57 lines
3.3 KiB
XML
57 lines
3.3 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
|
|
<chapter id="filter">
|
|
<title>Message Filter</title>
|
|
|
|
<section id="filter-introduction">
|
|
<title>Introduction</title>
|
|
<para>
|
|
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
|
|
<emphasis>which</emphasis> Message Channel to send to but only decides <emphasis>whether</emphasis> to send.
|
|
</para>
|
|
<para>
|
|
In Spring Integration, a Message Filter may be configured as a Message Endpoint that delegates to some
|
|
implementation of the <interfacename>MessageSelector</interfacename> interface. That interface is itself quite
|
|
simple: <programlisting language="java"><![CDATA[ public interface MessageSelector {
|
|
|
|
boolean accept(Message<?> message);
|
|
|
|
}]]></programlisting>
|
|
The <classname>MessageFilter</classname> constructor accepts a selector instance:
|
|
<programlisting language="java"><![CDATA[ MessageFilter filter = new MessageFilter(someSelector);]]></programlisting>
|
|
</para>
|
|
</section>
|
|
|
|
<section id="filter-namespace">
|
|
<title>The <filter> Element</title>
|
|
<para>
|
|
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:
|
|
<programlisting language="xml"><![CDATA[ <filter input-channel="input" ref="selector" output-channel="output"/>
|
|
|
|
<bean id="selector" class="example.MessageSelectorImpl"/>]]></programlisting>
|
|
</para>
|
|
<para>
|
|
Alternatively, the "method" attribute can be added at which point the "ref" may refer to any object.
|
|
The referenced method may expect either the <interfacename>Message</interfacename> 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 <emphasis>will</emphasis> be passed along to the output-channel. Otherwise, it will be dropped.
|
|
<programlisting language="xml"><![CDATA[ <filter input-channel="input" output-channel="output"
|
|
ref="exampleObject" method="someBooleanReturningMethod"/>
|
|
|
|
<bean id="exampleObject" class="example.SomeObject"/>]]></programlisting>
|
|
</para>
|
|
<note>
|
|
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 <emphasis>reactive</emphasis>
|
|
alternative to the more <emphasis>proactive</emphasis> approach of using a Message Router with a single
|
|
point-to-point input channel and multiple output channels.
|
|
</note>
|
|
</section>
|
|
|
|
</chapter> |