What is a Plug-in and how to use Java plug-ins with Struts?

By: Emiley J. Emailed: 1598 times Printed: 2051 times    

Latest comments
By: rohit kumar - how this program is work
By: Kirti - Hi..thx for the hadoop in
By: Spijker - I have altered the code a
By: ali mohammed - why we use the java in ne
By: ali mohammed - why we use the java in ne
By: mizhelle - when I exported the data
By: raul - no output as well, i'm ge
By: Rajesh - thanx very much...
By: Suindu De - Suppose we are executing

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;
}


Struts Home | All Struts Tutorials | Latest Struts Tutorials

Sponsored Links

If this tutorial doesn't answer your question, or you have a specific question, just ask an expert here. Post your question to get a direct answer.



Bookmark and Share

Comments(1)


1. View Comment

Thanks for the description. Am loading my actionservlet on first request only, at that time if I get a servletexception, I would like to show that error message in my page that is requested, how can that be done?

View Tutorial          By: sridevi at 2012-01-13 10:15:40

Your name (required):


Your email(required, will not be shown to the public):


Your sites URL (optional):


Your comments:



More Tutorials by Emiley J.
Error generating R.java from manifest
ERROR 1251: Client does not support authentication protocol requested by server; consider upgrading MySQL client
IIS and Tomcat - how to configure to work together
PHP Warning: Unknown(): Unable to load dynamic library '/usr/local/php4/lib/php/extensions/no-debug ......
Adding your own Application icon for your J2ME application (jar file)
Creating Menus in Code using VB.net
Traversing all files in a directory using Ruby
Using Proxy to connect to URLs in Ruby
POST a form in Ruby
Reading URL content using Ruby (HTTP)
dRuby client/server mode sample program
Benchmark module in Ruby
encoding and decoding in Ruby using Base64 Module
Module Abbrev in Ruby
Standard Library Packages in Ruby

More Tutorials in Struts
Configuring JDBC DataSources in Struts
Struts Classes
FAQ: Why was reload removed from Struts (since 1.1)?
FAQ: Why are my checkboxes not being set from ON to OFF?
Using JavaScript to submit a form in Struts
How to prepopulate a form in Struts
Simple example of using the requiredif Validator rule in Struts
Chaining actions in Struts
When is the best time to validate input in Struts
What is a Plug-in and how to use Java plug-ins with Struts?
7 Best Practices of Struts
Origin and Architecture of Struts
ActionErrors and ActionError in Struts
Tutorial on Struts Configuration File - struts-config.xml in Struts - from the book: Struts Survival Guide. Basics to Best Practices
Handling multiple buttons in HTML Form in Struts

More Latest News
Most Viewed Articles (in Struts )
Tutorial on Struts Configuration File - struts-config.xml in Struts - from the book: Struts Survival Guide. Basics to Best Practices
Configuring JDBC DataSources in Struts
The directories and files of a Struts application
Struts 1 vs Struts 2
Struts and Tiles - Steps to use Struts and Tiles
Using Multiple Message Resource Bundles in Struts
Using Checkbox & Radio Tags, html:select, html:options in Struts Forms
What is a Plug-in and how to use Java plug-ins with Struts?
How to prepopulate a form in Struts
Chaining actions in Struts
What is Struts? Which Version of Struts to use?
Is Struts the most popular web application framework for Java?
What is a Model View Controller (MVC) Model?
History of Struts
WebWork 2 is now Apache Struts 2
Most Emailed Articles (in Struts)
Editing struts-config.xml in a Struts Application
IncludeAction in Struts
Using JavaScript to submit a form in Struts
Simple example of using the requiredif Validator rule in Struts
What is Struts? Which Version of Struts to use?
MVC Architecture (Model 2 Architecture)
MVC with configurable controller
Struts-GUI and Struts Console
Origin and Architecture of Struts
FAQ: Why was reload removed from Struts (since 1.1)?
Chaining actions in Struts
When is the best time to validate input in Struts
Is Struts the most popular web application framework for Java?
WebWork 2 is now Apache Struts 2
Configuring JDBC DataSources in Struts