A message filter is a processor that eliminates undesired messages based on specific criteria. In FUSE Mediation Router, the message filter pattern, shown in Figure 5.2, is implemented by the filter() Java DSL command. The filter() command takes a single predicate argument, which controls the filter. When the predicate is true, the incoming message is allowed to proceed, and when the predicate is false, the incoming message is blocked.

Figure 5.2. Message Filter Pattern

The following example shows how to create a route from endpoint, seda:a, to endpoint, seda:b, that blocks all messages except for those messages whose foo header have the value, bar:

RouteBuilder builder = new RouteBuilder() {
    public void configure() {
        from("seda:a").filter(header("foo").isEqualTo("bar")).to("seda:b");
    }
};

To evaluate more complex filter predicates, you can invoke one of the supported scripting languages, such as XPath, XQuery, or SQL (see Languages for Expressions and Predicates in Defining Routes). The following example defines a route that blocks all messages except for those containing a person element whose name attribute is equal to James:

from("direct:start").
        filter().xpath("/person[@name='James']").
        to("mock:result");

The following example shows how to configure the route with an XPath predicate in XML (see Languages for Expressions and Predicates in Defining Routes):

<camelContext id="simpleFilterRoute" xmlns="http://activemq.apache.org/camel/schema/spring">
  <route>
    <from uri="seda:a"/>
    <filter>
      <xpath>$foo = 'bar'</xpath>
      <to uri="seda:b"/>
    </filter>
  </route>
</camelContext>