import lombok.Data;

@Data
public class Friend {
    private String name;
    private int age;
    private String sex;
}

public class FriendTest {
    private Friend friend = new Friend();

    @Before
    public void init(){
        friend.setAge(26);
        friend.setName("xiaofang");
        friend.setSex(null);
    }

@Test
    public void testObjectToJson(){
        String jsonString = JSONObject.toJSONString(friend);
        System.out.println(jsonString);
    }

@Test
    public void testJsonSerializer(){
        String jsonString = JSONObject.toJSONString(friend, SerializerFeature.WriteMapNullValue);
        System.out.println(jsonString);
    }