Servlet监视器

ServletContextListener：应用上下文生命周期监听器。用于监听Web应用的启动和销毁事件。

ServletContextAttributeListener ：应用上下文属性事件监听器。用于监听Web应用上下文中的属性改变的事件。

ServletRequestAttributeListener：请求属性事件监听器。用于监听请求中的属性改变的事件。

HttpSessionListener：会话生命周期监听器。用于监听会话的创建和销毁事件。

HttpSessionActivationListener：会话声明周期监听器。用于监听会话中的属性改变的事件。

HttpSessionAttributeListener：会话属性事件监视器。这是唯一不需要在web.xml中设定的Listener。

示例：显示用户登录，并实现踢人功能

用户：login.jsp --> loginServlet --> index.jsp

管理员： listUser.jsp -->kickUserServlet

监听器：SessionAttributeListener

Bean: User

User.java

package cn.class3g.domain;

public class User {
	private String username;
	private String password;
	
	public User(){
		
	} 
	public User(String _username, String _password){
		username=_username;
		password=_password;
	}
	
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
	
	
}

login.jsp

<form action="${pageContext.request.contextPath}/servlet/LoginServlet" method="post">
    	用户名:<input type="text" name="username" /> <br/>
    	密码:<input type="password" name="psssword" /><br/>
    	<input type="submit" value="登陆" />    	
    </form>

loginServlet.java

package cn.class3g.web.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import cn.class3g.domain.User;


public class LoginServlet extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		doPost(request,response);
	}

	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String username = request.getParameter("username");
		String password = request.getParameter("password");
		
		User user = new User(username,password);
		
		request.getSession().setAttribute("user",user);
		
		//用重定向，不用在转发
		response.sendRedirect(request.getContextPath()+"/index.jsp"); 		
	}

}

index.jsp

   欢迎你：${user.username }

SessionAttributeListener.java

package cn.class3g.web.listener;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

import cn.class3g.domain.User;


public class SessionAttributeListener implements HttpSessionAttributeListener {

	public void attributeAdded(HttpSessionBindingEvent arg0) {
		Object obj = arg0.getValue();
		if(obj instanceof User){
			HttpSession session = arg0.getSession();
			
			ServletContext context = session.getServletContext();
			Map map = (Map) context.getAttribute("map");
			
			if(map ==null){//第一个用户登陆，此时还没有创建Map集合，需要创建之且放入ServletContext对象中
				map = new HashMap();
				context.setAttribute("map", map);				
			}
			
			User user = (User) obj;
			
			//登陆用户的用户名作为key， 登陆用户的session对象作为value
			map.put(user.getUsername(),	session);			
		}
	}

	public void attributeRemoved(HttpSessionBindingEvent arg0) {
	}

	public void attributeReplaced(HttpSessionBindingEvent arg0) {
	}

}

listUser.jsp

<span style="font-size: 18px;"></span><p></p>
<pre class="html" name="code"><c:forEach var="me" items="${map }">
${me.key } 
<a href="${pageContext.request.contextPath }/servlet/KickUserServlet?username=${me.key}" >
<span style="white-space: pre;">	</span>踢死你
</a><br/><br/>


</c:forEach></pre>

KickUserServlet.java

package cn.class3g.web.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class KickUserServlet extends HttpServlet {


	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String username = request.getParameter("username");
		Map map = (Map) this.getServletContext().getAttribute("map");
		
		HttpSession session = (HttpSession) map.get(username);
		
		if(session !=null){ //开踢
			session.invalidate();
			map.remove(username);
		}
		
		request.getRequestDispatcher("/listUser.jsp").forward(request, response);
			
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		doGet(request,response);
	}

}

测试流程：

1. 模仿用户登录（多个）

2. 模仿管理员访问listUser.jsp ，踢人

3. 用户重新刷新index.jsp ,发现session已被销毁