In this tutorial, we will show how easily your application can be made extensible using the Struts 2 plugins mechanism. To keep the demonstration simple, our plugin will expose a JavaBean that writes a message. Your plugins can include any combination of JavaBeans, Actions, Interceptors, Results or whatever other resources you'd like to plugin.
At runtime, plugins are retrieved and referenced via an Interface. So, first, we should define an interface that our plugins can implement. This interface must be available to both your web application and the plugins. To reduce coupling between the web application and the plugins, keep the interface in a separate JAR.
Now that we have an interface to implement, lets create an actual plugin. At load time, the framework looks for JARs with contain a
struts-plugin.xml file at the root of the archive. To create a plugin, all we need to do is build a JAR and put the expected
struts-plugin.xml at the root.
To get started, let's create a class that implements our IMyPlugin interface.
Internally, the framework utilizes a number of JavaBeans. We can use the
bean element to add our own JavaBeans to the set managed by the framework.
Finally, to install the plugin, just drop the JAR file under WEB-INF/lib.
The JavaBeans configured by the
bean element can be retrieved via a Container provided by XWork 2. We can obtain a reference to the Container by using the
@Inject notation. (This notation is part of the Guice framework that XWork and Struts use under the covers.) The framework predefines a Container object, and the
@Inject notion tells the framework to set its Container object to the Action property.
We might want to supply a number of JavaBeans to the application this way. In the Action, we will obtain a reference to the entire set of JavaBeans that might have been plugged in. Then, we can scroll through the set, displaying each JavaBean's message.
As seen by the Action class code, it's important to define a unique interface for any beans that we plugin, so that we can identify our beans later.
In the same way that we plugged in this JavaBean, we could also plugin and configure Action classes, Interceptors, Results, or any other JAR-able resource that an application might utilize.
Let's do something with those plugins: