1.java代码如下:

packagecom.eduask;
 importjava.io.IOException;
 importjavax.servlet.ServletException;
 importjavax.servlet.http.HttpServlet;
 importjavax.servlet.http.HttpServletRequest;
 importjavax.servlet.http.HttpServletResponse;
 publicclassDemo5extendsHttpServlet{
 @Override
 protectedvoidservice(HttpServletRequest req, HttpServletResponse resp)
 throwsServletException, IOException {
 resp.setContentType("text/xml;charset=utf-8");
 String name=req.getParameter("username1");
 resp.setCharacterEncoding("utf-8");
 String msg="";
 if("tom".equals(name)) {
 msg="{'msg':'failed'}";
 resp.getWriter().write(msg);
 }else{
 msg="{'msg':'success'}";
 resp.getWriter().write(msg);
 }
 }
 }
 2.json.jsp代码如下:
 <body> <!-- function Person(id,name,pwd){
 this.id=id;
 this.name=name;
 this.pwd=pwd;
 }
 var p=new Person(1234,"tom","1234");
 
 alert(p.id+"--"+p.name+"--"+p["pwd"]);
 
 var p1={
 'id':234,
 'name':"jack",
 'pwd':"123"
 
 };
 function Person(){
 this.name="tom";
 this.age=23;
 }
 alert(p1.id+"--"+p1.name+"--"+p1["pwd"]);
 -->
 username1:<input type="text"id="username1"name="username1">
 <span id="msg1"></span>
 <script type="text/javascript">
 //===================post=====================================================
  varpost=document.getElementById("username1");
 post.οnblur=function(){
 varxhr=getXhr();
 xhr.onreadystatechange=function(){
 if(xhr.readyState==4) {
 if(xhr.status==200) {
 //字符串格式的数据
  varmsg1=xhr.responseText;
 //转换为json对象
 msg1=eval（"("+msg1+")")
 varspan=document.getElementById("msg1");
 //通过对象.属性名称获取属性的值
 span.innerHTML="<font color=red>"+msg1.msg+"</font>";
 }
 }
 }
 xhr.open("post","/ajax/Demo5");
 //设置消息头，告诉浏览器携带请求体数据
 xhr.setRequestHeader("content-type","application/x-www-form-urlencoded");
 /* */varspan=document.getElementById("msg1");
 span.innerHTML="正在加载数据...";
 xhr.send("username1="+post.value);
 }
 functiongetXhr(){
 varxhr;
 if(window.XMLHttpRequest) {
 xhr=newXMLHttpRequest();
 }else{
 xhr=newActiveXObject("microsoft.xmlhttp");
 }
 returnxhr;
 }
 </script>
 </body>
 3.web.xml配置文件如下:
 <!-- demo5 -->
 <servlet>
 <servlet-name>Demo5</servlet-name>
 <servlet-class>com.eduask.Demo5</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>Demo5</servlet-name>
 <url-pattern>/Demo5</url-pattern>
 </servlet-mapping>