Java Tutorial/Servlet/HttpSessionListener

Servlet HttpSessionListener

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* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements.  See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License.  You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

/**
 * Example listener for context-related application events, which were
 * introduced in the 2.3 version of the Servlet API.  This listener
 * merely documents the occurrence of such events in the application log
 * associated with our servlet context.
 *
 * @author Craig R. McClanahan
 * @version $Revision: 466607 $ $Date: 2006-10-21 17:09:50 -0600 (Sat, 21 Oct 2006) $
 */
public final class MyServlet
    implements ServletContextListener,
         HttpSessionAttributeListener, HttpSessionListener {

    // ----------------------------------------------------- Instance Variables

    /**
     * The servlet context with which we are associated.
     */
    private ServletContext context = null;

    // --------------------------------------------------------- Public Methods

    /**
     * Record the fact that a servlet context attribute was added.
     *
     * @param event The session attribute event
     */
    public void attributeAdded(HttpSessionBindingEvent event) {
  log("attributeAdded("" + event.getSession().getId() + "", "" +
      event.getName() + "", "" + event.getValue() + "")");
    }

    /**
     * Record the fact that a servlet context attribute was removed.
     *
     * @param event The session attribute event
     */
    public void attributeRemoved(HttpSessionBindingEvent event) {
  log("attributeRemoved("" + event.getSession().getId() + "", "" +
      event.getName() + "", "" + event.getValue() + "")");
    }

    /**
     * Record the fact that a servlet context attribute was replaced.
     *
     * @param event The session attribute event
     */
    public void attributeReplaced(HttpSessionBindingEvent event) {
  log("attributeReplaced("" + event.getSession().getId() + "", "" +
      event.getName() + "", "" + event.getValue() + "")");
    }

    /**
     * Record the fact that this web application has been destroyed.
     *
     * @param event The servlet context event
     */
    public void contextDestroyed(ServletContextEvent event) {
  log("contextDestroyed()");
  this.context = null;
    }

    /**
     * Record the fact that this web application has been initialized.
     *
     * @param event The servlet context event
     */
    public void contextInitialized(ServletContextEvent event) {
  this.context = event.getServletContext();
  log("contextInitialized()");
    }

    /**
     * Record the fact that a session has been created.
     *
     * @param event The session event
     */
    public void sessionCreated(HttpSessionEvent event) {
  log("sessionCreated("" + event.getSession().getId() + "")");
    }

    /**
     * Record the fact that a session has been destroyed.
     *
     * @param event The session event
     */
    public void sessionDestroyed(HttpSessionEvent event) {
  log("sessionDestroyed("" + event.getSession().getId() + "")");
    }

    // -------------------------------------------------------- Private Methods

    /**
     * Log a message to the servlet context application log.
     *
     * @param message Message to be logged
     */
    private void log(String message) {
  if (context != null)
      context.log("SessionListener: " + message);
  else
      System.out.println("SessionListener: " + message);
    }

    /**
     * Log a message and associated exception to the servlet context
     * application log.
     *
     * @param message Message to be logged
     * @param throwable Exception to be logged
     */
    private void log(String message, Throwable throwable) {
  if (context != null)
      context.log("SessionListener: " + message, throwable);
  else {
      System.out.println("SessionListener: " + message);
      throwable.printStackTrace(System.out);
  }
    }

}