Servlet的代码：

public void doPost(HttpServletRequest req,HttpServletResponse resp){

req.setCharacterEncoding("utf-8");

resp.setContentType("text/html;charset=utf-8");

PrintWriter out = resp.getWriter();

String str = "[{pid:1,pname:'吉林省'},{pid:2,pname:'辽宁省'},{pid:3,pname:'山东省'}]";

out.print(str);

//疑惑：JSON的key是否一定要用双引号？

}

window.onload = function(){

document.getElementById("checkusername").onclick = function(){

var username = document.getElementById("username").value;

//alert(username);

//1

var xmlReq = ajaxFunction();

//2

xmlReq.onreadystatechange = function(){

if(xmlReq.readyState==4){

if(xmlReq.status==200||xmlReq.status==304){

var data = xmlReq.responseText;

//使用eval()方法把JSON字符串转成对象

var dataObj = eval("("+data+")");

for (var i=0;i<dataObj.length ;i++ ){

alert(dataObj[i].pid+"=="+dataObj[i].pname);

}

}

}

}

//3

xmlReq.open("post","../registerServlet",true);

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

//4

xmlReq.send("username="+username);

}

}

例子：

Servlet的代码：

public void doPost(HttpServletRequest req,HttpServletResponse resp){

req.setCharacterEncoding("utf-8");

resp.setContentType("text/html;charset=utf-8");

PrintWriter out = resp.getWriter();

List<Province> list = new ArrayList<Province>();

Province p1 = new Province(1,"吉林省");

Province p2 = new Province(2,"辽宁省");

Province p3 = new Province(3,"山东省");

list.add(p1);

list.add(p2);

list.add(p3);

JsonConfig config = new JsonConfig();

//过滤list集合中存放的Province这个对象的pid属性不生成json

config.setExcludes(new String[]{"pid"});//把pid屏蔽掉

JSONArray jsonArray = JSONArray.fromObject(list,config);

out.print(jsonArray.toString());

/************以上是list，下面的是一个对象**********************/

Province p = new Province(1,"吉林省");

JsonConfig config1 = new JsonConfig();

config.setExcludes(new String[]{"pid"});

JSONObject jsonObject = JSONObject.fromObject(p,config1);

System.out.println(jsonObject.toString());

}