Any Java class that you want to initialize when Struts application starts up and destroy when the application shuts down is a Java Plug-in. You can create a Java Plug-in by creating a Java class that implements org.apache.struts.action.Plugin interface.

Example: 
public interface PlugIn {
public void init(ActionServlet servlet,
ApplicationConfig config)
throws ServletException;
public void destroy();
}

Why you need a Plug-in?
Before Struts 1.1 (in Struts 1.0), you had to subclass ActionServlet to initialize application resources at startup time. With plugin mechanism (in Struts 1.1), you create Plugin classes and configure them.

Generic mechanism
Struts framework itself uses plugin mechanism for supporting Validator and Tiles. The plug-in must be declared in struts-config.xml via <plug-in> element.

example plug-in's in struts-example (sample application)

<plug-in className="org.apache.struts.plugins.ModuleConfigVerifier"/>
<plug-in className="org.apache.struts.webapp.example.memory.MemoryDatabasePlugIn">
<set-property property="pathname" value="/WEB-INF/database.xml"/>
</plug-in>

<plug-in className="org.apache.struts.validator.ValidatorPlugIn">
<set-property property="pathnames" value="/WEB-INF/validator-rules.xml, /WEB-INF/validation.xml"/>
</plug-in>

How do Plug-in's get called? During startup of a Struts application, ActionServlet calls init() method of each Plug-in configured. Plug-ins are called in the order they are configured in struts-config.xml file. Here is a sample application that shows how init() of MemoryDatabasePlugin in struts-example sample application.

public final class MemoryDatabasePlugIn implements PlugIn {
...
/**
* Initialize and load our initial database from persistent storage.
*
* @param servlet The ActionServlet for this web application
* @param config The ApplicationConfig for our owning module
*
* @exception ServletException if we cannot configure ourselves correctly
*/
public void init(ActionServlet servlet, ModuleConfig config)
throws ServletException {
log.info("Initializing memory database plug in from '" + pathname + "'");
// Remember our associated configuration and servlet
this.config = config;
this.servlet = servlet;
// Construct a new database and make it available
database = new MemoryUserDatabase();
try {
String path = calculatePath();
if (log.isDebugEnabled()) {
log.debug(" Loading database from '" + path + "'");
}
database.setPathname(path);
database.open();
} catch (Exception e) {
log.error("Opening memory database", e);
throw new ServletException("Cannot load database from '" + pathname + "'", e);
}
// Make the initialized database available
servlet.getServletContext().setAttribute(Constants.DATABASE_KEY,
database);
// Setup and cache other required data
setupCache(servlet, config);
}

//destroy() of MemoryDatabasePlugin in struts-example1 (sample code)
public final class MemoryDatabasePlugIn implements PlugIn {
...
/**
* Gracefully shut down this database, releasing any resources
* that were allocated at initialization.
*/
public void destroy() {
log.info("Finalizing memory database plug in");
if (database != null) {
try {
database.close();
} catch (Exception e) {
log.error("Closing memory database", e);
}
}
servlet.getServletContext().removeAttribute(Constants.DATABASE_KEY);
database = null;
servlet = null;
database = null;
config = null;
}

2008-09-17

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