Introduction :- A popular technique for organizing the execution of complex processing flows is the "Chain of Responsibility" pattern. In this pattern the different “commands” are combined into a single chain with a series of processing elements and the context parameter is passed along with all the parameter. All the commands contain a single execute method. A Boolean variable determines whether the command executes successfully or not. This pattern promotes the idea of loose coupling, a common programming practice.

From the version 1.3, the struts replaces the RequestProcessor with ComposableRequestProcessor class which follows the Chain of Responsibility pattern (CoR) design pattern. It is configured via the following context initialization parameters:

• org.apache.struts.chain.CATALOG_NAME - Name of the Catalog in which we will look up the Command to be executed for each request. If not specified, the default value is struts.


• org.apache.struts.chain.COMMAND_NAME - Name of the Command which we will execute for each request, to be looked up in the specified Catalog. If not specified, the default value is servlet-standard.

Each step of the processing is represented as a separate Command in the Chain. User can easily customize by changing, removing and replacing the chain commands. The Command classes are following packages:-

1. org.apache.struts.chain.commands.

2. org.apache.struts.chain.commands.servlet packages.

The following table will listed out all the command objects of CoR.

The default Chain-config file supplied by the apache is as follows:-

<?xml version="1.0" ?>
<!--
$Id: chain-config.xml 481833 2006-12-03 17:32:52Z niallp $

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.
-->
<!--
This file contains definitions of the standard Chain Of Responsibility chains that emulate Struts 1.2 processing functionality. These chains are defined in a catalog named "struts" so that the application can use the default catalog for its own purposes, without any potential for name clashes.

$Id: chain-config.xml 481833 2006-12-03 17:32:52Z niallp $
-->

<catalog name="struts">

    <define name="lookup" className="org.apache.commons.chain.generic.LookupCommand"/>
<!-- ========== Servlet Complete Request Chain ========================= -->

<chain name="servlet-standard">

<!-- Establish exception handling filter -->
    <command className="org.apache.struts.chain.commands.ExceptionCatcher" catalogName="struts" exceptionCommand="servlet-exception"/>

    <lookup catalogName="struts" name="process-action" optional="false"/>

    <lookup catalogName="struts" name="process-view" optional="false"/>

</chain>

<!-- ========== Action Processing chain ======================== -->
<chain name="process-action">

<!--
This chain attempts to emulate (most of) the standard request processing in the standard org.apache.struts.action.RequestProcessor class, by performing the corresponding tasks in individual Commands
that are composable. The following list defines a cross reference between the processXxx methods and the Commands that perform the corresponding functionality:

processMultipart Integrated into servlet and legacy classes

processPath SelectAction (which also does processMapping)

processException ExceptionCatcher / ExceptionHandler

processLocale SelectLocale

processContent SetContentType

processNoCache RequestNoCache

processPreprocess LookupCommand with optional="true". Multiple occurrences of this can easily be added, to support additional processing hooks at any point in the chain without modifying the standard definition.

processCachedMessages RemoveCachedMessages

processMapping SelectAction (which also does processPath)

processRoles AuthorizeAction

processActionForm CreateActionForm

processPopulate PopulateActionForm

processValidate ValidateActionForm / SelectInput

processForward SelectForward

processInclude SelectInclude / PerformInclude

processActionCreate CreateAction

processActionPerform ExecuteAction
-->
    <!-- Look up optional preprocess command -->

    <lookup catalogName="struts" name="servlet-standard-preprocess" optional="true"/>

    <!-- Identify the Locale for this request -->
    <command className="org.apache.struts.chain.commands.servlet.SelectLocale"/>

    <!-- Set (if needed) the URI of the original request -->
    <command className="org.apache.struts.chain.commands.servlet.SetOriginalURI"/>

    <!-- Set (if needed) no cache HTTP response headers -->
    <command className="org.apache.struts.chain.commands.servlet.RequestNoCache"/>

    <!-- Set (if needed) the HTTP response content type -->
    <command className="org.apache.struts.chain.commands.servlet.SetContentType"/>

    <!-- Remove messages cached in the Session -->
    <command className="org.apache.struts.chain.commands.RemoveCachedMessages"/>

    <!-- Identify the ActionConfig for this request -->
    <command className="org.apache.struts.chain.commands.servlet.SelectAction"/>

    <!-- Authorize the selected ActionConfig for this request -->
    <command className="org.apache.struts.chain.commands.servlet.AuthorizeAction"/>

    <!-- Create (if needed) the ActionForm for this request -->
    <command className="org.apache.struts.chain.commands.CreateActionForm"/>

    <!-- Populate the ActionForm for this request -->
    <command className="org.apache.struts.chain.commands.servlet.PopulateActionForm"/>

    <!-- Validate the ActionForm for this request -->
    <command className="org.apache.struts.chain.commands.servlet.ValidateActionForm"/>

    <!-- Select the appropriate ForwardConfig for return to input page -->
    <command className="org.apache.struts.chain.commands.servlet.SelectInput"/>

    <!-- Lookup and execute a chain command if the current ActionConfig is so-configured. -->
    <command className="org.apache.struts.chain.commands.ExecuteCommand"/>

    <!-- Select the appropriate ForwardConfig for action mappings that only have an ActionForward -->
    <command className="org.apache.struts.chain.commands.servlet.SelectForward"/>

    <!-- Select the include uri (if any) for the current action mapping -->
    <command className="org.apache.struts.chain.commands.SelectInclude"/>

    <!-- Perform the include (if needed) -->
    <command className="org.apache.struts.chain.commands.servlet.PerformInclude"/>

    <!-- Create (if needed) the Action for this request -->
    <command className="org.apache.struts.chain.commands.servlet.CreateAction"/>

    <!-- Execute the Action for this request -->
    <command className="org.apache.struts.chain.commands.servlet.ExecuteAction"/>

</chain>

<!-- ========== View Processing chain ======================== -->
<chain name="process-view">

    <!-- Lookup and execute a chain command if the current ForwardConfig is so-configured. -->
    <command className="org.apache.struts.chain.commands.ExecuteForwardCommand"/>

    <!-- Follow the returned ForwardConfig (if any) -->
    <command className="org.apache.struts.chain.commands.servlet.PerformForward"/>

</chain>

<!-- ========== Servlet Exception Handler Chain ======================== -->

<chain name="servlet-exception">

<!--
This chain is designed to be invoked (by o.a.s.c.ExceptionCatcher) if an unhandled exception is thrown by any subsequent command in a processing chain (including the one that invokes a Struts action). The standard definition of this chain supports the exception mapping of Struts 1.1, but can be replaced in order to handle exceptions differently.
-->

    <!-- Execute the configured exception handler (if any) -->
    <command className="org.apache.struts.chain.commands.servlet.ExceptionHandler"/>

    <!-- Follow the returned ForwardConfig (if any) -->
    <command className="org.apache.struts.chain.commands.servlet.PerformForward"/>

</chain>
</catalog>