General Extension Points

This section discusses some extension points that affect the overall framework, not necessarily one particular layer. Arguably the most important of these is the PlugIn mechanism.

Using the PlugIn Mechanism

The Struts framework provides a mechanism to allow components to be plugged in and loaded dynamically. This feature was added in Version 1.1 and is supported through the use of the org.apache.struts.action.PlugIn interface. Any Java class can function as a plug-in, as long as it implements the PlugIn interface.

A plug-in is simply any Java class that you need to initialize when the Struts application starts up, and destroy when the application shuts down.

The PlugIn interface contains two methods, as shown in Example 9-1.

Example 9-1. The org.apache.struts.action.PlugIn interface

public interface PlugIn {
  * Notification that the specified module is being started.
  public void init(ActionServlet servlet, ModuleConfig config)
        throws ServletException;

  * Notification that the module is being shut down.
  public void destroy( );

During startup of a Struts application, the ActionServlet calls the init() method for each PlugIn that is configured; the framework supports configuration of one or more PlugIns for each application. Initialization routines that your plug- in needs to perform should be done during the init() method. This is a good time to initialize a database connection or establish a connection to a remote system, for example.[8]

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