As alluded to in the overview, the MessageSource interface is itself a marker interface for any "source" of Messages. The two sub-interfaces - PollableSource and SubscribableSource - accommodate two types of source: those that must be polled and those that send Messages on their own. An example of the first type would be a source that represents a directory in the File-system, and an example of the second type would be an inbound RMI invocation.

The PollableSource interface defines a single method for receiving Message objects.

public interface PollableSource<T> extends MessageSource<T> {
    Message<T> receive();
}

The BlockingSource interface extends PollableSource and adds a single method with a timeout:

Message<T> receive(long timeout);

Spring Integration also provides a MethodInvokingSource implementation that serves as an adapter for invoking any arbitrary method on a plain Object (i.e. there is no need to implement an interface). To use the MethodInvokingSource, provide the Object reference and the method name.

MethodInvokingSource source = new MethodInvokingSource();
source.setObject(new SourceObject());
source.setMethodName("sourceMethod");
Message<?> result = source.receive();

It is also possible to configure a MethodInvokingSource in XML by providing a bean reference in the "source" attribute of a <channel-adapter> element along with a "method" attribute.

<channel-adapter source="sourceObject" method="sourceMethod" channel="someChannel"/>