简单ajax应用实例：ajax+jsp实现用户名验证(2)

//回调函数
function callback(){

   //判断对象的状态是否交互完成
   if(xmlhttp.readyState == 4){
  
      //判断http的交互是否成功
      if(xmlhttp.status == 200){
     
         //获取服务器端返回的数据（文本）
         var resbonseText = xmlhttp.responseText;
        
         //将数据显示在页面上
         var divNode = document.getElementById("result");
        
         //设置元素节点中的html内容
         divNode.innerHTML = responseText;
        
      }
     
   }
}

后台servlet程序代码：

public class AJAXServer extends HttpServlet{
    protected void doPost(httpServletRequest request, HttpServletResbonse resbonse)throws ServletException,IOException{
    doGet(request, resbonse);
    }
   
    protected void doGet(httpServletRequest request, HttpServletResbonse resbonse)throws ServletException,IOException{
    try{
        request.setCharacterEncoding("utf-8");
        resbonse.setContentType("text/html; charset=utf-8");
        PrintWriter out = resbonse.getWriter();
        String old = request.getParameter("name");
        if(old == null || old.length() == 0){
        out.println("用户名不能为空！");
        }else{
             String name = old;
             if(name.equals("zhangsan")){
             out.println("用户名[" + name + "]已经存在！");
             }else{
                 out.println("用户名[" + name + "]尚未存在，可以使用！");
             }
        }
    }catch(Exception e){
       e.printStackTrace();
    }
   
    }
}