Java代码  

1. public class User {  
2.       
3.     private String username;  
4.     private String password;  
5.       
6.     public String getUsername() {  
7.         return username;  
8.     }  
9.     public void setUsername(String username) {  
10.         this.username = username;  
11.     }  
12.     public String getPassword() {  
13.         return password;  
14.     }  
15.     public void setPassword(String password) {  
16.         this.password = password;  
17.     }  
18.       
19. }  

普通JavaBean(以User为例)转成json格式 
1.转成JSONArray类型 
User user = new User(); 
user.setUsername("cxl"); 
user.setPassword("1234"); 
JSONArray json = JSONArray.fromObject(user); 
System.out.println(json);//[{"password":"1234","username":"cxl"}] 
response.getWriter().print(json.toString()); 
在js文件取数据 
$.getJSON("http://localhost:8080/jQueryDemo/servlet/UserServlet",null,function(data) { 
      alert(data[0].username); 
      alert(data[0].password); 
}); 
2.转成JSONObject类型 
JSONObject jsonObj = JSONObject.fromObject(user); 
System.out.println(jsonObj);//{"password":"1234","username":"cxl"} 
response.getWriter().print(jsonObj); 
在js文件取数据 
alert(data.username); 
alert(data.password); 

List转成json格式 
List<User> users = new ArrayList<User>(); 
User user = new User(); 
user.setUsername("cxl"); 
user.setPassword("1234"); 
User u = new User(); 
u.setUsername("lhl"); 
u.setPassword("1234"); 
users.add(user); 
users.add(u); 
1.转成JSONArray类型 
JSONArray json = JSONArray.fromObject(users); 
System.out.println(json.toString()); 
//[{"password":"1234","username":"cxl"},{"password":"1234","username":"lhl"}] 
response.getWriter().print(json.toString()); 
js中取数据：alert(alert(data[0].username)); 
2.转成JSONObject类型 
不可直接使用JSONObject jsonObj = JSONObject.fromObject(users); 
可通过下列方式 
JSONObject jsonObj = new JSONObject(); 
jsonObj.put("users", users); 
jsonObj.put("u", u); 
System.out.println(jsonObj); 
{"users":[{"password":"1234","username":"cxl"},{"password":"1234","username":"lhl"}], 
  "u":{"password":"1234","username":"lhl"}} 
response.getWriter().print(jsonObj); 
js中取数据：alert(data.users[0].username); 
           alert(data.u[0].username); 

Map转成json格式 
Map<String,Object> map = new HashMap<String,Object>(); 
map.put("users", users); 
map.put("u", u); 
1.转成JSONArray类型 
JSONArray json = JSONArray.fromObject(map); 
System.out.println(json.toString());// 
[{"users":[{"password":"1234","username":"cxl"},{"password":"1234","username":"lhl"}],"u":{"password":"1234","username":"lhl"}}] 
response.getWriter().print(json.toString); 
js中取数据：alert(data[0].users[0].username); 
2.转成JSONObject类型 
JSONObject json = JSONObject.fromObject(map); 
System.out.println(json);// 
{"user":[{"password":"1234","username":"cxl"},{"password":"1234","username":"lhl"}],"u":{"password":"1234","username":"lhl"}} 
response.getWriter().print(json); 
js中取数据：alert(data.user[0].username); 

JavaBean,List,Map转成json格式