Message Filters are used to decide whether a Message should be passed along or dropped based on some criteria such as a Message Header value or even content within the Message itself. Therefore, a Message Filter is similar to a router, except that for each Message received from the filter's input channel, that same Message may or may not be sent to the filter's output channel. Unlike the router, it makes no decision regarding which Message Channel to send to but only decides whether to send. As you will see momentarily, the Filter does also support a discard channel, so in certain cases it can play the role of a very simple router (or "switch") based on a boolean condition. In Spring Integration, a Message Filter may be configured as a Message Endpoint that delegates to some implementation of the MessageSelector interface. That interface is itself quite simple: message); }]]> The MessageFilter constructor accepts a selector instance:

The <filter> element is used to create a Message-selecting endpoint. In addition to "input-channel" and "output-channel" attributes, it requires a "ref". The "ref" may point to a MessageSelector implementation: ]]> Alternatively, the "method" attribute can be added at which point the "ref" may refer to any object. The referenced method may expect either the Message type or the payload type of inbound Messages. The return value of the method must be a boolean value. Any time the method returns 'true', the Message will be passed along to the output-channel. ]]> If the selector or adapted POJO method returns false, there are a few settings that control the fate of the rejected Message. By default (if configured like the example above), the rejected Messages will be silently dropped. If rejection should instead indicate an error condition, then set the 'throw-exception-on-rejection' flag to true: ]]> If you want the rejected messages to go to a specific channel, provide that reference as the 'discard-channel': ]]> A common usage for Message Filters is in conjunction with a Publish Subscribe Channel. Many filter endpoints may be subscribed to the same channel, and they decide whether or not to pass the Message for the next endpoint which could be any of the supported types (e.g. Service Activator). This provides a reactive alternative to the more proactive approach of using a Message Router with a single Point-to-Point input channel and multiple output channels. Using a "ref" attribute is generally recommended if the custom filter implementation can be reused in other <filter> definitions. However if the custom filter implementation should be scoped to a single <filter> element, provide an inner bean definition: ]]> Using both the "ref" attribute and an inner handler definition in the same <filter> configuration is not allowed, as it creates an ambiguous condition, and it will therefore result in an Exception being thrown.