初始Servlet監聽器

Servlet監聽器簡介
Servlet監聽器的作用是監聽Web容器的有效事件，由容器管理。利用Listener接口監聽在容器中的某個執行程序，並更具應用程序的需求做出適當的響應。

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監聽Servlet上下文
Servlet上下文監聽可以監聽ServletContext對象的創建、刪除和添加屬性，以及刪除和修改操作，該監聽器需要用到下面兩個接口。

ServletContextListener接口：監聽ServletContext的創建和刪除。

class myServletContextListener implements ServletContextListener{

    //通知正在收聽的對象應用程序已經被加載及初始化
    @Override
    public void contextInitialized(ServletContextEvent servletContextEvent) {

    }

    //通知正在收聽的對象應用程序已經被載出，即關閉
    @Override
    public void contextDestroyed(ServletContextEvent servletContextEvent) {

    }
}

ServletAttributeListener接口：監聽ServletContext屬性的修改、刪除和增加

class myServletAttributeListener implements ServletContextAttributeListener{
    //若有對象加入Application范圍，通知正在收聽的對象
    @Override
    public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {

    }
    //若有對象從Application范圍移除，通知正在收聽的對象
    @Override
    public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {

    }
    //若有對象在Application范圍內的一個對象被另一個對象替換，通知正在收聽的對象
    @Override
    public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {

    }
}

class myServletAttributeListener2 implements ServletRequestAttributeListener{

    @Override
    public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }
}

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監聽HTTP會話

HttpSessionListener接口：監聽HTTP會話的創建及銷毀

class myHttpSessionListener implements HttpSessionListener{

    //通知正在收聽的對象，session已經被加載及初始化
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {

    }
    //通知正在收聽的對象，session被載出
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {

    }
}

HttpSessionActivationListener接口：監聽HTTP會話active和passivate情況

class myHttpSessionActivationListener implements HttpSessionActivationListener{
    //通知正在收聽的對象，其session狀態變為有效狀態
    @Override
    public void sessionWillPassivate(HttpSessionEvent httpSessionEvent) {

    }
    //通知正在收聽的對象，其session狀態變為無效狀態
    @Override
    public void sessionDidActivate(HttpSessionEvent httpSessionEvent) {

    }
}

HttpSessionBindingListener接口：監聽HTTP會話中對象的綁定信息，它是唯一不需要再web.xml中設置Listener的。

class myHttpBindingListener implements HttpSessionBindingListener{
    //當有對象加入session的范圍時，會被自動調用
    @Override
    public void valueBound(HttpSessionBindingEvent httpSessionBindingEvent) {

    }
    //當有對象從session的范圍移除時，會被自動取消調用
    @Override
    public void valueUnbound(HttpSessionBindingEvent httpSessionBindingEvent) {

    }
}

HttpSessionAttributeListener接口：監聽HTTP會話中屬性的設置請求。

class myHttpSessionAttributeListener implements HttpSessionAttributeListener {

    //若有對象加入Session范圍，通知正在收聽的對象
    @Override
    public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {

    }

    //若有對象從Session范圍移除，通知正在收聽的對象
    @Override
    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {

    }

    //若有對象在Session范圍內的一個對象被另一個對象替換，通知正在收聽的對象
    @Override
    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {

    }
}

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監聽Servlet請求

ServletRequestListener接口：

class myServletRequestListener implements ServletRequestListener{

    //通知正在收聽的對象，ServletRequest已經被加載及初始化
    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {

    }
    //通知正在收聽的對象，ServletRequest已經被載出，即關閉
    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {

    }
}

ServletRequestAttributeListener接口：

class myServletRequestAttributeListener implements ServletRequestAttributeListener{
    //若有對象加入Request范圍，通知正在收聽的對象
    @Override
    public void attributeAdded(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }
    //若有對象從Request范圍移除，通知正在收聽的對象
    @Override
    public void attributeRemoved(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }
    //若有對象在Request范圍內的一個對象被另一個對象替換，通知正在收聽的對象
    @Override
    public void attributeReplaced(ServletRequestAttributeEvent servletRequestAttributeEvent) {

    }
}

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下面使用一個小例子：使用監聽器查看在線用戶
UserInfoList類：

package com.java.model;

import java.util.ArrayList;

/**
 * Created with IntelliJ IDEA.
 * Created by YEN on 2016/7/10 and 22:05.
 */
public class UserInfoList {
    private static UserInfoList userInfoList=new UserInfoList();
    private ArrayList arrayList=null;
    public UserInfoList(){
        this.arrayList=new ArrayList();
    }
    public static UserInfoList getInstance(){
        return userInfoList;
    }
    //增加用戶
    public boolean addUserInfo(String username){
        if(username!=null){
            this.arrayList.add(username);
            return true;
        }else return false;
    }
    //獲取用戶列表
    public ArrayList getUserList(){
        return arrayList;
    }
    //移除用戶
    public void removeUserInfo(String username){
        if(username!=null){
            arrayList.remove(username);
        }
    }
}

UserInfoTrace類：

package com.java.model;

import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

/**
 * Created with IntelliJ IDEA.
 * Created by YEN on 2016/7/10 and 22:12.
 */

public class UserInfoTrace implements HttpSessionBindingListener{
    private String username;
    private UserInfoList container=UserInfoList.getInstance();
    public UserInfoTrace(){
        this.username="";
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    @Override
    public void valueBound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("上線................"+this.username);
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("下線................"+this.username);
        if(username!=""){
            container.removeUserInfo(username);
        }
    }
}

login.jsp頁面

<%--
  Created by IntelliJ IDEA.
  User: YEN
  Date: 2016/7/9
  Time: 16:29
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
    <script>
        function checkForm() {
            if(document.getElementById("username").value==null || document.getElementById("username").value==""){
                  alert("用戶名不能為空！");
                  return false;
              }
            if(document.getElementById("password").value==null || document.getElementById("password").value==""){
                alert("密碼不能為空！");
                return false;
            }
            return true;
        }
    </script>
</head>
<body>
<form name="form1" action="<%=request.getContextPath()%>/showUserInfo.jsp" method="get" onsubmit="return checkForm()">
    username:<input type="text" name="username" id="username">   <br>
    password:<input type="password" name="password" id="password">   <br>
    <input type="submit" value="提交">
</form>
</body>
</html>

showUserInfo.jsp頁面

<%--
  Created by IntelliJ IDEA.
  User: YEN
  Date: 2016/7/10
  Time: 22:16
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@page import="com.java.model.UserInfoList" language="java" %>
<%@page import="com.java.model.UserInfoTrace" language="java" %>
<%@ page import="java.util.ArrayList" %>
<html>
<head>
    <title>UserInfoPage</title>
</head>
<body>
<%
    UserInfoList userInfoList=UserInfoList.getInstance();  //獲得UserInfoList類的對象
    UserInfoTrace userInfoTrace=new UserInfoTrace();       //創建UserInfoTrace類的對象
    request.setCharacterEncoding("UTF-8");
    String username=request.getParameter("username");
    userInfoTrace.setUsername(username);
    session.setAttribute("list",userInfoTrace);
    userInfoList.addUserInfo(username);
    session.setMaxInactiveInterval(30);                   //session過期時間為30s
%>
<textarea rows="8" cols="20">
    <%
        ArrayList arrayList=userInfoList.getUserList();
        if(arrayList!=null && arrayList.size()>0 ){
            for (int i = 0; i < arrayList.size(); i++) {
                out.print(arrayList.get(i));
            }
        }
    %>
</textarea>
</body>
</html>

運行結果：

一：在login.jsp頁面輸入信息

二：點擊登陸按鈕

三：在登陸一個用戶