As described above, when a MessageHandler is registered with the message bus, it is hosted by an endpoint and thereby subscribed to a channel. Often it is necessary to provide additional dynamic logic to determine what messages the handler should receive. The MessageSelector strategy interface fulfills that role.

public interface MessageSelector {
    boolean accept(Message<?> message);
}

A MessageEndpoint can be configured with zero or more selectors, and will only receive messages that are accepted by each selector. Even though the interface is simple to implement, a couple common selector implementations are provided. For example, the PayloadTypeSelector provides similar functionality to Datatype Channels (as described in the section called “Configuring Message Channels”) except that in this case the type-matching can be done by the endpoint rather than the channel.

PayloadTypeSelector selector = new PayloadTypeSelector(String.class, Integer.class);
assertTrue(selector.accept(new StringMessage("example")));
assertTrue(selector.accept(new GenericMessage<Integer>(123)));
assertFalse(selector.accept(new GenericMessage<SomeObject>(someObject)));

Another simple but useful MessageSelector provided out-of-the-box is the UnexpiredMessageSelector. As the name suggests, it only accepts messages that have not yet expired.

Essentially, using a selector provides reactive routing whereas the Datatype Channel and Message Router provide proactive routing. However, selectors accommodate additional uses. For example, the MessageChannel's 'purge' method accepts a selector:

channel.purge(someSelector);

There is even a ChannelPurger utility class whose purge operation is a good candidate for Spring's JMX support:

ChannelPurger purger = new ChannelPurger(new ExampleMessageSelector(), channel);
purger.purge();

Implementations of MessageSelector might provide opportunities for reuse on channels in addition to endpoints. For that reason, Spring Integration provides a simple selector-wrapping ChannelInterceptor that accepts one or more selectors in its constructor.

MessageSelectingInterceptor interceptor =
        new MessageSelectingInterceptor(selector1, selector2);
channel.addInterceptor(interceptor);