servlet监听完成统计在线人数，显示在线人员列表(附源码)

ServletContext事件监听器---->针对applicationScope

 ServletContextListener(*)

对整个Web应用的装载和卸载进行监听。

 ServletContextAttributeListener

对ServletContext中的信息存放、删除和替换进行监听。

ServletContext就是Servlet上下文监听，在web中表示的是对启动服务和销毁服务进行监听，需要实现的接口：

ServletContextListener接口，实现的就是对上下午进行监听：

void contextInitialized(ServletContextEvent sce)：启动上下文时的监听

void contextDestroyed(ServletContextEvent sce)：销毁上下文时进行的监听

除了对上下文的启动和销毁进行监听的之外，还可以对上下文的属性进行监听：ServletContextAttributeListener接口。

void attributeAdded(ServletContextAttributeEvent event)：设置上下文属性监听

void attributeRemoved(ServletContextAttributeEvent event)：移除上下文属性的监听

void attributeReplaced(ServletContextAttributeEvent event)：修改上下文属性的监听

ServletContextAttributeEvent：事件，可以通过事件取得属性的内容和名称。

·取得属性名称：public java.lang.String getName()

·取得属性的值：public java.lang.Object getValue()

效果如下图：

当登录一个账号时

打开另一个浏览器，再登录一个账号

如上图，我们可以看到，程序已经完成了统计在线人数和显示人员列表的功能，那么他的实现流程是什么呢？

我们可以通过ServletContextListener完成在线人数的统计和显示在线人数列表，首先listener和filter一样要在web.xml中进行描述。

代码如下：

<listener>  
  		<listener-class>net.jvsun.ListenerTest</listener-class>  
	</listener>

为了测试这个程序，我们也必须完成用户登录功能。

public class JDBCHelper {
	public static final String DRIVER = "oracle.jdbc.driver.OracleDriver";
	public static final String URL = "jdbc:oracle:thin:@localhost:1521:xxx";
	public static final String DBNAME = "scott";
	public static final String PASSWORD = "xxx";
	public static Connection getConn() throws Exception{
		Class.forName(DRIVER);
		Connection conn = DriverManager.getConnection(URL, DBNAME, PASSWORD);
		return conn;
	}
}

public class UserPOJO implements Serializable{
	private static final long serialVersionUID = 7554548269035753256L;
	private int id;
	private String username;
	private String password;
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	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;
	}
	public UserPOJO(int id, String username, String password) {
		super();
		this.id = id;
		this.username = username;
		this.password = password;
	}
	public UserPOJO(String username, String password) {
		this.username = username;
		this.password = password;
	}
	public UserPOJO() {
		super();
		// TODO Auto-generated constructor stub
	}
	
}

public class UserDAO {
	public UserPOJO login(String username, String password) {
		UserPOJO user=null;
		Connection conn = null;
		PreparedStatement pstate = null;
		ResultSet res = null;
		try {
			conn=JDBCHelper.getConn();
			String sql="select id,username from userinfo where username=? and password=?";
			pstate = conn.prepareStatement(sql);
			pstate.setString(1, username);
			pstate.setString(2, password);
			res = pstate.executeQuery();
			while(res.next()){
				user=new UserPOJO(res.getInt(1),username,null);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}finally{
			try {
				res.close();
				pstate.close();
				conn.close();
			} catch (SQLException e) {
				e.printStackTrace();
			}
			
		}
		return user;
	}
}

public class UserServlet extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String path = request.getContextPath();
		response.setContentType("text/html; charset=utf-8");
		request.setCharacterEncoding("utf-8");
		String username = request.getParameter("username");
		String password = request.getParameter("password");
		ServletContext application = this.getServletContext();//取得application对象
		List<String> list = (List<String>) application.getAttribute("allUser");
		if(list.indexOf(username) == -1){
			UserPOJO pojo = new UserDAO().login(username, password);
			if(null != pojo){
				HttpSession session = request.getSession(true);//取得session对象
				session.setAttribute("userName", username);
			}
			
		}
		response.sendRedirect(path+"/index.jsp");
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doGet(req, resp);
	}

}

public class ListenerTest implements ServletContextListener,HttpSessionAttributeListener,HttpSessionListener{
	ServletContext application = null;
	public void contextDestroyed(ServletContextEvent event) {
		System.out.println("服务器关闭");
	}

	public void contextInitialized(ServletContextEvent event) {
		List<String> list = new ArrayList<String>();
		//用来保存所有已登录的用户
		application =  event.getServletContext();
		//取得application对象
		application.setAttribute("allUser", list);
		//将集合设置到application范围属性中去
		
	}

	public void attributeAdded(HttpSessionBindingEvent se) {
		List<String> list = (List<String>)application.getAttribute("allUser");
		//假设：用户登陆成功之后，只将户名设置到session中
		String userName = (String)se.getValue();
		//取得用户名
		if(list.indexOf(userName) == -1){
			//表示此用户之前没有登陆
			list.add(userName);
			application.setAttribute("allUser", list);
		}
	}

	public void attributeRemoved(HttpSessionBindingEvent se) {
		List<String> list = (List<String>)application.getAttribute("allUser");
		list.remove((String)se.getValue());
		application.setAttribute("allUser", list);
	}

	public void attributeReplaced(HttpSessionBindingEvent se) {
		
	}

	public void sessionCreated(HttpSessionEvent event) {
		
	}

	public void sessionDestroyed(HttpSessionEvent event) {
		
	}
}

<%@page contentType="text/html; charset=utf-8"%>
<%@page import="java.util.*" %>
<%
	String path  = request.getContextPath();
 %>
 
<html>
	<body>
	
		<form action="<%=path %>/Login" method="post">
			账号：
			<input type="text" name="username" />
			<br />
			密码：
			<input type="password" name="password" />
			<br />
			<input type="submit" value="提交" />
		</form>

	</body>
	
</html>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>"> 
    <title></title>
  </head>
  
  <body>
	<ul>
	<%
		System.out.println(application.getAttribute("allUser"));
		if(null != application.getAttribute("allUser")){
			List<String> list = (List<String>)application.getAttribute("allUser");
			%>
			<h2>在线人数:<%=list.size() %></h2>
			<%
			for(String s:list){
			%>
			<a>姓名：</a><%=s %><a>---->此时在线</a><br>
			
			<%
			}
		}
	 %>
	 </ul>
	<hr/>
	<a href="<%=path %>/logout.jsp">注销</a>
  </body>
</html>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    <title>logout</title>
  </head>
  <body>
    <%
    	session.removeAttribute("username");
    	session.invalidate();
    	response.sendRedirect("login.jsp");
    %>
  </body>
</html>