xdocs/index.xml - commons-chain - Git at Google

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    Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
   the License.  You may obtain a copy of the License at

        http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
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 <document>

   <properties>
     <title>Overview</title>
     <author email="dev@commons.apache.org">Commons Documentation Team</author>
     <author email="martinc@apache.org">Martin Cooper</author>
     <author email="craigmcc@apache.org">Craig McClanahan</author>
   </properties>

   <body>

   <section name="Commons Chain">

       <p>A popular technique for organizing the execution of complex
       processing flows is the "Chain of Responsibility" pattern, as
       described (among many other places) in the classic "Gang of Four"
       design patterns book. Although the fundamental API contracts
       required to implement this design patten are extremely simple, it
       is useful to have a base API that facilitates using the pattern,
       and (more importantly) encouraging composition of command
       implementations from multiple diverse sources.</p>

       <p>Towards that end, the Chain API models a computation as a
       series of "commands" that can be combined into a "chain". The API
       for a command consists of a single method
       (<code>execute()</code>), which is passed a "context" parameter
       containing the dynamic state of the computation, and whose return
       value is a boolean that determines whether or not processing for
       the current chain has been completed (true), or whether
       processing should be delegated to the next command in the chain
       (false).</p>

       <p>The "context" abstraction is designed to isolate command
       implementations from the environment in which they are run (such
       as a command that can be used in either a Servlet or Portlet,
       without being tied directly to the API contracts of either of
       these environments). For commands that need to allocate resources
       prior to delegation, and then release them upon return (even if a
       delegated-to command throws an exception), the "filter" extension
       to "command" provides a <code>postprocess()</code> method for
       this cleanup. Finally, commands can be stored and looked up in a
       "catalog" to allow deferral of the decision on which command (or
       chain) is actually executed.</p>

       <p>To maximize the usefulness of the Chain of Responsibility
       pattern APIs, the fundamental interface contracts are defined in
       a manner with zero dependencies other than an appropriate JDK.
       Convenience base class implementations of these APIs are
       provided, as well as more specialized (but optional)
       implementations for the web environment (i.e. servlets and
       portlets).</p>

       <p>Given that command implementations are designed to conform
       with these recommendations, it should be feasible to utilize the
       Chain of Responsibility APIs in the "front controller" of a web
       application framework (such as Struts), but also be able to use
       it in the business logic and persistence tiers to model complex
       computational requirements via composition. In addition,
       separation of a computation into discrete commands that operate
       on a general purpose context allows easier creation of commands
       that are unit testable, because the impact of executing a command
       can be directly measured by observing the corresponding state
       changes in the context that is supplied.</p>

   </section>

   <section name="Documentation">
     <p>
       <ul>
         <li>The <a href="api-release/index.html">Javadoc</a> of the latest Release.</li>
         <li>The <a href="cookbook.html">Cookbook</a> containing "recipes" for using the chain.</li>
         <li>The <a href="source-repository.html">SVN repository</a> can be browsed.</li>
       </ul>
     </p>
   </section>

   <section name="Downloading Chain">
     <p>See the <a href="downloads.html">Downloads</a> page for current/previous
        releases.
     </p>
   </section>

   <section name="Support">
     <p>
       The <a href="mail-lists.html">commons mailing lists</a> act as the main support forum.
       The user list is suitable for most library usage queries.
       The dev list is intended for the development discussion.
       Please remember that the lists are shared between all commons components,
       so prefix your email by [chain].
     </p>

     <p>
       Issues may be reported via <a href="issue-tracking.html">ASF JIRA</a>.
     </p>
   </section>

 </body>
 </document>
	<?xml version="1.0" encoding="UTF-8"?>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one or more
	contributor license agreements. See the NOTICE file distributed with
	this work for additional information regarding copyright ownership.
	The ASF licenses this file to You under the Apache License, Version 2.0
	(the "License"); you may not use this file except in compliance with
	the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->

	<document>

	<properties>
	<title>Overview</title>
	<author email="dev@commons.apache.org">Commons Documentation Team</author>
	<author email="martinc@apache.org">Martin Cooper</author>
	<author email="craigmcc@apache.org">Craig McClanahan</author>
	</properties>

	<body>

	<section name="Commons Chain">

	<p>A popular technique for organizing the execution of complex
	processing flows is the "Chain of Responsibility" pattern, as
	described (among many other places) in the classic "Gang of Four"
	design patterns book. Although the fundamental API contracts
	required to implement this design patten are extremely simple, it
	is useful to have a base API that facilitates using the pattern,
	and (more importantly) encouraging composition of command
	implementations from multiple diverse sources.</p>

	<p>Towards that end, the Chain API models a computation as a
	series of "commands" that can be combined into a "chain". The API
	for a command consists of a single method
	(<code>execute()</code>), which is passed a "context" parameter
	containing the dynamic state of the computation, and whose return
	value is a boolean that determines whether or not processing for
	the current chain has been completed (true), or whether
	processing should be delegated to the next command in the chain
	(false).</p>

	<p>The "context" abstraction is designed to isolate command
	implementations from the environment in which they are run (such
	as a command that can be used in either a Servlet or Portlet,
	without being tied directly to the API contracts of either of
	these environments). For commands that need to allocate resources
	prior to delegation, and then release them upon return (even if a
	delegated-to command throws an exception), the "filter" extension
	to "command" provides a <code>postprocess()</code> method for
	this cleanup. Finally, commands can be stored and looked up in a
	"catalog" to allow deferral of the decision on which command (or
	chain) is actually executed.</p>

	<p>To maximize the usefulness of the Chain of Responsibility
	pattern APIs, the fundamental interface contracts are defined in
	a manner with zero dependencies other than an appropriate JDK.
	Convenience base class implementations of these APIs are
	provided, as well as more specialized (but optional)
	implementations for the web environment (i.e. servlets and
	portlets).</p>

	<p>Given that command implementations are designed to conform
	with these recommendations, it should be feasible to utilize the
	Chain of Responsibility APIs in the "front controller" of a web
	application framework (such as Struts), but also be able to use
	it in the business logic and persistence tiers to model complex
	computational requirements via composition. In addition,
	separation of a computation into discrete commands that operate
	on a general purpose context allows easier creation of commands
	that are unit testable, because the impact of executing a command
	can be directly measured by observing the corresponding state
	changes in the context that is supplied.</p>

	</section>

	<section name="Documentation">
	<p>
	<ul>
	<li>The <a href="api-release/index.html">Javadoc</a> of the latest Release.</li>
	<li>The <a href="cookbook.html">Cookbook</a> containing "recipes" for using the chain.</li>
	<li>The <a href="source-repository.html">SVN repository</a> can be browsed.</li>
	</ul>
	</p>
	</section>

	<section name="Downloading Chain">
	<p>See the <a href="downloads.html">Downloads</a> page for current/previous
	releases.
	</p>
	</section>

	<section name="Support">
	<p>
	The <a href="mail-lists.html">commons mailing lists</a> act as the main support forum.
	The user list is suitable for most library usage queries.
	The dev list is intended for the development discussion.
	Please remember that the lists are shared between all commons components,
	so prefix your email by [chain].
	</p>

	<p>
	Issues may be reported via <a href="issue-tracking.html">ASF JIRA</a>.
	</p>
	</section>

	</body>
	</document>