/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed 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.
 */

package org.springframework.web.util;

import java.io.Serializable;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

Servlet HttpSessionListener that automatically exposes the session mutex when an HttpSession gets created. To be registered as a listener in web.xml. 
The session mutex is guaranteed to be the same object during the entire lifetime of the session, available under the key defined by the SESSION_MUTEX_ATTRIBUTE constant. It serves as a safe reference to synchronize on for locking on the current session. 
In many cases, the HttpSession reference itself is a safe mutex
as well, since it will always be the same object reference for the
same active logical session. However, this is not guaranteed across
different servlet containers; the only 100% safe way is a session mutex.
Author:
Juergen Hoeller
See Also:
WebUtils.SESSION_MUTEX_ATTRIBUTE
WebUtils.getSessionMutex(HttpSession)
setSynchronizeOnSession.setSynchronizeOnSession
Since:
1.2.7/**
 * Servlet HttpSessionListener that automatically exposes the session mutex
 * when an HttpSession gets created. To be registered as a listener in
 * {@code web.xml}.
 *
 * <p>The session mutex is guaranteed to be the same object during
 * the entire lifetime of the session, available under the key defined
 * by the {@code SESSION_MUTEX_ATTRIBUTE} constant. It serves as a
 * safe reference to synchronize on for locking on the current session.
 *
 * <p>In many cases, the HttpSession reference itself is a safe mutex
 * as well, since it will always be the same object reference for the
 * same active logical session. However, this is not guaranteed across
 * different servlet containers; the only 100% safe way is a session mutex.
 *
 * @author Juergen Hoeller
 * @since 1.2.7
 * @see WebUtils#SESSION_MUTEX_ATTRIBUTE
 * @see WebUtils#getSessionMutex(javax.servlet.http.HttpSession)
 * @see org.springframework.web.servlet.mvc.AbstractController#setSynchronizeOnSession
 */
public class HttpSessionMutexListener implements HttpSessionListener {

	@Override
	public void sessionCreated(HttpSessionEvent event) {
		event.getSession().setAttribute(WebUtils.SESSION_MUTEX_ATTRIBUTE, new Mutex());
	}

	@Override
	public void sessionDestroyed(HttpSessionEvent event) {
		event.getSession().removeAttribute(WebUtils.SESSION_MUTEX_ATTRIBUTE);
	}


	
The mutex to be registered.
Doesn't need to be anything but a plain Object to synchronize on.
Should be serializable to allow for HttpSession persistence.
/**
	 * The mutex to be registered.
	 * Doesn't need to be anything but a plain Object to synchronize on.
	 * Should be serializable to allow for HttpSession persistence.
	 */
	@SuppressWarnings("serial")
	private static class Mutex implements Serializable {
	}

}