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Struts 2 plugins contain classes and configuration that extend, replace, or add to existing Struts framework functionality. A plugin can be installed by adding its JAR file to the application's class path, in addition to the JAR files to fulfill whatever dependencies the plugin itself may have. To configure the plugin, the JAR should contain a struts-plugin.xml file, which follows the same format as an ordinary struts.xml file.

Since a plugin can contain the struts-plugin.xml file, it has the ability to:

  • Define new packages with results, interceptors, and/or actions
  • Override framework constants
  • Introduce new extension point implementation classes

Many popular but optional features of the framework are distributed as plugins. An application can retain all the plugins provided with the distribution, or just include the ones it uses. Plugins can be used to organize application code or to distribute code to third-parties.

Packages defined in a plugin can have parent packages that are defined in another plugin. Plugins may define configuration elements with classes not contained in the plugin. Any classes not included in the plugin's JAR must be on the application's classpath at runtime. As from Struts 2.3.5

The framework loads its default configuration first, then any plugin configuration files found in others JARs on the classpath, and finally the "bootstrap" struts.xml.

  1. struts-default.xml (bundled in the Core JAR)
  2. struts-plugin.xml (as many as can be found in other JARs)
  3. struts.xml (provided by your application)

Since the struts.xml file is always loaded last, it can make use of any resources provided by the plugins bundled with the distribution, or any other plugins available to an application.

Static resources

To include static resources in your plugins add them under "/static" in your jar. And include them in your page using "/struts" as the path, like in the following example:

Assuming /static/main.css is inside a plugin jar, to add it to the page:

<@s.url value="/struts/main.css" var="css" />
<link rel="stylesheet" type="text/css" href="%{#css}" />

Read also StaticContentLoader JavaDoc.

Extension Points

Extension points allow a plugin to override a key class in the Struts framework with an alternate implementation. For example, a plugin could provide a new class to create Action classes or map requests to Actions.

The following extension points are available in Struts 2:

Type Property Scope Description
com.opensymphony.xwork2.ObjectFactory struts.objectFactory singleton Creates actions, results, and interceptors
com.opensymphony.xwork2.factory.ActionFactory struts.objectFactory.actionFactory singleton Dedicated factory used to create Actions, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ResultFactory struts.objectFactory.resultFactory singleton Dedicated factory used to create Results, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.InterceptorFactory struts.objectFactory.interceptorFactory singleton Dedicated factory used to create Interceptors, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ConverterFactory struts.objectFactory.converterFactory singleton Dedicated factory used to create TypeConverters, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ValidatorFactory struts.objectFactory.validatorFactory singleton Dedicated factory used to create Validators, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.ActionProxyFactory struts.actionProxyFactory singleton Creates the ActionProxy
com.opensymphony.xwork2.util.ObjectTypeDeterminer struts.objectTypeDeterminer singleton Determines what the key and element class of a Map or Collection should be
org.apache.struts2.dispatcher.mapper.ActionMapper struts.mapper.class singleton Determines the ActionMapping from a request and a URI from an ActionMapping
org.apache.struts2.dispatcher.multipart.MultiPartRequest struts.multipart.parser per request Parses a multipart request (file upload)
org.apache.struts2.views.freemarker.FreemarkerManager struts.freemarker.manager.classname singleton Loads and processes Freemarker templates
org.apache.struts2.views.velocity.VelocityManager struts.velocity.manager.classname singleton Loads and processes Velocity templates
com.opensymphony.xwork2.validator.ActionValidatorManager struts.actionValidatorManager singleton Main interface for validation managers (regular and annotation based). Handles both the loading of configuration and the actual validation (since 2.1)
com.opensymphony.xwork2.util.ValueStackFactory struts.valueStackFactory singleton Creates value stacks (since 2.1)
com.opensymphony.xwork2.reflection.ReflectionProvider struts.reflectionProvider singleton Provides reflection services, key place to plug in a custom expression language (since 2.1)
com.opensymphony.xwork2.reflection.ReflectionContextFactory struts.reflectionContextFactory singleton Creates reflection context maps used for reflection and expression language operations (since 2.1)
com.opensymphony.xwork2.config.PackageProvider N/A singleton All beans registered as PackageProvider implementations will be automatically included in configuration building (since 2.1)
com.opensymphony.xwork2.util.PatternMatcher struts.patternMatcher singleton Matches patterns, such as action names, generally used in configuration (since 2.1)
org.apache.struts2.views.dispatcher.DefaultStaticContentLoader struts.staticContentLoader singleton Loads static resources (since 2.1)
com.opensymphony.xwork2.conversion.impl.XWorkConverter struts.xworkConverter singleton Handles conversion logic and allows to load custom converters per class or per action
com.opensymphony.xwork2.TextProvider struts.xworkTextProvider default Allows provide custom TextProvider for whole application
com.opensymphony.xwork2.LocaleProvider struts.localeProvider singleton DEPRECATED! Allows provide custom TextProvider for whole application - instead this endpoint use struts.localeProviderFactory
com.opensymphony.xwork2.LocaleProviderFactory struts.localeProviderFactory singleton Allows provide custom LocaleProvider for whole application
org.apache.struts2.components.UrlRenderer struts.urlRenderer singleton Allows provide custom implementation of environment specific URL rendering/creating class
com.opensymphony.xwork2.UnknownHandlerManager struts.unknownHandlerManager singleton Implementation of this interface allows handle logic of unknown Actions, Methods or Results
org.apache.struts2.views.util.UrlHelper struts.view.urlHelper singleton Helper class used with URLRenderer to provide exact logic for building URLs
com.opensymphony.xwork2.FileManagerFactory struts.fileManagerFactory singleton Used to create FileManager instance to access files on the File System as also to monitor if reload is needed, can be implemented / overwritten to meet specific an application server needs
com.opensymphony.xwork2.conversion.impl.CollectionConverter struts.converter.collection singleton Converter used to convert any object to Collection and back
com.opensymphony.xwork2.conversion.impl.ArrayConverter struts.converter.array singleton Converter used to convert any object to Array and back
com.opensymphony.xwork2.conversion.impl.DateConverter singleton Converter used to convert any object to Date and back
com.opensymphony.xwork2.conversion.impl.NumberConverter struts.converter.number singleton Converter used to convert any object to Number and back
com.opensymphony.xwork2.conversion.impl.StringConverter struts.converter.string singleton Converter used to convert any object to String and back
com.opensymphony.xwork2.conversion.ConversionPropertiesProcessor singleton Process Properties to create converters
com.opensymphony.xwork2.conversion.ConversionPropertiesProcessor struts.converter.file.processor singleton Process <class> file create converters</class>
com.opensymphony.xwork2.conversion.ConversionAnnotationProcessor struts.converter.annotation.processor singleton Process TypeConversion annotation to create converters
com.opensymphony.xwork2.conversion.TypeConverterCreator struts.converter.creator singleton Creates user converters
com.opensymphony.xwork2.conversion.TypeConverterHolder struts.converter.holder singleton Holds user converters' instances
com.opensymphony.xwork2.util.TextParser struts.expression.parser singleton Used to parse expressions like ${} or %{} but it is up tp the TextParser's implementation what kind of opening char to use (#, $, %, etc)
com.opensymphony.xwork2.ExcludedPatternsChecker struts.excludedPatterns.checker request Used across different interceptors to check if given string matches one of the excluded patterns
com.opensymphony.xwork2.AcceptedPatternsChecker struts.acceptedPatterns.checker request Used across different interceptors to check if given string matches one of the accepted patterns
org.apache.struts2.util.ContentTypeMatcher struts.contentTypeMatcher singleton Matches content type of uploaded files (since 2.3.22)
com.opensymphony.xwork2.LocalizedTextProvider struts.localizedTextProvider singleton Provides access to resource bundles used to localise messages (since 2.5.11)

Plugin Examples

Let's look at two similar but different plugins bundled with the core distribution.

Sitemesh plugin

SiteMesh is a popular alternative to Tiles. SiteMesh provides a common look-and-feel to an application's pages by automatically wrapping a plain page with common elements like headers and menubars.

The sitemesh-plugin.jar contains several classes, a standard JAR manifest, and a plugin configuration file.

 + org
   + apache
     + struts2
       + sitemesh
         + FreeMarkerPageFilter.class
         + TemplatePageFilter.class
         + VelocityPageFilter.class
 + struts-plugin.xml

While the SiteMesh Plugin doesn't provide any new results, interceptors, or actions, or even extend any Struts integration points, it does need to know what settings have been enabled in the Struts framework. Therefore, its struts-plugin.xml looks like this:

Error rendering macro 'code': Invalid value specified for parameter 'java.lang.NullPointerException'
<?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
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
	"-//Apache Software Foundation//DTD Struts Configuration 2.5//EN"
    <bean class="org.apache.struts2.sitemesh.FreemarkerPageFilter" static="true" optional="true"/>
    <bean class="org.apache.struts2.sitemesh.VelocityPageFilter" static="true" optional="true"/>

The two bean elements, with the "static" flag enabled, tell Struts to inject the current settings and framework objects into static property setters on startup. This allows, for example, the FreeMarkerPageFilter class to get an instance of the Struts FreemarkerManager and the current encoding setting.

Tiles plugin

Tiles is a popular alternative to SiteMesh. Tiles provides a common look-and-feel to an application's pages by breaking the page down into common fragments or "tiles".

The tiles-plugin.jar contains several classes, a standard JAR manifest, and a configuration file.

 + org
   + apache
     + struts2
       + tiles
         + StrutsTilesListener.class
         + StrutsTileUtilImpl.class
       + views
         + tiles
           + TilesResult.class
   + struts-plugin.xml

Since the Tiles Plugin does need to register configuration elements, a result class, it provides a struts-plugin.xml file.

Plugin Registry

(tick) For a list of bundled plugins, see the Plugin Reference Documentation. For more about bundled and third-party plugins, visit the Apache Struts Plugin Registry.

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