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valuemap.put(user.getUsername(),session);}}public void attributeRemoved(HttpSessionBindingEvent arg0) {}public void attributeReplaced(HttpSessionBindingEvent arg0) {}}

listUser.jsp

<c:forEach var="me" items="${map }"> ${me.key }   <a href="${pageContext.request.contextPath }/servlet/KickUserServlet?username=${me.key}" > 踢死你 </a><br/><br/></c:forEach> 

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