%@ page language="java" contentType="text/html; charset=UTF-8"
  pageEncoding="UTF-8"%>
!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
html>
head>
meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
title>登錄/title>
/head>
body>
  用戶名：input type="text" name="username" id="username">br>
  密碼:input type="password" name="password" id="password">
  a href="javascript:chkUser();">登錄/a>
  div id="res">/div>
/body>

script type="text/javascript">
  var xhr;
  /**
  * 創建XMLHttpRequest對象
  */
 function createXMLHttpRequest(){
  //1、創建XMLHttpRequest對象
    if(window.XMLHttpRequest){
      //非IE內核瀏覽器
     xhr=new XMLHttpRequest(); 
    }else{
      //IE瀏覽器
      try{
        xhr=new ActiveXObject("Msxml2.XMLHTTP"); 
      }catch(e){
       //IE低版本
        xhr=new ActiveXObject("Microsoft.XMLHTTP"); 
      }
    }
 }
  
  /**
  * 發送請求，用來檢查用戶名、密碼是否正確
  */
  function chkUser(){
    
    //1、創建XMLHttpRequest
    createXMLHttpRequest();
    
    //2、獲得用戶名、密碼
    var username=document.getElementById("username").value;
    var password=document.getElementById("password").value;
    
    //3、與服務器建立連接：open
    var url="login?username="+username+"password="+password;
    //方式1：get提交
    //xhr.open("GET",url,true);
    
    //方式2：post提交
    var url2="login";
    xhr.open("POST",url2,true);
    
    
    //4、設置回調函數，獲得服務器響應的數據
    xhr.onreadystatechange=function(){
      /*
      readyState狀態碼：
      0：未初始化
      1：正在加載
      2：加載完成
      3：請求進行中
      4：請求完成
      */
      if(xhr.readyState==4){
        //status,200表示響應正常
        if(xhr.status==200){
         //alert("從服務器返回的值為："+xhr.responseText);
         var res=xhr.responseText;
         if(res=='0'){
           document.getElementById("res").innerHTML="登錄成功";
         }else{
           document.getElementById("res").innerHTML="font color='red'>登錄失敗/font>";
         }
         
        }else{
          alert("出現在了異常:"+xhr.response.Text);
        }
        
      }
    }
    //5、發送請求
    //方式1：get方式
    //xhr.send(null);
    
    //方式2:post方式：
    //Post提交需要設置http請求頭
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xhr.send("username="+username+"password="+password);
    
    
  }
 
/script>

/html> 

package com.newer.login.web;

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 com.newer.login.bean.User;
import com.newer.login.service.UserService;

/**
 * Servlet implementation class LoginServlet
 */
public class LoginServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;

  UserService userService = new UserService();

  /**
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
   *   response)
   */
  protected void doGet(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException {
    doPost(request,response);
  
  }

  /**
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
   *   response)
   */
  protected void doPost(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException {
    // 1、獲得頁面參數
    String username = request.getParameter("username");
    String password = request.getParameter("password");

    System.out.println("獲得請求的參數username:"+username);
    System.out.println("獲得請求的參數password:"+password);    
    // 2、封裝User對象
    User user = new User();
    user.setUsername(username);
    user.setPassword(password);

    // 3、調用服務類，完成用戶名、密碼的校驗
    User u = userService.login(user);

    /*
     * 傳統方式 if(u!=null){ //表示登錄成功 request.setAttribute("user", user);
     * //跳轉至首頁... }else{ //登錄失敗，跳轉登錄頁面
     * 
     * }
     */
    // ajax響應

    PrintWriter out = response.getWriter();

    if (u != null) {
      //0成功，1失敗
      out.print(0);
    }else{
      out.print(1);
    }
    out.close();

  }

}