1.jsonlib需要引入的包也多，代码也相对多一些。

2.Gson：谷歌的

3.FastJson：阿里巴巴的，这几个也是性能最好的一个。(推荐)
jsonlib类

Jsonlib：package json
import java.util.ArrayList;
import java.util.List;
import net.sf.json.JSONArray;
import user.User;
 
public class Jsonlib {
    public static void main(String[] args) {
        User user1 = new User();
        user1.setUsername("111");
        user1.setPassword("111");
 
        User user2 = new User();
        user2.setUsername("222");
        user2.setPassword("222");
        List<User> lists = new ArrayList<User>();
 
        lists.add(user1);
        lists.add(user2);
 
        JSONArray fromObject1 = JSONArray.fromObject(user1);
        JSONArray fromObject2 = JSONArray.fromObject(lists);
 
        String str1 = fromObject1.toString();
        String str2 = fromObject2.toString();
        System.out.println(str1);
        //打印结果
        //[{"password":"111","username":"111"}]
        System.out.println(str2);
        //打印结果
        //[{"password":"111","username":"111"},{"password":"222","username":"222"}]
    }
}

jar包

Gson：

package json;
import java.util.ArrayList;
import java.util.List;
import com.google.gson.Gson;
import user.User;
public class Gson_a {
 
    public static void main(String[] args) {
 
        User user1 = new User();
        user1.setUsername("111");
        user1.setPassword("111");
 
        User user2 = new User();
        user2.setUsername("222");
        user2.setPassword("222");
        List<User> lists = new ArrayList<User>();
        lists.add(user1);
        lists.add(user2);
 
    
        Gson gson = new Gson();
        String json1 = gson.toJson(user1);
        System.out.println(json1);
        //打印结果
        //{"username":"111","password":"111"}
        String json2 = gson.toJson(lists);
        System.out.println(json2);
        //打印结果
        //[{"username":"111","password":"111"},{"username":"222","password":"222"}]        
    }
}

推荐使用FastJson：
pom依赖

<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson -->
<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.68</version>
</dependency>

FastJsonTest

import java.util.ArrayList;
import java.util.List;
import com.alibaba.fastjson.JSON;
import user.User;
public class FastJson {
    public static void main(String[] args) {
        User user1 = new User();
        user1.setUsername("111");
        user1.setPassword("111");
 
        //1、单个对象转换成json
        String jsonString = JSON.toJSONString(user1);
        System.out.println(jsonString);
        //打印结果
        //{"password":"111","username":"111"}
 
        User user2 = new User();
        user2.setUsername("222");
        user2.setPassword("222");
        List<User> lists = new ArrayList<User>();
        lists.add(user1);
        lists.add(user2);
        String jsonString2 = JSON.toJSONString(lists);
        System.out.println(jsonString2);
        //打印结果
        //[{"password":"111","username":"111"},{"password":"222","username":"222"}]
    }
}