Running Java Code when a Web App Starts or Stops

You can run Java code when a web application starts and stops by implementing ServletContextListener in your class. ServletContextListener requires two methods (an init and a destroy) and an entry in web.xml configuring the listener.

If you like, you can persist data beyond the initialization by creating static variables in your class.

See details here: http://www.java-tips.org/java-ee-tips/java-servlet/how-to-work-with-servletcontextlistener.html

Here's a quick version

package org.peter;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletContextEvent;

public class ServletListener implements ServletContextListener {

    private ServletContext context = null;
    static ArrayList lf = null;

    /*This method is invoked when the Web Application has been removed
    and is no longer able to accept requests
     */
    public void contextDestroyed(ServletContextEvent event) {
        //Output a simple message to the server's console
        System.out.println("The web app has been removed");
        this.context = null;
    }

    //This method is invoked when the Web Application
    //is ready to service requests
    public void contextInitialized(ServletContextEvent event) {
        this.context = event.getServletContext();
        System.out.println("Web app started up");
        lf.add("foo");
    } // end of Initialized
}

And the entry in web.xml:

<listener>
        <listener-class>
          org.peter.ServletListener
        </listener-class>
    </listener>