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 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);

}