AJAX无法定时刷新（解决立即结贴)

jsp文件中有如下两个javascript函数：   function getXHR(){
      try {
        xhr = new XMLHttpRequest();
      } catch (trymicrosoft) {
        try {
          xhr = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (othermicrosoft) {
          try {
            xhr = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (failed) {
            xhr = false;
          }
        }
      }
      return xhr;
    }
    function requestData(){
        try{
          var x=getXHR();
          if(x){
            //alert("send data!");
            x.onreadyStatechange=processMessage;
            x.open("GET","messageProcessAction.do");
            x.send(null);
            window.setTimeout("requestData()",3000);
          }
        }catch(e){
          alert("error3");
        }
      }
我的意图是让浏览器每三秒向服务器发送一次请求，但测试的结果发现，服务器只收到了一次请求（浏览器也的确每隔三秒做了一些事情，但服务器却只收到一次请求）。请问这是为什么，如何实现每三秒向服务器发送一次请求？

解决方案 »

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setTimeout用window.setInterval代替。
//alert("send data!");把注释去了看看是什么效果啊
window.setTimeout("requestData()",3000);或者把里边的括号也去了试试，我看不出问题在哪里
还是不行，现在把完整的代码帖给大家：test.jsp:<%@taglib uri="/WEB-INF/struts-html.tld" prefix="html"%>
<%@page contentType="text/html; charset=gb2312"%>
<html:html>
  <head>
  <title>AJAX测试</title>
<script type="text/javascript">
    var xhr = false;
    //var online=true;
    //var count=0;    function getXHR(){
      var x;
      try {
        x = new XMLHttpRequest();
      } catch (trymicrosoft) {
        try {
          x = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (othermicrosoft) {
          try {
            x = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (failed) {
            x = false;
          }
        }
      }
      return x;
    }    function init(){
        window.setInterval("requestData()",3000);
    }
      function requestData(){
        try{
          xhr=getXHR();
          if(xhr){
            //alert("send data!");
            xhr.onreadyStatechange=processMessage;
            xhr.open("GET","messageProcessAction.do");
            xhr.send(null);

          }
        }catch(e){
          alert("error3");
        }
      }      function processMessage(){
        if(xhr.readyState==4){
          if(xhr.status==200){
            var message=xhr.responseText;
            var msgArea=document.getElementById("msgArea");
            msgArea.innerHTML=message+msgArea.innerHTML;
            //message="";
          }
        }
      }  </script>
  </head>
  <body onload="init()">
  <textarea cols="20" rows="10" id="msgArea">  </textarea>
  <br />as;dlkfja;lskdjf;eptoiyg;'l\f[gplhd\f;ghl'dsflk
  <br />lkjoiertwkj;sadfkjg';lkhdfghfdjffkjgfdgjsd
  </body>
</html:html>
服务器端有一struts的Action：import org.apache.struts.action.ActionMapping;
import org.apache.struts.action.ActionForm;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.struts.action.ActionForward;
import imtest.form.MessageForm;
import org.apache.struts.action.Action;import java.io.*;public class MessageProcessAction extends Action {
    public ActionForward execute(ActionMapping mapping, ActionForm form,
                                 HttpServletRequest request,
                                 HttpServletResponse response) {
        MessageForm messageForm = (MessageForm) form;
        int counter=1;
        System.out.println("有连接！");        return mapping.findForward("success");
    }
}mapping.findForward("success"); 转发给另一个jsp页面 reply.jsp：<%!
  int counter=0;
%>
<%
  out.println(counter++);
%>现在的问题是：打开浏览器，访问test.jsp ，服务端只输出一次“有链接！”，浏览器的textArea 中却不断的有数字填充（如：0 0 0 0...），在此打开浏览器，访问test.jsp ，服务端又输出一次“有链接！”，textArea 中却不断的有数字填充（这次变成：0 0 0 0...）。百思不得其解，浏览器应该是不断的发送请求啊！！！！！！
重新访问test.jsp，textArea中的数字会＋1
import org.apache.struts.action.ActionMapping;
import org.apache.struts.action.ActionForm;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.struts.action.ActionForward;
import imtest.form.MessageForm;
import org.apache.struts.action.Action;import java.io.*;public class MessageProcessAction extends Action {
    public ActionForward execute(ActionMapping mapping, ActionForm form,
                                 HttpServletRequest request,
                                 HttpServletResponse response) {
        int counter=1;
        System.out.println("有连接！");        return null;
    }
}
返回空依然输出一次“有连接！”，这样的话，浏览器中textarea也就不会有任何数据显示了，期待哥们把环境搭起来！
function processMessage(){
        if(xhr.readyState==4){
           alert(xhr.status);//试试它返回的是多少啊
          if(xhr.status==200){
            var message=xhr.responseText;
            var msgArea=document.getElementById("msgArea");
            msgArea.innerHTML=message+msgArea.innerHTML;
            //message="";
          }
        }
      }
<%@taglib uri="/WEB-INF/struts-html.tld" prefix="html"%>
<%@page contentType="text/html; charset=gb2312"%>
<html:html>
  <head>
  <title>AJAX测试</title>
<script type="text/javascript">
    var xhr = false;
    //var online=true;
    //var count=0;    function getXHR(){
      var x;
      try {
        x = new XMLHttpRequest();
      } catch (trymicrosoft) {
        try {
          x = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (othermicrosoft) {
          try {
            x = new ActiveXObject("Microsoft.XMLHTTP");
          } catch (failed) {
            x = false;
          }
        }
      }
      return x;
    }    function init(){
        window.setInterval("requestData()",3000);
    }
    var jj=1;
      function requestData(){
        try{
          xhr=new ActiveXObject("Msxml2.XMLHTTP");
          if(xhr){
            //alert("send data!");
            xhr.onreadyStatechange=processMessage;
            xhr.open("GET","messageProcessAction.do");
            jj++;
            xhr.send(null);
          }
        }catch(e){
          alert("error3");
        }
      }      function processMessage(){
        if(xhr.readyState==4){
          if(xhr.status==200){
            var message=xhr.responseText;
            var msgArea=document.getElementById("msgArea");
            msgArea.innerHTML=message+msgArea.innerHTML;
            //message="";
          }
        }
      }  </script>
  </head>
  <body >
  <textarea cols="20" rows="10" id="msgArea">  </textarea>
  <%
  System.out.println("访问JSP了");
  %>
  <br />
  <br />
  <script>
   init();
  </script>
  </body>
</html:html>package com.awu.struts.action;
import org.apache.struts.action.ActionMapping;
import org.apache.struts.action.ActionForm;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.struts.action.ActionForward;
import org.apache.struts.action.Action;
import java.io.*;public class MessageProcessAction extends Action {
    public ActionForward execute(ActionMapping mapping, ActionForm form,
                                 HttpServletRequest request,
                                 HttpServletResponse response) {
     System.out.println("访问Action了");
        int counter=1;
        PrintWriter out=null;
try {
out = response.getWriter();
out.flush();
out.println("fff");
//out.println(Integer.parseInt(request.getParameter("id"))+100+"");
out.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
        System.out.println("有连接！");
        return null;
    }
}
我的现在是这样子的,不停的打印中
浏览器端的确在不停的打印（即使关闭掉服务器），但我的意图是让服务器也不停的输出：访问Action了
有连接！现在只输出一次，迷惑！！
真的假的！我用的JB开发环境，tomcat5.5 作的服务器！
崩溃了，拉倒！结贴。谢谢 AWUSOFT 的大力支持！