ajax与json配合使用最简单实现

//ajax声明

var xmlHttp;
  function createXMLHttpRequest() {    

     if (window.ActiveXObject) {
         xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
     }
     else if (window.XMLHttpRequest) {
         xmlHttp = new XMLHttpRequest();
     }
}
//javascript调用执行back()方法启动ajax
  function back()
{ 
     var url="/websys/main/actionurl.do?paraname=123";
     createXMLHttpRequest();
     xmlHttp.open("GET", url);
     xmlHttp.onreadystatechange =callback;
     xmlHttp.send(null);
    
  }

//回掉函数
function callback(){
  if (xmlHttp.readyState == 4) {
       if (xmlHttp.status == 200) {
   var resstr=xmlHttp.responseText;

   var ress=eval("("+resstr+")");
           if(ress.msg==false){
                    alert(ress.value);

           }else{
            alert(ress.value);
           }
       }
    
}

//后台action

public ActionForward execute(.......)throws IOException

{

     String paravalue=rerquest.getParameter("paraname");

     PrintWriter out=response.getWriter();

    if(paravalue.equals("123"))

{

      out.print("{msg:'success',value:'oknow!'}");

      return null;

}

else

{

     out.print("{msg:error,value:'error!'}");}

     return null;

}

return null;
}