Plugins architecture

• Static resources
• Extension Points
• Plugin Examples
  • Sitemesh plugin
  • Tiles plugin
• Developing new extension point
  • Extension point provided by the Core
  • Plugin-defined extension points
• Plugin Registry

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 and optionally a struts-deferred.xml, which follow the same format as an ordinary struts.xml file.

Since a plugin can contain these xml files, they have the ability to:

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

Whilst configuration from struts-plugin.xml is loaded prior to struts.xml, struts-deferred.xml is loaded after all other configuration is loaded. This makes it useful for defining plugin extension points.

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 struts-plugin.xml files found in others JARs on the classpath, the “bootstrap” struts.xml, then finally any struts-deferred.xml files.

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)
4. struts-deferred.xml (as many as can be found in other JARs)

Since the struts.xml file is loaded between struts-plugin.xml and before any potential plugin extension points in struts-deferred.xml, 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 “/static” 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="/static/main.css" var="css" />
<link rel="stylesheet" type="text/css" href="%{#css}" />

Read also Static Content and JavaDoc of StaticContentLoader.

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:

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.

 + META-INF/
   + 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:

<?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.
 */
-->
<!DOCTYPE struts PUBLIC
	"-//Apache Software Foundation//DTD Struts Configuration 2.5//EN"
	"http://struts.apache.org/dtds/struts-2.5.dtd">
    
<struts>
    <bean class="org.apache.struts2.sitemesh.FreemarkerPageFilter" static="true" optional="true"/>
    <bean class="org.apache.struts2.sitemesh.VelocityPageFilter" static="true" optional="true"/>
</struts>

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.

 + META-INF/
   + 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.

Developing new extension point

An extension point it’s a name which will be used to locate other beans by the name and inject them into a given place. Extension point isn’t a bean, you cannot inject it. It’s a bridge between your interface/class and the final implementation, provided either by a plugin or by a user.

If needed you can define your own extension point. This can happen in two ways: either define it in the Struts Core, or define the extension in the plugin you are developing.

Extension point provided by the Core

First step is to name your extension point, basically we are using a pattern like follow:

struts.<component | functionallity>.<name of the extebsion point>

e.g.: struts.date.formatter.

Now you must provide an interface or a class as the extension point, it will be used by others to implement they own behaviour of the extension point, e.g.: org.apache.struts2.components.date.DateFormatter.

The next step is to tie the name of the extension point with the interface/class. It happens in StrutsBeanSelectionProvider

by using alias() method as below:

alias(DateFormatter.class, StrutsConstants.STRUTS_DATE_FORMATTER, builder, props, Scope.SINGLETON);

as you can see you must provide:

• Class<?> class behind the extension point, which is used by users to implement the extension point
• String a key, the name of the extension point
• ContainerBuilder builder used to setup the Container
• Properties additional properties when needed
• Scope scope of the extension point, all the beans extneding the extension point must have the same scope

Having all that set, you can define where to use the extension point by injecting beans implementing it by using @Inject

@Inject
public void setDateFormatter(DateFormatter dateFormatter) {
    this.dateFormatter = dateFormatter;
}

you can use optional = true if the implementation is not required, yet please remember to handle null in such case.

All the above steps have defined a new extension point, now you can implement a bean which will be bridged throughout the extension point with the @Inject. Just implement the interface/class used to define the extension point and define the bean in struts-default.xml:

public class DateTimeFormatterAdapter implements DateFormatter {

    @Override
    public String format(TemporalAccessor temporal, String format) {
        DateTimeFormatter dtf;
        Locale locale = ActionContext.getContext().getLocale();
        if (format == null) {
            dtf = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM)
                .withLocale(locale);
        } else {
            dtf = DateTimeFormatter.ofPattern(format, locale);
        }
        return dtf.format(temporal);
    }

}

<bean type="org.apache.struts2.components.date.DateFormatter" 
      name="dateTimeFormatter" 
      class="org.apache.struts2.components.date.DateTimeFormatterAdapter" 
      scope="singleton"/>

The name attribute it’s of your choice, it just needs to be unique.

And the final step is to use the new bean with the extension point, this will happen in default.properties:

struts.date.formatter=dateTimeFormatter

and done!

If a user would like to provide its own implementation they just needs to implement the interface, define a bean in struts.xml with a name and then tie it to the extension point overriding the one provided by the framework:

<bean type="org.apache.struts2.components.date.DateFormatter" 
      name="myDateTimeFormatter" 
      class="com.company.date.MyDateTimeFormatterAdapter" 
      scope="singleton"/>

struts.date.formatter=myDateTimeFormatter

Plugin-defined extension points

It’s very much like above except that the plugin must provide a bean-selection configuration option in struts-deferred.xml. The bean-selection option represents an implementation of a class org.apache.struts2.config.AbstractBeanSelectionProvider with no-arguments constructor:

public class VelocityBeanSelectionProvider extends AbstractBeanSelectionProvider {

    @Override
    public void register(ContainerBuilder builder, LocatableProperties props) throws ConfigurationException {
        alias(VelocityManager.class, VelocityConstants.STRUTS_VELOCITY_MANAGER_CLASSNAME, builder, props);
    }

}

The class defines extension points by implementing register() method and using alias() method to register them.

And finally it must be added to the struts-deferred.xml:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
	"-//Apache Software Foundation//DTD Struts Configuration 2.6//EN"
	"http://struts.apache.org/dtds/struts-2.6.dtd">
    
<struts>
  ...
  
  <bean-selection name="velocityBeans" class="org.apache.struts2.views.velocity.VelocityBeanSelectionProvider"/>

</struts>

And now other plugins or user application can use the new extension point represented by VelocityConstants.STRUTS_VELOCITY_MANAGER_CLASSNAME.

Plugin Registry

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.