type of payload, forcing you to ensure the payload type of the input messages. The first thing that comes to mind may be to use a message filter. However, all that message filter can 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 and conversion to the required data type. This would work, but a simpler way to accomplish 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 accepts only messages that contain a certain payload type, provide the data type’s fully-qualified class name in the channel element’s datatype attribute, as the following example shows:

<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 in the preceding example would accept messages whose payload is java.lang.Integer or java.lang.Double. Multiple types can be provided as a comma-delimited list, as the following example shows:

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

So the numberChannel in the preceding example accepts only 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. However, if you have defined an integrationConversionService bean, it is used in an attempt to convert the message’s payload to the acceptable type.

You can even register custom converters. For example, suppose you send a message with a String payload to the numberChannel we configured above. You might handle the message as follows:

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

Typically this would be a perfectly legal operation. However, since we use Datatype Channel, the result of such operation would generate an exception similar to the following:

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]
…

The exception happens because we require the payload type to be a Number, but we sent a String. So we need something to convert a String to a Number. For that, we can implement a converter similar to the following example:

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

Then we can register it as a converter with the Integration Conversion Service, as the following example shows:

<int:converter ref="strToInt"/>

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

When the converter element is parsed, it creates the integrationConversionService bean if one is not already defined. With that converter in place, the send operation would now be successful, because the datatype channel uses that converter to convert the String payload to an Integer.

For more information regarding payload type conversion, see Section 10.1.6, “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. It provides the converter with access to the message headers (in case the conversion might need information from the headers, such as content-type). The method can return only 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, you can declare a <bean/> of type MessageConverter with an ID of datatypeChannelMessageConverter, and that converter is used by all channels with a datatype.

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

<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 is unbounded. To avoid issues such as running out of memory, we highly recommend that you 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 12.4, “Message Store”.

	Important
	The capacity attribute is not allowed when the message-store attribute is used.

When a QueueChannel receives a Message, it adds the message to the message store. When a Message is polled from a QueueChannel, it is removed from the message store.

By default, a QueueChannel stores its messages in an in-memory queue, which can lead to the lost message scenario mentioned earlier. However, Spring Integration provides persistent stores, such as the JdbcChannelMessageStore.

You can configure a message store for any QueueChannel by adding the message-store attribute, as the following example shows:

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

<bean id="channelStore" class="o.s.i.jdbc.store.JdbcChannelMessageStore">
    <property name="dataSource" ref="dataSource"/>
    <property name="channelMessageStoreQueryProvider" ref="queryProvider"/>
</bean>

The Spring Integration JDBC module also provides a schema Data Definition Language (DDL) for a number of popular databases. These schemas are located in the org.springframework.integration.jdbc.store.channel package of that module (spring-integration-jdbc).

	Important
	One important feature is that, with any transactional persistent store (such as JdbcChannelMessageStore), as long as the poller has a transaction configured, a message removed from the store can be permanently removed only if the transaction completes successfully. Otherwise the transaction rolls back, and the Message is not lost.

Many other implementations of the message store are available as the growing number of Spring projects related to "NoSQL" data stores come to provide underlying support for these stores. You can also provide your own implementation of the MessageGroupStore interface if you cannot find one that meets your particular needs.

Since version 4.0, we recommend that QueueChannel instances be configured to use a ChannelMessageStore, if possible. These are generally optimized for this use, as compared to a general message store. If the ChannelMessageStore is a ChannelPriorityMessageStore, the messages are received in FIFO within priority order. The notion of priority is determined by the message store implementation. For example, the following example shows the Java configuration for the Section 25.2.1, “MongoDB Channel Message Store”:

@Bean
public BasicMessageGroupStore mongoDbChannelMessageStore(MongoDbFactory mongoDbFactory) {
    MongoDbChannelMessageStore store = new MongoDbChannelMessageStore(mongoDbFactory);
    store.setPriorityEnabled(true);
    return store;
}

@Bean
public PollableChannel priorityQueue(BasicMessageGroupStore mongoDbChannelMessageStore) {
    return new PriorityChannel(new MessageGroupQueue(mongoDbChannelMessageStore, "priorityQueue"));
}

	Note
	Pay attention to the MessageGroupQueue class. That is a BlockingQueue implementation to use the MessageGroupStore operations.

The same implementation with Java DSL might look like the following example:

@Bean
public IntegrationFlow priorityFlow(PriorityCapableChannelMessageStore mongoDbChannelMessageStore) {
    return IntegrationFlows.from((Channels c) ->
            c.priority("priorityChannel", mongoDbChannelMessageStore, "priorityGroup"))
            ....
            .get();
}

Another option to customize the QueueChannel environment is provided by the ref attribute of the <int:queue> sub-element or its particular constructor. This attribute supplies the reference to any java.util.Queue implementation. For example, a Hazelcast distributed IQueue can be configured as follows:

@Bean
public HazelcastInstance hazelcastInstance() {
    return Hazelcast.newHazelcastInstance(new Config()
                                           .setProperty("hazelcast.logging.type", "log4j"));
}

@Bean
public PollableChannel distributedQueue() {
    return new QueueChannel(hazelcastInstance()
                              .getQueue("springIntegrationQueue"));
}

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 publishes in the sender’s thread), as follows:

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

If you provide a resequencer or aggregator downstream from a PublishSubscribeChannel, you can set the apply-sequence property on the channel to true. Doing so indicates that the channel should set the sequence-size and sequence-number message headers as well as the correlation ID prior to passing along the messages. For example, if there are five subscribers, the sequence-size would be set to 5, and the messages would have sequence-number header values ranging from 1 to 5.

Along with the Executor, you can also configure an ErrorHandler. By default, the PublishSubscribeChannel uses a MessagePublishingErrorHandler implementation to send an error to the MessageChannel from the errorChannel header or into the global errorChannel instance. If an Executor is not configured, the ErrorHandler is ignored and exceptions are thrown directly to the caller’s thread.

If you provide a Resequencer or Aggregator downstream from a PublishSubscribeChannel, you can set the apply-sequence property on the channel to true. Doing so indicates that the channel should set the sequence-size and sequence-number message headers as well as the correlation ID prior to passing along the messages. For example, if there are five subscribers, the sequence-size would be set to 5, and the messages would have sequence-number header values ranging from 1 to 5.

The following example shows how to set the apply-sequence header to true:

<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, when the flag is set to true, the channel creates new Message instances with the same payload reference but different header values.

To create an ExecutorChannel, add the <dispatcher> sub-element with a task-executor attribute. The attribute’s value can reference any TaskExecutor within the context. For example, doing so enables configuration of a thread pool for dispatching messages to subscribed handlers. As mentioned earlier, doing so breaks the single-threaded execution context between sender and receiver so that any active transaction context is not shared by the invocation of the handler (that is, the handler may throw an Exception, but the send invocation has already returned successfully). The following example shows how to use the dispatcher element and specify an executor in the task-executor attribute:

<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 earlier in the section called “DirectChannel Configuration”. The same defaults apply. Consequently, the channel has a round-robin load-balancing strategy with failover enabled unless explicit configuration is provided for one or both of those attributes, as the following example shows: <int:channel id="executorChannelWithoutFailover"> <int:dispatcher task-executor="someExecutor" failover="false"/> </int:channel>

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

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

By default, the channel consults the priority header of the message. However, you can instead provide a custom Comparator reference. 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 and capacity are not allowed in that case). The message store must be a PriorityCapableChannelMessageStore. Implementations of the PriorityCapableChannelMessageStore are currently provided for Redis, JDBC, and MongoDB. See the section called “QueueChannel Configuration” and Section 12.4, “Message Store” for more information. You can find sample configuration in Section 21.4.3, “Backing Message Channels”.

A RendezvousChannel is created when the queue sub-element is a <rendezvous-queue>. It does not provide any additional configuration options to those described earlier, and its queue does not accept any capacity value, since it is a zero-capacity direct handoff queue. The following example shows how to declare a RendezvousChannel:

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

Any channel can be configured with a scope attribute, as the following example shows:

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

Message channels may also have interceptors, as described in Section 6.1.3, “Channel Interceptors”. The <interceptors/> sub-element can be added to a <channel/> (or the more specific element types). You can provide the ref attribute to reference any Spring-managed object that implements the ChannelInterceptor interface, as the following example shows:

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

In general, we recommend defining 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. Consider the following pair of examples:

<int:channel-interceptor pattern="input*, thing2*, thing1, !cat*" order="3">
    <bean class="thing1.thing2SampleInterceptor"/>
</int:channel-interceptor>

<int:channel-interceptor ref="myInterceptor" pattern="input*, thing2*, thing1, !cat*" order="3"/>

<bean id="myInterceptor" class="thing1.thing2SampleInterceptor"/>

Each <channel-interceptor/> element lets you define a global interceptor, which is applied on all channels that match any patterns defined by the pattern attribute. In the preceding case, the global interceptor is applied on the thing1 channel and all other channels that begin with thing2 or input but not to channels starting with thing3 (since version 5.0).

	Warning
	The addition of this syntax to the pattern causes one possible (though perhaps unlikely) problem. If you have a bean named !thing1 and you included a pattern of !thing1 in your channel interceptor’s pattern patterns, it no longer matches. The pattern now matches all beans not named thing1. In this case, you can escape the ! in the pattern with \. The pattern \!thing1 matches a bean named !thing1.

The order attribute lets you manage where this interceptor is injected when there are multiple interceptors on a given channel. For example, channel inputChannel could have individual interceptors configured locally (see below), as the following example shows:

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

A reasonable question is "how is a global interceptor injected in relation to other interceptors configured locally or through other global interceptor definitions?" The current implementation provides a simple mechanism for defining the order of interceptor execution. A positive number in the order attribute ensures interceptor injection after any existing interceptors, while a negative number ensures that the interceptor is injected before existing interceptors. This means that, in the preceding example, the global interceptor is 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 both interceptors' order attributes. 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).

Starting with version 4.3.15, you can configure the spring.integration.postProcessDynamicBeans = true property to apply any global interceptors to dynamically created MessageChannel beans. See Section E.4, “Global Properties” for more information.

As mentioned earlier, Spring Integration provides a simple wire tap interceptor. You can configure a wire tap on any channel within an <interceptors/> element. Doing so 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 the payload and headers variables. Alternatively, to log the full message toString() result, provide a value of true for the log-full-message attribute. By default, it is false so that only the payload is logged. Setting it to true enables logging of all headers in addition to the payload. The expression option provides the most flexibility (for example, expression="payload.user.name").

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. By default, wire tap as a component is not invoked asynchronously. Instead, Spring Integration focuses on a single unified approach to configuring asynchronous behavior: the message channel. What makes certain parts of the message flow synchronous or asynchronous 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 tasks:

It is essentially a variation of the bridge pattern, 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 depends on the type of message channel that channelB is. We have the following options: direct channel, pollable channel, and executor channel. The last two break the thread boundary, making communication over such channels asynchronous, 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 synchronous or asynchronous. It is consistent with other components within the framework (such as message publisher) and adds a level of consistency and simplicity by sparing you from worrying in advance (other than writing thread-safe code) about whether a particular piece of code should be implemented as synchronous or asynchronous. The actual wiring of two pieces of code (say, component A and component B) over a message channel is what makes their collaboration synchronous or asynchronous. You may even want to change from synchronous to asynchronous in the future, and message channel lets 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 asynchronous by default, you should keep in mind that 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. However we doe not enforce asynchronous behavior by default. There are a number of use cases that would break if we did, including that you might not want to break a transactional boundary. Perhaps you use the wire tap pattern 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 the WireTap class) references a channel. 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 asks the interceptor if it is OK to intercept each channel that is a candidate, based on the supplied pattern. You can also add runtime protection in the interceptor methods to ensure that the channel is not one that is referenced by the interceptor. The WireTap uses both of these techniques.

Starting with version 4.3, the WireTap has additional constructors that take a channelName instead of a MessageChannel instance. This can be convenient for Java configuration and when channel auto-creation logic is being used. The target MessageChannel bean is resolved from the provided channelName later, on the first interaction with the interceptor.

	Important
	Channel resolution requires a BeanFactory, so the wire tap instance must be a Spring-managed bean.

This late-binding approach also allows simplification of typical wire-tapping patterns with Java DSL configuration, as the following example shows:

@Bean
public PollableChannel myChannel() {
    return MessageChannels.queue()
            .wireTap("loggingFlow.input")
            .get();
}

@Bean
public IntegrationFlow loggingFlow() {
    return f -> f.log();
}

Wire taps can be made conditional by using the selector or selector-expression attributes. The selector references a MessageSelector bean, which can determine at runtime whether the message should go to the tap channel. Similarly, the selector-expression is a boolean SpEL expression that performs the same purpose: If the expression evaluates to true, the message is sent to the tap channel.

It is possible to configure a global wire tap as a special case of the the section called “Global Channel Interceptor Configuration”. To do so, 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 they do for the channel-interceptor. The following examlpe shows how to configure a global wire tap:

<int:wire-tap pattern="input*, thing2*, thing1" 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. To do so, set the pattern attribute to the target channel name. For example, you can use this technique to configure a test case to verify messages on a channel.

Advice objects, in an advice-chain on a poller, advise the whole polling task (both message retrieval and processing). These "around advice" methods do not have access to any context for the poll — only the poll itself. This is fine for requirements such as making a task transactional or skipping a poll due to some external condition, as discussed earlier. What if we wish to take some action depending on the result of the receive part of the poll or if we want to adjust the poller depending on conditions? For those instances, Spring Integration offers "Smart" Polling.

Version 4.2 introduced the AbstractMessageSourceAdvice. Any Advice objects in the advice-chain that subclass this class are applied only to the receive operation. Such classes implement the following methods:

	Thread safety
	If an advice mutates the MessageSource, you should not configure the poller with a TaskExecutor. If an advice mutates the source, such mutations are not thread safe and could cause unexpected results, especially with high frequency pollers. If you need to process poll results concurrently, consider using a downstream ExecutorChannel instead of adding an executor to the poller.

Advice Chain Ordering

You should understand how the advice chain is processed during initialization. Advice objects that do not extend AbstractMessageSourceAdvice are applied to the whole poll process and are all invoked first, in order, before any AbstractMessageSourceAdvice. Then AbstractMessageSourceAdvice objects are invoked in order around the MessageSource receive() method. If you have, for example, Advice objects a, b, c, d, where b and d are AbstractMessageSourceAdvice, the objects are applied in the following order: a, c, b, d. Also, if a MessageSource is already a Proxy, the AbstractMessageSourceAdvice is invoked after any existing Advice objects. If you wish to change the order, you must wire up the proxy yourself.

This advice is a simple implementation of AbstractMessageSourceAdvice. When used in conjunction with a DynamicPeriodicTrigger, it adjusts the polling frequency, depending on whether or not the previous poll resulted in a message or not. The poller must also have a reference to the same DynamicPeriodicTrigger.

	Important: Async Handoff
	SimpleActiveIdleMessageSourceAdvice modifies the trigger based on the receive() result. This works only if the advice is called on the poller thread. It does not work if the poller has a task-executor. To use this advice where you wish to use async operations after the result of a poll, do the async handoff later, perhaps by using an ExecutorChannel.

This advice allows the selection of one of two triggers based on whether a poll returns a message or not. Consider a poller that uses a CronTrigger. CronTrigger instances are immutable, so they cannot be altered once constructed. Consider a use case where we want to use a cron expression to trigger a poll once each hour but, if no message is received, poll once per minute and, when a message is retrieved, revert to using the cron expression.

The advice (and poller) use a CompoundTrigger for this purpose. The trigger’s primary trigger can be a CronTrigger. When the advice detects that no message is received, it adds the secondary trigger to the CompoundTrigger. When the CompoundTrigger instance’s nextExecutionTime method is invoked, it delegates to the secondary trigger, if present. Otherwise, it delegates to the primary trigger.

The poller must also have a reference to the same CompoundTrigger.

The following example shows the configuration for the hourly cron expression with a fallback to every minute:

<int:inbound-channel-adapter channel="nullChannel" auto-startup="false">
    <bean class="org.springframework.integration.endpoint.PollerAdviceTests.Source" />
    <int:poller trigger="compoundTrigger">
        <int:advice-chain>
            <bean class="org.springframework.integration.aop.CompoundTriggerAdvice">
                <constructor-arg ref="compoundTrigger"/>
                <constructor-arg ref="secondary"/>
            </bean>
        </int:advice-chain>
    </int:poller>
</int:inbound-channel-adapter>

<bean id="compoundTrigger" class="org.springframework.integration.util.CompoundTrigger">
    <constructor-arg ref="primary" />
</bean>

<bean id="primary" class="org.springframework.scheduling.support.CronTrigger">
    <constructor-arg value="0 0 * * * *" /> <!-- top of every hour -->
</bean>

<bean id="secondary" class="org.springframework.scheduling.support.PeriodicTrigger">
    <constructor-arg value="60000" />
</bean>

	Important: Async Handoff
	CompoundTriggerAdvice modifies the trigger based on the receive() result. This works only if the advice is called on the poller thread. It does not work if the poller has a task-executor. To use this advice where you wish to use async operations after the result of a poll, do the async handoff later, perhaps by using an ExecutorChannel.