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Added documentation for MessageHandlerChain and the <chain/> element (INT-486)

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Mark Fisher
2008-11-24 04:12:17 +00:00
parent 039c124dad
commit 9abe52d9f7
2 changed files with 51 additions and 0 deletions
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spring-integration-reference/src/chain.xml Normal file
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<?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="chain">
<title>Message Handler Chain</title>
<section id="chain-introduction">
<title>Introduction</title>
<para>
The <classname>MessageHandlerChain</classname> is an implementation of
<interfacename>MessageHandler</interfacename> that can be configured as a single Message Endpoint while
actually delegating to a chain of other handlers, such as Filters, Transformers, Splitters, and so on.
This can lead to a much simpler configuration when several handlers need to be connected in a fixed, linear
progression. For example, it is fairly common to provide a Transformer before other components. Similarly, when
providing a <emphasis>Filter</emphasis> before some other component in a chain, you are essentially creating a
<ulink url="http://www.eaipatterns.com/MessageSelector.html">Selective Consumer</ulink>. In either case, the
chain only requires a single input-channel and a single output-channel as opposed to the configuration of
channels for each individual component.
<tip>
Spring Integration's <emphasis>Filter</emphasis> provides a boolean property 'throwExceptionOnRejection'. When
providing multiple Selective Consumers on the same point-to-point channel with different acceptance criteria,
this value should be set to 'true' (the default is false) so that the dispatcher will know that the Message was
rejected and as a result will attempt to pass the Message on to other subscribers. If the Exception were not
thrown, then it would appear to the dispatcher as if the Message had been passed on successfully even though
the Filter had <emphasis>dropped</emphasis> the Message to prevent further processing.
</tip>
</para>
<para>
The handler chain simplifies configuration while internally maintaining the same degree of loose coupling between
components, and it is trivial to modify the configuration if at some point a non-linear arrangement is required.
</para>
</section>
<section id="chain-namespace">
<title>The &lt;chain&gt; Element</title>
<para>
The &lt;chain&gt; element provides an 'input-channel' attribute, and if the last element in the chain is capable
of producing reply messages, it may optionally provide an 'output-channel' attribute. The sub-elements are then
filters, transformers, splitters, and service-activators. The last element may be a router.
<programlisting language="xml"><![CDATA[ <chain input-channel="input" output-channel="output">
<filter ref="someSelector" throw-exception-on-rejection="true"/>
<header-enricher error-channel="customErrorChannel">
<header name="foo" value="bar"/>
</header-enricher>
<service-activator ref="someService" method="someMethod"/>
</chain>]]></programlisting>
</para>
</section>
</chapter>

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spring-integration-reference/src/spring-integration-reference.xml
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<xi:include href="./splitter.xml"/>
<xi:include href="./aggregator.xml"/>
<xi:include href="./resequencer.xml"/>
<xi:include href="./chain.xml"/>
<xi:include href="./gateway.xml"/>
<xi:include href="./bridge.xml"/>
<xi:include href="./file.xml"/>