Ajax读取后台返回值respnseText为空,该怎么处理

Ajax读取后台返回值respnseText为空
大家好，最近在做一个项目，需要简单应用Ajax。前台用的是Jsp，javascript，后台用的是SSH。
我从前台通过Ajax向后台传值时，后台可以得到传输的值，但是当后台向前台写值时，前台在读取responseText时，总是读取为空。并且xmlHttp.status返回为0.   但是我后台明明是写了数据的。IE和Chrome都试过了，不行

前台代码  right.jsp中js代码

var xmlHttp=false;


function sendMessage()

{


  try

  {

    xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");

  }

  catch(e)

  {

      try

      {

        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); 

      }

      catch(E)

      {

        xmlHttp = false; 

      }

  }

  

  if (!xmlHttp && typeof XMLHttpRequest!='undefined'){

  xmlHttp = new XMLHttpRequest(); 

  }

  

  if(xmlHttp)

  {

     // do something

     var url="/OverTime/addOvertimeTemp.action";

     

     //data to post

     var workerids="";

     var isProductAbout=0;

     var chxs=document.getElementsByName("chkbox");  

     for(var i=0;i<chxs.length;i++){

         if(chxs[i].checked){

             workerids+=chxs[i].value;

             if(i<chxs.length-1)

             {

               workerids+=",";

             }

             

             if(chxs[i].title=="3")

             {

               isProductAbout=1;

             }

         }

     }

        

     var reason=document.getElementById("addOtptempVo.reason").value;

     //var workfanbu=0;

     var worktypeid=document.getElementById("addOtptempVo.worktypeid").value;

     var worklen=document.getElementById("addOtptempVo.worklen").value;

     var workshift=document.getElementById("addOtptempVo.worklen").value;

     

     if(worktypeid=="4")

     {

       worklen=0;

     }

     else

     {

        workshift=0;

     }

     

     var postinfo="";//construted by the values mentioned before.

     

     //为postinfo添加VO的各个属性键值对

     postinfo+="addOtptempVo.stfIds="+ workerids;

     postinfo+="&addOtptempVo.reason="+reason;

     postinfo+="&addOtptempVo.isProductabout="+isProductAbout;

     postinfo+="&addOtptempVo.worktypeid="+worktypeid;

     postinfo+="&addOtptempVo.worklen="+worklen;

     postinfo+="&addOtptempVo.workshift="+workshift;

     

     

     xmlHttp.open("POST",url,false);//原来是true，网上看到一种解决方案，改为false,仍然不行。

     xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

     xmlHttp.send(postinfo);

     

     //alert(workerids);

     alert("addOtptempVo.stfIds="+ workerids+" "+"&addOtptempVo.reason="+reason+" "+"&addOtptempVo.isProductabout="+isProductAbout+" "+"&addOtptempVo.worktypeid="+worktypeid+" "+"&addOtptempVo.worklen="+worklen+" "+"&addOtptempVo.workshift="+workshift);

     xmlHttp.onreadystatechange=getMessage;

  }

  else

  {

    alert("xmlHttp无法创建!");

  }


}


function getMessage()

{

  alert(xmlHttp.statusText);

  alert("getmessage:xmlHttp.readyState:"+xmlHttp.readyState+";xmlHttp.Status"+xmlHttp.status);

  if(xmlHttp.readyState==4)

  {

    if(xmlHttp.status==200||xmlHttp.status==0)