实现功能：在输入框中输入字符，用AJAX传到后台Servlet处理后加上随机数，并返回到前台显示。

一、写前台jsp页面index.jsp

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
 <head>
  <title>My JSP 'index.jsp' starting page</title>
  <script type="text/javascript">
  /*
    ajax 的几个步骤：
    1、建立XmlHttpRequest对象
    2、设置回调函数
    3、使用Open方法建立与服务器的连接
    4、向服务器发送数据
    5、在回调函数中针对不同响应状态进行处理
  */
    var xmlHttp;
    function createXMLHttpRequest(){  //1建立XmlHttpRequest对象
      if(window.ActiveXObject){
        try{
          xmlHttp = new ActiveXObject("Microsoft.XMLHttp");
        }catch(e){
          alert("Error!!!");
        }
      }else{
        xmlHttp = new XMLHttpRequest();
      }
    }
    function showMes(){   //2设置回调函数
      if(xmlHttp.readyState==4){ //数据接收完成并可以使用
        if(xmlHttp.status==200){ //http状态OK
        //5、在回调函数中针对不同响应状态进行处理
          document.getElementById("sp").innerHTML = xmlHttp.responseText; //服务器的响应内容
        }else{
          alert("出错："+xmlHttp.statusText); //HTTP状态码对应的文本
        }
      }
    }
    /**
    //这是GET方法传送
    function getMes(){
      createXMLHttpRequest();
      var txt = document.getElementById("txt").value;
      var url="servlet/AjaxServlet?txt="+txt;
      url = encodeURI(url); //转换码后再传输
      xmlHttp.open("GET",url,true); //3使用Open方法建立与服务器的连接
      xmlHttp.onreadystatechange=showMes;
      xmlHttp.send(null); //4向服务器发送数据
    }
    */
    /**
    *这是post方法
    */
    function postMes(){
      createXMLHttpRequest();
      var txt = document.getElementById("txt").value;
      var url = "servlet/AjaxServlet";
      var params = "username="+txt;
    // alert(params);
      xmlHttp.open("POST",url,true);
      xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");
      xmlHttp.send(params);
      xmlHttp.onreadystatechange = showMes;
    }
  </script>
 </head>
 <body>
  <input type="text" id="txt"/>
  <input type="button" value="query" onclick="postMes()" />
  <span id="sp"></span>
 </body>
</html>

 

二、写后台Servlet加random随机数，关键代码如下：

 

public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    request.setCharacterEncoding("utf-8"); //用utf-8转换获得传输过来的码
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String txt = request.getParameter("txt");
//   String tx = new String(txt.getBytes("iso-8859"),"utf-8");
    out.print("txt="+txt+Math.random());
    out.flush();
    out.close();
}
/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    String username = request.getParameter("username");
//   String txt = new String(username.getBytes("ISO-8859-1"),"UTF-8");
    String txt = new String(username);
    out.print("txt="+txt+":"+Math.random());
    out.flush();
    out.close();
}