6. Messaging Channels

6.1 Message Channels

While the Message plays the crucial role of encapsulating data, it is the MessageChannel that decouples message producers from message consumers.

6.1.1 The MessageChannel Interface

Spring Integration’s top-level MessageChannel interface is defined as follows:

public interface MessageChannel {

    boolean send(Message message);

    boolean send(Message message, long timeout);
}

When sending a message, the return value is true if the message is sent successfully. If the send call times out or is interrupted, it returns false.

PollableChannel

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.

SubscribableChannel

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);

}

6.1.2 Message Channel Implementations

Spring Integration provides several different message channel implementations. The following sections briefly describe each one.

PublishSubscribeChannel

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]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.

QueueChannel

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.

PriorityChannel

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.

RendezvousChannel

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]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.

DirectChannel

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]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]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.

ExecutorChannel

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]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.

Scoped Channel

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.

6.1.3 Channel Interceptors

One of the advantages of a messaging architecture is the ability to provide common behavior and capture meaningful information about the messages passing through the system in a non-invasive way. Since the Message instances are sent to and received from MessageChannel instances, those channels provide an opportunity for intercepting the send and receive operations. The ChannelInterceptor strategy interface, shown in the following listing, provides methods for each of those operations:

public interface ChannelInterceptor {

    Message<?> preSend(Message<?> message, MessageChannel channel);

    void postSend(Message<?> message, MessageChannel channel, boolean sent);

    void afterSendCompletion(Message<?> message, MessageChannel channel, boolean sent, Exception ex);

    boolean preReceive(MessageChannel channel);

    Message<?> postReceive(Message<?> message, MessageChannel channel);

    void afterReceiveCompletion(Message<?> message, MessageChannel channel, Exception ex);
}

After implementing the interface, registering the interceptor with a channel is just a matter of making the following call:

channel.addInterceptor(someChannelInterceptor);

The methods that return a Message instance can be used for transforming the Message or can return null to prevent further processing (of course, any of the methods can throw a RuntimeException). Also, the preReceive method can return false to prevent the receive operation from proceeding.

[Note]Note

Keep in mind that receive() calls are only relevant for PollableChannels. In fact, the SubscribableChannel interface does not even define a receive() method. The reason for this is that when a Message is sent to a SubscribableChannel, it is sent directly to zero or more subscribers, depending on the type of channel (for example, a PublishSubscribeChannel sends to all of its subscribers). Therefore, the preReceive(...), postReceive(...), and afterReceiveCompletion(...) interceptor methods are invoked only when the interceptor is applied to a PollableChannel.

Spring Integration also provides an implementation of the Wire Tap pattern. It is a simple interceptor that sends the Message to another channel without otherwise altering the existing flow. It can be very useful for debugging and monitoring. An example is shown in the section called “Wire Tap”.

Because it is rarely necessary to implement all of the interceptor methods, the interface provides no-op methods (methods returning void method have no code, the Message-returning methods return the Message as-is, and the boolean method returns true).

[Tip]Tip

The order of invocation for the interceptor methods depends on the type of channel. As described earlier, the queue-based channels are the only ones where the receive method is intercepted in the first place. Additionally, the relationship between send and receive interception depends on the timing of the separate sender and receiver threads. For example, if a receiver is already blocked while waiting for a message, the order could be as follows: preSend, preReceive, postReceive, postSend. However, if a receiver polls after the sender has placed a message on the channel and has already returned, the order would be as follows: preSend, postSend (some-time-elapses), preReceive, postReceive. The time that elapses in such a case depends on a number of factors and is therefore generally unpredictable (in fact, the receive may never happen). The type of queue also plays a role (for example, rendezvous versus priority). In short, you cannot rely on the order beyond the fact that preSend precedes postSend and preReceive precedes postReceive.

Starting with Spring Framework 4.1 and Spring Integration 4.1, the ChannelInterceptor provides new methods: afterSendCompletion() and afterReceiveCompletion(). They are invoked after send()' and 'receive() calls, regardless of any exception that is raised, which allow for resource cleanup. Note that the channel invokes these methods on the ChannelInterceptor list in the reverse order of the initial preSend() and preReceive() calls.

Starting with version 5.1, global channel interceptors now apply to dynamically registered channels - such as through beans that are initialized by using beanFactory.initializeBean() or IntegrationFlowContext when using the Java DSL. Previously, interceptors were not applied when beans were created after the application context was refreshed.

Also, starting with version 5.1, ChannelInterceptor.postReceive() is no longer called when no message is received; it is no longer necessary to check for a null Message<?>. Previously, the method was called. If you have an interceptor that relies on the previous behavior, implement afterReceiveCompleted() instead, since that method is invoked, regardless of whether a message is received or not.

6.1.4 MessagingTemplate

When the endpoints and their various configuration options are introduced, Spring Integration provides a foundation for messaging components that enables non-invasive invocation of your application code from the messaging system. However, it is sometimes necessary to invoke the messaging system from your application code. For convenience when implementing such use cases, Spring Integration provides a MessagingTemplate that supports a variety of operations across the message channels, including request and reply scenarios. For example, it is possible to send a request and wait for a reply, as follows:

MessagingTemplate template = new MessagingTemplate();

Message reply = template.sendAndReceive(someChannel, new GenericMessage("test"));

In the preceding example, a temporary anonymous channel would be created internally by the template. The sendTimeout and receiveTimeout properties may also be set on the template, and other exchange types are also supported. The following listing shows the signatures for such methods:

public boolean send(final MessageChannel channel, final Message<?> message) { ...
}

public Message<?> sendAndReceive(final MessageChannel channel, final Message<?> request) { ...
}

public Message<?> receive(final PollableChannel<?> channel) { ...
}
[Note]Note

A less invasive approach that lets you invoke simple interfaces with payload or header values instead of Message instances is described in Section 10.4.1, “Enter the GatewayProxyFactoryBean.

6.1.5 Configuring Message Channels

To create a message channel instance, you can use the <channel/> element, as follows:

<int:channel id="exampleChannel"/>

The default channel type is point-to-point. To create a publish-subscribe channel, use the <publish-subscribe-channel/> element, as follows:

<int:publish-subscribe-channel id="exampleChannel"/>

When you use the <channel/> element without any sub-elements, it creates a DirectChannel instance (a SubscribableChannel).

However, you can alternatively provide a variety of <queue/> sub-elements to create any of the pollable channel types (as described in Section 6.1.2, “Message Channel Implementations”). The following sections shows examples of each channel type.

DirectChannel Configuration

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>

Datatype Channel Configuration

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.

QueueChannel Configuration

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]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]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]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]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"));
}

PublishSubscribeChannel Configuration

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]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.

ExecutorChannel

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]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>

PriorityChannel Configuration

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”.

RendezvousChannel 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 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>

Scoped Channel Configuration

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

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

Channel Interceptor Configuration

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.

Global Channel Interceptor Configuration

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]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

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.

Wire Tap

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]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:

  • Intercept a message flow by tapping into a channel (for example, channelA)
  • Grab each message
  • Send the message to another channel (for example, channelB)

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]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]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();
}

Conditional Wire Taps

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.

Global Wire Tap Configuration

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]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.

6.1.6 Special Channels

If namespace support is enabled, two special channels are defined within the application context by default: errorChannel and nullChannel. The nullChannel acts like /dev/null, logging any message sent to it at the DEBUG level and returning immediately. Any time you face channel resolution errors for a reply that you do not care about, you can set the affected component’s output-channel attribute to nullChannel (the name, nullChannel, is reserved within the application context). The errorChannel is used internally for sending error messages and may be overridden with a custom configuration. This is discussed in greater detail in Section E.3, “Error Handling”.

See also Section 11.2, “Message Channels” in the Java DSL chapter for more information about message channel and interceptors.

6.2 Poller

This section describes how polling works in Spring Integration.

6.2.1 Polling Consumer

When Message Endpoints (Channel Adapters) are connected to channels and instantiated, they produce one of the following instances:

The actual implementation depends on the type of channel to which these endpoints connect. A channel adapter connected to a channel that implements the org.springframework.messaging.SubscribableChannel interface produces an instance of EventDrivenConsumer. On the other hand, a channel adapter connected to a channel that implements the org.springframework.messaging.PollableChannel interface (such as a QueueChannel) produces an instance of PollingConsumer.

Polling consumers let Spring Integration components actively poll for Messages rather than process messages in an event-driven manner.

They represent a critical cross-cutting concern in many messaging scenarios. In Spring Integration, polling consumers are based on the pattern with the same name, which is described in the book Enterprise Integration Patterns, by Gregor Hohpe and Bobby Woolf. You can find a description of the pattern on the book’s website.

6.2.2 Pollable Message Source

Spring Integration offers a second variation of the polling consumer pattern. When inbound channel adapters are used, these adapters are often wrapped by a SourcePollingChannelAdapter. For example, when retrieving messages from a remote FTP Server location, the adapter described in Section 18.4, “FTP Inbound Channel Adapter” is configured with a poller to periodically retrieve messages. So, when components are configured with pollers, the resulting instances are of one of the following types:

This means that pollers are used in both inbound and outbound messaging scenarios. Here are some use cases in which pollers are used:

  • Polling certain external systems, such as FTP Servers, Databases, and Web Services
  • Polling internal (pollable) message channels
  • Polling internal services (such as repeatedly executing methods on a Java class)
[Note]Note

AOP advice classes can be applied to pollers, in an advice-chain, such as a transaction advice to start a transaction. Starting with version 4.1, a PollSkipAdvice is provided. Pollers use triggers to determine the time of the next poll. The PollSkipAdvice can be used to suppress (skip) a poll, perhaps because there is some downstream condition that would prevent the message being processed. To use this advice, you have to provide it with an implementation of a PollSkipStrategy. Starting with version 4.2.5, a SimplePollSkipStrategy is provided. To use it, you can add an instance as a bean to the application context, inject it into a PollSkipAdvice, and add that to the poller’s advice chain. To skip polling, call skipPolls(). To resume polling, call reset(). Version 4.2 added more flexibility in this area. See Section 6.2.4, “Conditional Pollers for Message Sources”.

This chapter is meant to only give a high-level overview of polling consumers and how they fit into the concept of message channels (see Section 6.1, “Message Channels”) and channel adapters (see Section 6.3, “Channel Adapter”). For more information regarding messaging endpoints in general and polling consumers in particular, see Section 10.1, “Message Endpoints”.

6.2.3 Deferred Acknowledgment Pollable Message Source

Starting with version 5.0.1, certain modules provide MessageSource implementations that support deferring acknowledgment until the downstream flow completes (or hands off the message to another thread). This is currently limited to the AmqpMessageSource and the KafkaMessageSource provided by the spring-integration-kafka extension project.

With these message sources, the IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK header (see Section 7.2.1, “MessageHeaderAccessor API”) is added to the message. The value of the header is an instance of AcknowledgmentCallback, as the following example shows:

@FunctionalInterface
public interface AcknowledgmentCallback {

    void acknowledge(Status status);

    boolean isAcknowledged();

    void noAutoAck();

    default boolean isAutoAck();

    enum Status {

        /**
         * Mark the message as accepted.
         */
        ACCEPT,

        /**
         * Mark the message as rejected.
         */
        REJECT,

        /**
         * Reject the message and requeue so that it will be redelivered.
         */
        REQUEUE

    }

}

Not all message sources (for example, Kafka) support the REJECT status. It is treated the same as ACCEPT.

Applications can acknowledge a message at any time, as the following example shows:

Message<?> received = source.receive();

...

StaticMessageHeaderAccessor.getAcknowledgmentCallback(received)
        .acknowledge(Status.ACCEPT);

If the MessageSource is wired into a SourcePollingChannelAdapter, when the poller thread returns to the adapter after the downstream flow completes, the adapter checks whether the acknowledgment has already been acknowledged and, if not, sets its status to ACCEPT it (or REJECT if the flow throws an exception). The status values are defined in the AcknowledgmentCallback.Status enumeration.

Spring Integration provides MessageSourcePollingTemplate to perform ad-hoc polling of a MessageSource. This, too, takes care of setting ACCEPT or REJECT on the AcknowledgmentCallback when the MessageHandler callback returns (or throws an exception). The following example shows how to poll with the MessageSourcePollingTemplate:

MessageSourcePollingTemplate template =
    new MessageSourcePollingTemplate(this.source);
template.poll(h -> {
    ...
});

In both cases (SourcePollingChannelAdapter and MessageSourcePollingTemplate), you can disable auto ack/nack by calling noAutoAck() on the callback. You might do this if you hand off the message to another thread and wish to acknowledge later. Not all implementations support this (for example, Apache Kafka does not, because the offset commit has to be performed on the same thread).

6.2.4 Conditional Pollers for Message Sources

This section covers how to use conditional pollers.

Background

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.

"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:

  • beforeReceive(MessageSource<?> source) This method is called before the MessageSource.receive() method. It lets you examine and reconfigure the source. Returning false cancels this poll (similar to the PollSkipAdvice mentioned earlier).
  • Message<?> afterReceive(Message<?> result, MessageSource<?> source) This method is called after the receive() method. Again, you can reconfigure the source or take any action (perhaps depending on the result, which can be null if there was no message created by the source). You can even return a different message
[Important]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.

[Important]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.

SimpleActiveIdleMessageSourceAdvice

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]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.

CompoundTriggerAdvice

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]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.

6.3 Channel Adapter

A channel adapter is a message endpoint that enables connecting a single sender or receiver to a message channel. Spring Integration provides a number of adapters to support various transports, such as JMS, file, HTTP, web services, mail, and more. Upcoming chapters of this reference guide discuss each adapter. However, this chapter focuses on the simple but flexible method-invoking channel adapter support. There are both inbound and outbound adapters, and each may be configured with XML elements provided in the core namespace. These provide an easy way to extend Spring Integration, as long as you have a method that can be invoked as either a source or a destination.

6.3.1 Configuring An Inbound Channel Adapter

An inbound-channel-adapter element can invoke any method on a Spring-managed object and send a non-null return value to a MessageChannel after converting the method’s output to a Message. When the adapter’s subscription is activated, a poller tries to receive messages from the source. The poller is scheduled with the TaskScheduler according to the provided configuration. To configure the polling interval or cron expression for an individual channel adapter, you can provide a poller element with one of the scheduling attributes, such as fixed-rate or cron. The following example defines two inbound-channel-adapter instances:

<int:inbound-channel-adapter ref="source1" method="method1" channel="channel1">
    <int:poller fixed-rate="5000"/>
</int:inbound-channel-adapter>

<int:inbound-channel-adapter ref="source2" method="method2" channel="channel2">
    <int:poller cron="30 * 9-17 * * MON-FRI"/>
</int:channel-adapter>

See also Section 6.3.3, “Channel Adapter Expressions and Scripts”.

[Note]Note

If no poller is provided, then a single default poller must be registered within the context. See Section 10.1.4, “Endpoint Namespace Support” for more detail.

[Important]Important: Poller Configuration

Some inbound-channel-adapter types are backed by a SourcePollingChannelAdapter, which means they contain a poller configuration that polls the MessageSource (to invoke a custom method that produces the value that becomes a Message payload) based on the configuration specified in the Poller. The following example shows the configuration of two pollers:

<int:poller max-messages-per-poll="1" fixed-rate="1000"/>

<int:poller max-messages-per-poll="10" fixed-rate="1000"/>

In the the first configuration, the polling task is invoked once per poll, and, during each task (poll), the method (which results in the production of the message) is invoked once, based on the max-messages-per-poll attribute value. In the second configuration, the polling task is invoked 10 times per poll or until it returns null, thus possibly producing ten messages per poll while each poll happens at one-second intervals. However, what happens if the configuration looks like the following example:

<int:poller fixed-rate="1000"/>

Note that there is no max-messages-per-poll specified. As we cover later, the identical poller configuration in the PollingConsumer (for example, service-activator, filter, router, and others) would have a default value of -1 for max-messages-per-poll, which means "execute the polling task non-stop unless the polling method returns null (perhaps because there are no more messages in the `QueueChannel)`" and then sleep for one second.

However, in the SourcePollingChannelAdapter, it is a bit different. The default value for max-messages-per-poll is 1, unless you explicitly set it to a negative value (such as -1). This makes sure that the poller can react to lifecycle events (such as start and stop) and prevents it from potentially spinning in an infinite loop if the implementation of the custom method of the MessageSource has a potential to never return null and happens to be non-interruptible.

However, if you are sure that your method can return null and you need to poll for as many sources as available per each poll, you should explicitly set max-messages-per-poll to a negative value, as the following example shows:

<int:poller max-messages-per-poll="-1" fixed-rate="1000"/>

6.3.2 Configuring An Outbound Channel Adapter

An outbound-channel-adapter element can also connect a MessageChannel to any POJO consumer method that should be invoked with the payload of messages sent to that channel. The following example shows how to define an outbound channel adapter:

<int:outbound-channel-adapter channel="channel1" ref="target" method="handle"/>

<beans:bean id="target" class="org.MyPojo"/>

If the channel being adapted is a PollableChannel, you must provide a poller sub-element, as the following example shows:

<int:outbound-channel-adapter channel="channel2" ref="target" method="handle">
    <int:poller fixed-rate="3000" />
</int:outbound-channel-adapter>

<beans:bean id="target" class="org.MyPojo"/>

You should use a ref attribute if the POJO consumer implementation can be reused in other <outbound-channel-adapter> definitions. However, if the consumer implementation is referenced by only a single definition of the <outbound-channel-adapter>, you can define it as an inner bean, as the following example shows:

<int:outbound-channel-adapter channel="channel" method="handle">
    <beans:bean class="org.Foo"/>
</int:outbound-channel-adapter>
[Note]Note

Using both the ref attribute and an inner handler definition in the same <outbound-channel-adapter> configuration is not allowed, as it creates an ambiguous condition. Such a configuration results in an exception being thrown.

Any channel adapter can be created without a channel reference, in which case it implicitly creates an instance of DirectChannel. The created channel’s name matches the id attribute of the <inbound-channel-adapter> or <outbound-channel-adapter> element. Therefore, if channel is not provided, id is required.

6.3.3 Channel Adapter Expressions and Scripts

Like many other Spring Integration components, the <inbound-channel-adapter> and <outbound-channel-adapter> also provide support for SpEL expression evaluation. To use SpEL, provide the expression string in the expression attribute instead of providing the ref and method attributes that are used for method-invocation on a bean. When an expression is evaluated, it follows the same contract as method-invocation where: the expression for an <inbound-channel-adapter> generates a message any time the evaluation result is a non-null value, while the expression for an <outbound-channel-adapter> must be the equivalent of a void-returning method invocation.

Starting with Spring Integration 3.0, an <int:inbound-channel-adapter/> can also be configured with a SpEL <expression/> (or even with a <script/>) sub-element, for when more sophistication is required than can be achieved with the simple expression attribute. If you provide a script as a Resource by using the location attribute, you can also set refresh-check-delay, which allows the resource to be periodically refreshed. If you want the script to be checked on each poll, you would need to coordinate this setting with the poller’s trigger, as the following example shows:

<int:inbound-channel-adapter ref="source1" method="method1" channel="channel1">
    <int:poller max-messages-per-poll="1" fixed-delay="5000"/>
    <script:script lang="ruby" location="Foo.rb" refresh-check-delay="5000"/>
</int:inbound-channel-adapter>

See also the cacheSeconds property on the ReloadableResourceBundleExpressionSource when using the <expression/> sub-element. For more information regarding expressions, see Appendix A, Spring Expression Language (SpEL). For scripts, see Section 10.8, “Groovy support” and Section 10.7, “Scripting Support”.

[Important]Important

The <int:inbound-channel-adapter/> is endpoint starts a message flow by periodically triggering to poll some underlying MessageSource. Since, at the time of polling, there is no message object, expressions and scripts do not have access to a root Message, so there are no payload or headers properties that are available in most other messaging SpEL expressions. The script can generate and return a complete Message object with headers and payload or only a payload, which is added to a message with basic headers.

6.4 Messaging Bridge

A messaging bridge is a relatively trivial endpoint that connects two message channels or channel adapters. For example, you may want to connect a PollableChannel to a SubscribableChannel so that the subscribing endpoints do not have to worry about any polling configuration. Instead, the messaging bridge provides the polling configuration.

By providing an intermediary poller between two channels, you can use a messaging bridge to throttle inbound messages. The poller’s trigger determines the rate at which messages arrive on the second channel, and the poller’s maxMessagesPerPoll property enforces a limit on the throughput.

Another valid use for a messaging bridge is to connect two different systems. In such a scenario, Spring Integration’s role is limited to making the connection between these systems and managing a poller, if necessary. It is probably more common to have at least a transformer between the two systems, to translate between their formats. In that case, the channels can be provided as the input-channel and output-channel of a transformer endpoint. If data format translation is not required, the messaging bridge may indeed be sufficient.

6.4.1 Configuring a Bridge with XML

You can use the <bridge> element is used to create a messaging bridge between two message channels or channel adapters. To do so, provide the input-channel and output-channel attributes, as the following example shows:

<int:bridge input-channel="input" output-channel="output"/>

As mentioned above, a common use case for the messaging bridge is to connect a PollableChannel to a SubscribableChannel. When performing this role, the messaging bridge may also serve as a throttler:

<int:bridge input-channel="pollable" output-channel="subscribable">
     <int:poller max-messages-per-poll="10" fixed-rate="5000"/>
 </int:bridge>

You can use a similar mechanism to connecting channel adapters. The following example shows a simple "echo" between the stdin and stdout adapters from Spring Integration’s stream namespace:

<int-stream:stdin-channel-adapter id="stdin"/>

 <int-stream:stdout-channel-adapter id="stdout"/>

 <int:bridge id="echo" input-channel="stdin" output-channel="stdout"/>

Similar configurations work for other (potentially more useful) Channel Adapter bridges, such as file-to-JMS or mail-to-file. Upcoming chapters cover the various channel adapters.

[Note]Note

If no output-channel is defined on a bridge, the reply channel provided by the inbound message is used, if available. If neither an output nor a reply channel is available, an exception is thrown.

6.4.2 Configuring a Bridge with Java Configuration

The following example shows how to configure a bridge in Java by using the @BridgeFrom annotation:

@Bean
public PollableChannel polled() {
    return new QueueChannel();
}

@Bean
@BridgeFrom(value = "polled", poller = @Poller(fixedDelay = "5000", maxMessagesPerPoll = "10"))
public SubscribableChannel direct() {
    return new DirectChannel();
}

The following example shows how to configure a bridge in Java by using the @BridgeTo annotation:

@Bean
@BridgeTo(value = "direct", poller = @Poller(fixedDelay = "5000", maxMessagesPerPoll = "10"))
public PollableChannel polled() {
    return new QueueChannel();
}

@Bean
public SubscribableChannel direct() {
    return new DirectChannel();
}

Alternately, you can use a BridgeHandler, as the following example shows:

@Bean
@ServiceActivator(inputChannel = "polled",
        poller = @Poller(fixedRate = "5000", maxMessagesPerPoll = "10"))
public BridgeHandler bridge() {
    BridgeHandler bridge = new BridgeHandler();
    bridge.setOutputChannelName("direct");
    return bridge;
}

6.4.3 Configuring a Bridge with the Java DSL

You can use the Java Domain Specific Language (DSL) to configure a bridge, as the following example shows:

@Bean
public IntegrationFlow bridgeFlow() {
    return IntegrationFlows.from("polled")
            .bridge(e -> e.poller(Pollers.fixedDelay(5000).maxMessagesPerPoll(10)))
            .channel("direct")
            .get();
}