4. Messaging Channels

4.1 Message Channels

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

4.1.1 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 will be true if the message is sent successfully. If the send call times out or is interrupted, then it will return false.


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

public interface PollableChannel extends MessageChannel {

    Message<?> receive();

    Message<?> receive(long timeout);


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


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

public interface SubscribableChannel extends MessageChannel {

    boolean subscribe(MessageHandler handler);

    boolean unsubscribe(MessageHandler handler);


4.1.2 Message Channel Implementations

Spring Integration provides several different Message Channel implementations. Each is briefly described in the sections below.


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

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


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


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

public QueueChannel(int capacity)

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


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


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


Keep in mind that all of these queue-based channels are storing messages in-memory only by default. When persistence is required, you can either provide a message-store attribute within the queue element to reference a persistent MessageStore implementation, or you can replace the local channel with one that is backed by a persistent broker, such as a JMS-backed channel or Channel Adapter. The latter option allows you to take advantage of any JMS provider’s implementation for message persistence, and it will be discussed in Chapter 20, JMS Support. However, when buffering in a queue is not necessary, the simplest approach is to rely upon the DirectChannel discussed next.

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


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

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

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


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

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

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

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

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

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

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


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


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


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

Scoped Channel

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

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

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

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

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

4.1.3 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 s are being sent to and received from MessageChannels, those channels provide an opportunity for intercepting the send and receive operations. The ChannelInterceptor strategy interface 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 calling:


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.


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 will be sent directly to one or more subscribers depending on the type of channel (e.g. a PublishSubscribeChannel sends to all of its subscribers). Therefore, the preReceive(..) and postReceive(..) interceptor methods are only invoked 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, a ChannelInterceptorAdapter class is also available for sub-classing. It provides no-op methods (the void method is empty, the Message returning methods return the Message as-is, and the boolean method returns true). Therefore, it is often easiest to extend that class and just implement the method(s) that you need as in the following example.

public class CountingChannelInterceptor extends ChannelInterceptorAdapter {

    private final AtomicInteger sendCount = new AtomicInteger();

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

The order of invocation for the interceptor methods depends on the type of channel. As described above, 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 separate sender and receiver threads. For example, if a receiver is already blocked while waiting for a message the order could be: preSend, preReceive, postReceive, postSend. However, if a receiver polls after the sender has placed a message on the channel and already returned, the order would be: 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!). Obviously, the type of queue also plays a role (e.g. rendezvous vs. priority). The bottom line is that you cannot rely on the order beyond the fact that preSend will precede postSend and preReceive will precede 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()/receive() calls, regardless of any exception that is raised, thus allowing for resource cleanup. Note, the Channel invokes these methods on the ChannelInterceptor List in the reverse order of the initial preSend()/preReceive() calls.

4.1.4 MessagingTemplate

As you will see 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, sometimes it is 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/reply scenarios. For example, it is possible to send a request and wait for a reply.

MessagingTemplate template = new MessagingTemplate();

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

In that 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.

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

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

public Message<?> receive(final PollableChannel<?> channel) { ...

A less invasive approach that allows you to invoke simple interfaces with payload and/or header values instead of Message instances is described in Section 8.3.1, “Enter the GatewayProxyFactoryBean”.

4.1.5 Configuring Message Channels

To create a Message Channel instance, you can use the <channel/> element:

<int:channel id="exampleChannel"/>

The default channel type is Point to Point. To create a Publish Subscribe channel, use the <publish-subscribe-channel/> element:

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

When using the <channel/> element without any sub-elements, it will create 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 inSection 4.1.2, “Message Channel Implementations”). Examples of each are shown below.

DirectChannel Configuration

As mentioned above, DirectChannel is the default type.

<int:channel id="directChannel"/>

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

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

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

Datatype Channel Configuration

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

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

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

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

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

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

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

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

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

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

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

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

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

<int:converter ref="strToInt"/>

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

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


For more information regarding Payload Type Conversion, please read Section 8.1.6, “Payload Type Conversion”.

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

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

QueueChannel Configuration

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

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

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

Persistent QueueChannel Configuration

Since a QueueChannel provides the capability to buffer Messages, but does so in-memory only by default, it also introduces a possibility that Messages could be lost in the event of a system failure. To mitigate this risk, a QueueChannel may be backed by a persistent implementation of the MessageGroupStore strategy interface. For more details on MessageGroupStore and MessageStore see Section 9.4, “Message Store”.

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

By default, a QueueChannel stores its Messages in an in-memory Queue and can therefore lead to the lost message scenario mentioned above. However Spring Integration provides persistent stores, such as the JdbcMessageStore.

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

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

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

The Spring Integration JDBC module also provides schema 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).


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

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

Since version 4.0, it is recommended that QueueChannel s are configured to use a ChannelMessageStore if possible. These are generally optimized for this use, when compared with a general message store. If the ChannelMessageStore is a ChannelPriorityMessageStore the messages will be received in FIFO within priority order. The notion of priority is determined by the message store implementation. For example the Java Configuration for the Section 22.3.1, “MongoDB Channel Message Store”:

public BasicMessageGroupStore mongoDbChannelMessageStore(MongoDbFactory mongoDbFactory) {
    MongoDbChannelMessageStore store = new MongoDbChannelMessageStore(mongoDbFactory);
    return store;

public PollableChannel priorityQueue(BasicMessageGroupStore mongoDbChannelMessageStore) {
   return new QueueChannel(new MessageGroupQueue(mongoDbChannelMessageStore, "priorityQueue"));

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

The same with Java DSL may look like:

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

Another option to customize the QueueChannel environment is provided by the ref attribute of the <int:queue> sub-element. This attribute implies the reference to any java.util.Queue implementation. An implementation is provided by the Project Reactor and its reactor.queue.PersistentQueue implementation for the IndexedChronicle:

public QueueChannel reactorQueue() {
    return new QueueChannel(new PersistentQueueSpec<Message<?>>()
                    .codec(new JavaSerializationCodec<Message<?>>())
                    .basePath(System.getProperty("java.io.tmpdir") + "/reactor-queue")

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 simply publishes in the sender’s thread):

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

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

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

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


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

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

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

<int:channel id="executorChannelWithoutFailover">
    <int:dispatcher task-executor="someExecutor" failover="false"/>

PriorityChannel Configuration

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

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

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

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

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

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 above, and its queue does not accept any capacity value since it is a 0-capacity direct handoff queue.

<int:channel id="rendezvousChannel"/>

Scoped Channel Configuration

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

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

Channel Interceptor Configuration

Message channels may also have interceptors as described in Section 4.1.3, “Channel Interceptors”. The <interceptors/> sub-element can be added within a <channel/> (or the more specific element types). Provide the ref attribute to reference any Spring-managed object that implements the ChannelInterceptor interface:

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

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

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. Look at the example below:

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


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

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

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

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

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


Note 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).

Wire Tap

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

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

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

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

A little more on Wire Tap

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

  • intercept a message flow by tapping into a channel (e.g., channelA)
  • grab each message
  • send the message to another channel (e.g., channelB)

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

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


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

Conditional Wire Taps

Wire taps can be made conditional, 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 will be 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”. Simply configure a top level wire-tap element. Now, in addition to the normal wire-tap namespace support, the pattern and order attributes are supported and work in exactly the same way as with the channel-interceptor

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

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

4.1.6 Special Channels

If namespace support is enabled, there are two special channels defined within the application context by default: errorChannel and nullChannel. The nullChannel acts like /dev/null, simply logging any Message sent to it at DEBUG level and returning immediately. Any time you face channel resolution errors for a reply that you don’t 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 F.4, “Error Handling”.

4.2 Poller

4.2.1 Polling Consumer

When Message Endpoints (Channel Adapters) are connected to channels and instantiated, they produce one of the following 2 instances:

The actual implementation depends on which type of channel these Endpoints are connected to. A channel adapter connected to a channel that implements the org.springframework.messaging.SubscribableChannel interface will produce an instance of EventDrivenConsumer. On the other hand, a channel adapter connected to a channel that implements the org.springframework.messaging.PollableChannel interface (e.g. a QueueChannel) will produce an instance of PollingConsumer.

Polling Consumers allow Spring Integration components to actively poll for Messages, rather than to 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 at:


4.2.2 Pollable Message Source

Furthermore, in Spring Integration a second variation of the Polling Consumer pattern exists. When Inbound Channel Adapters are being 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 15.4, “FTP Inbound Channel Adapter” is configured with a poller to retrieve messages periodically. So, when components are configured with Pollers, the resulting instances are of one of the following types:

This means, Pollers are used in both inbound and outbound messaging scenarios. Here are some use-cases that illustrate the scenarios in which Pollers are used:

  • Polling certain external systems such as FTP Servers, Databases, Web Services
  • Polling internal (pollable) Message Channels
  • Polling internal services (E.g. repeatedly execute methods on a Java class)

AOP Advice classes can be applied to pollers, in an advice-chain. An example being 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 to be processed properly. To use this advice, you have to provide it with an implementation of a PollSkipStrategy. Version 4.2 added more flexibility in this area - see Section 4.2.3, “Conditional Pollers for Message Sources”.

This chapter is meant to only give a high-level overview regarding Polling Consumers and how they fit into the concept of message channels - Section 4.1, “Message Channels” and channel adapters - Section 4.3, “Channel Adapter”. For more in-depth information regarding Messaging Endpoints in general and Polling Consumers in particular, please seeSection 8.1, “Message Endpoints”.

4.2.3 Conditional Pollers for Message Sources


Advice objects, in an advice-chain on a poller, advise the whole polling task (message retrieval and processing). These "around advice" methods do not have access to any context for the poll, just 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 above. 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?

"Smart" Polling

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

beforeReceive(MessageSource<?> source)

This method is called before the MessageSource.receive() method. It enables you to examine and or reconfigure the source at this time. Returning false cancels this poll (similar to the PollSkipAdvice mentioned above).

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!


This advice is a simple implementation of AbstractMessageSourceAdvice, when used in conjunction with a DynamicPeriodicTrigger, it adjuststhe 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

This advice modifies the trigger based on the receive() result. This will only work if the advice is called on the poller thread. It will 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.

[Important]Advice Chain Ordering

It is important to 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, say Advice objects a, b, c, d, where b and d are AbstractMessageSourceAdvice, they will be applied in the order a, c, b, d. Also, if a MessageSource is already a Proxy, the AbstractMessageSourceAdvice will be invoked after any existing Advice objects. If you wish to change the order, you should wire up the proxy yourself.

4.3 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 out of the box to support various transports, such as JMS, File, HTTP, Web Services, Mail, and more. Those will be discussed in upcoming chapters of this reference guide. 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 destination.

4.3.1 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 it to a Message. When the adapter’s subscription is activated, a poller will attempt to receive messages from the source. The poller will be scheduled with the TaskScheduler according to the provided configuration. To configure the polling interval or cron expression for an individual channel-adapter, provide a poller element with one of the scheduling attributes, such as fixed-rate or cron.

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

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

Also see Section 4.3.3, “Channel Adapter Expressions and Scripts”.


If no poller is provided, then a single default poller must be registered within the context. See Section 8.1.4, “Namespace Support” for more detail.

[Important]Important: Poller Configuration

Some inbound-channel-adapter types are backed by a SourcePollingChannelAdapter which means they contain Poller configuration which will poll the MessageSource (invoke a custom method which produces the value that becomes a Message payload) based on the configuration specified in the Poller.

For example:

<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 will be invoked once per poll and during such task (poll) the method (which results in the production of the Message) will be invoked once based on the max-messages-per-poll attribute value. In the second configuration the polling task will be invoked 10 times per poll or until it returns null thus possibly producing 10 Messages per poll while each poll happens at 1 second intervals. However what if the configuration looks like this:

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

Note there is no max-messages-per-poll specified. As you’ll learn later the identical poller configuration in the PollingConsumer (e.g., service-activator, filter, router etc.) would have a default value of -1 for max-messages-per-poll which means "execute poling task non-stop unless polling method returns null (e.g., no more Messages in the QueueChannel)" and then sleep for 1 second.

However in the SourcePollingChannelAdapter it is a bit different. The default value for max-messages-per-poll will be set to 1 by default unless you explicitly set it to a negative value (e.g., -1). It is done so to make sure that poller can react to a LifeCycle events (e.g., start/stop) and prevent it from potentially spinning in the infinite loop if the implementation of the custom method of the MessageSource has a potential to never return null and happened to be non-interruptible.

However if you are sure that your method can return null and you need the behavior where you want to poll for as many sources as available per each poll, then you should explicitly set max-messages-per-poll to a negative value.

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

4.3.2 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.

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

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

If the channel being adapted is a PollableChannel, provide a poller sub-element:

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

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

Using a "ref" attribute is generally recommended if the POJO consumer implementation can be reused in other <outbound-channel-adapter> definitions. However if the consumer implementation is only referenced by a single definition of the <outbound-channel-adapter>, you can define it as inner bean:

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

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 will result in an Exception being thrown.

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

4.3.3 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 via 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> will generate a message anytime 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 <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 using the location attribute, you can also set the refresh-check-delay allowing the resource to be refreshed periodically. If you want the script to be checked on each poll, you would need to coordinate this setting with the poller’s trigger:

<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"/>

Also see the cacheSeconds property on the ReloadableResourceBundleExpressionSource when using the <expression/> sub-element. For more information regarding expressions see Appendix A, Spring Expression Language (SpEL), and for scripts - Section 8.7, “Groovy support” and Section 8.6, “Scripting support”.


The <int:inbound-channel-adapter/> is an endpoint that starts a message flow via periodic triggering to poll some underlying MessageSource. Since, at the time of polling, there is not yet a message object, expressions and scripts don’t have access to a root Message, so there are no payload or headers properties that are available in most other messaging SpEL expressions. Of course, the script can generate and return a complete Message object with headers and payload, or just a payload, which will be added to a message with basic headers.

4.4 Messaging Bridge

4.4.1 Introduction

A Messaging Bridge is a relatively trivial endpoint that simply 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, a Messaging Bridge can be used to throttle inbound Messages. The poller’s trigger will determine the rate at which messages arrive on the second channel, and the poller’s "maxMessagesPerPoll" property will enforce 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 would be 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, and in that case, the channels would 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.

4.4.2 Configuring Bridge

The <bridge> element is used to create a Messaging Bridge between two Message Channels or Channel Adapters. Simply provide the "input-channel" and "output-channel" attributes:

<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, and 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"/>

Connecting Channel Adapters is just as easy. Here is a simple echo example 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"/>

Of course, the configuration would be similar for other (potentially more useful) Channel Adapter bridges, such as File to JMS, or Mail to File. The various Channel Adapters will be discussed in upcoming chapters.


If no output-channel is defined on a bridge, the reply channel provided by the inbound Message will be used, if available. If neither output or reply channel is available, an Exception will be thrown.