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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:

com.opensymphony.xwork2.ObjectFactorystruts.objectFactorysingletonCreates actions, results, and interceptors
com.opensymphony.xwork2.factory.ActionFactorystruts.objectFactory.actionFactorysingletonDedicated factory used to create Actions, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ResultFactorystruts.objectFactory.resultFactorysingletonDedicated factory used to create Results, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.InterceptorFactorystruts.objectFactory.interceptorFactorysingletonDedicated factory used to create Interceptors, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ConverterFactorystruts.objectFactory.converterFactorysingletonDedicated factory used to create TypeConverters, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.factory.ValidatorFactorystruts.objectFactory.validatorFactorysingletonDedicated factory used to create Validators, you can implement/extend existing one instead of defining new ObjectFactory
com.opensymphony.xwork2.ActionProxyFactorystruts.actionProxyFactorysingletonCreates the ActionProxy
com.opensymphony.xwork2.util.ObjectTypeDeterminerstruts.objectTypeDeterminersingletonDetermines what the key and element class of a Map or Collection should be
org.apache.struts2.dispatcher.mapper.ActionMapperstruts.mapper.classsingletonDetermines the ActionMapping from a request and a URI from an ActionMapping
org.apache.struts2.dispatcher.multipart.MultiPartRequeststruts.multipart.parserper requestParses a multipart request (file upload)
org.apache.struts2.views.freemarker.FreemarkerManagerstruts.freemarker.manager.classnamesingletonLoads and processes Freemarker templates
org.apache.struts2.views.velocity.VelocityManagerstruts.velocity.manager.classnamesingletonLoads and processes Velocity templates
com.opensymphony.xwork2.validator.ActionValidatorManagerstruts.actionValidatorManagersingletonMain 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.ValueStackFactorystruts.valueStackFactorysingletonCreates value stacks (since 2.1)
com.opensymphony.xwork2.reflection.ReflectionProviderstruts.reflectionProvidersingletonProvides reflection services, key place to plug in a custom expression language (since 2.1)
com.opensymphony.xwork2.reflection.ReflectionContextFactorystruts.reflectionContextFactorysingletonCreates reflection context maps used for reflection and expression language operations (since 2.1)
com.opensymphony.xwork2.config.PackageProviderN/AsingletonAll beans registered as PackageProvider implementations will be automatically included in configuration building (since 2.1)
com.opensymphony.xwork2.util.PatternMatcherstruts.patternMatchersingletonMatches patterns, such as action names, generally used in configuration (since 2.1)
org.apache.struts2.views.dispatcher.DefaultStaticContentLoaderstruts.staticContentLoadersingletonLoads static resources (since 2.1)
com.opensymphony.xwork2.conversion.impl.XWorkConverterstruts.xworkConvertersingletonHandles conversion logic and allows to load custom converters per class or per action
com.opensymphony.xwork2.TextProviderstruts.xworkTextProviderdefaultAllows provide custom TextProvider for whole application
com.opensymphony.xwork2.LocaleProviderstruts.localeProvidersingletonAllows provide custom TextProvider for whole application
org.apache.struts2.components.UrlRendererstruts.urlRenderersingletonAllows provide custom implementation of environment specific URL rendering/creating class
com.opensymphony.xwork2.UnknownHandlerManagerstruts.unknownHandlerManagersingletonImplementation of this interface allows handle logic of unknown Actions, Methods or Results
org.apache.struts2.views.util.UrlHelperstruts.view.urlHelpersingletonHelper class used with URLRenderer to provide exact logic for building URLs
com.opensymphony.xwork2.FileManagerFactorystruts.fileManagerFactorysingletonUsed 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.CollectionConverterstruts.converter.collectionsingletonConverter used to convert any object to Collection and back
com.opensymphony.xwork2.conversion.impl.ArrayConverterstruts.converter.arraysingletonConverter used to convert any object to Array and back
com.opensymphony.xwork2.conversion.impl.DateConverterstruts.converter.datesingletonConverter used to convert any object to Date and back
com.opensymphony.xwork2.conversion.impl.NumberConverterstruts.converter.numbersingletonConverter used to convert any object to Number and back
com.opensymphony.xwork2.conversion.impl.StringConverterstruts.converter.stringsingletonConverter used to convert any object to String and back
com.opensymphony.xwork2.conversion.ConversionPropertiesProcessorstruts.conversion.properties.processorsingletonProcess Properties to create converters
com.opensymphony.xwork2.conversion.ConversionPropertiesProcessorstruts.converter.file.processorsingletonProcess <class>-conversion.properties file create converters</class>
com.opensymphony.xwork2.conversion.ConversionAnnotationProcessorstruts.converter.annotation.processorsingletonProcess TypeConversion annotation to create converters
com.opensymphony.xwork2.conversion.TypeConverterCreatorstruts.converter.creatorsingletonCreates user converters
com.opensymphony.xwork2.conversion.TypeConverterHolderstruts.converter.holdersingletonHolds user converters' instances
com.opensymphony.xwork2.util.TextParserstruts.expression.parsersingletonUsed to parse expressions like ${foo.bar} or %{bar.foo} but it is up tp the TextParser's implementation what kind of opening char to use (#, $, %, etc)
com.opensymphony.xwork2.ExcludedPatternsCheckerstruts.excludedPatterns.checkerrequestUsed across different interceptors to check if given string matches one of the excluded patterns
com.opensymphony.xwork2.AcceptedPatternsCheckerstruts.acceptedPatterns.checkerrequestUsed across different interceptors to check if given string matches one of the accepted patterns
org.apache.struts2.util.ContentTypeMatcherstruts.contentTypeMatchersingletonMatches content type of uploaded files (since 2.3.22)

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.

   + manifest.mf
 + 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:

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.

   + manifest.mf
 + 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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