FastJson

import com.alibaba.fastjson.JSON;
Group group = new Group();
group.setId(0L);
group.setName("admin");
User guestUser = new User();
guestUser.setId(2L);
guestUser.setName("guest");
User rootUser = new User();
rootUser.setId(3L);
rootUser.setName("root");
group.add(guestUser);
group.add(rootUser);
String jsonString = JSON.toJSONString(group);
System.out.println(jsonString);
// {"id":0,"name":"admin","users":[{"id":2,"name":"guest"},{"id":3,"name":"root"}]}Copy to clipboardErrorCopied

String jsonString = ...;
Group group = JSON.parseObject(jsonString, Group.class);
@Data
public class Group {
    private Long       id;
    private String     name;
    private List<User> users = new ArrayList<User>();
}
@Data
public class User {
    private Long   id;
    private String name;
}