//客户端请求
$(document).ready(function(){
    $("#c").click(function(){
        /*$.getJSON("http://localhost/test?jsoncallback=?", function(data){
            alert(data["a"]);
        });*/
        $.ajax({
            url:"http://localhost/test/index.jsp",
            type:"GET",
            cache:false,
            dataType:"jsonp",
            jsonp:"jsoncallback",
            success:function(data){
                alert(data['a']);
            }
        });
    });
});

//服务端响应
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<%String jsoncallback = request.getParameter("jsoncallback");
out.println(jsoncallback+"({a:'b'})");
//注意输出内容为：获取的jsoncallback值+括号，括号中为json字符串
%>