Spring Boot 2.返回JSON格式数据

package com.example.helloworld;

public class User {
    private int id;
    private String username;
    private String password;

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

package com.example.helloworld;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloWorldController {

    @GetMapping("/helloworld")
    String helloWorld() {
        return "hello world";
    }

    @RequestMapping("getUser")
    public User getUser() {
        User user = new User();
        user.setId(1);
        user.setUsername("hello spring boot");
        user.setPassword("123456");
        return user;
    }
}

 @RequestMapping("getUserList")
    public List<User> getUserList() {
        List<User> userList = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            User user = new User();
            user.setId(i);
            user.setUsername("hello spring boot" + i);
            user.setPassword("123456");
            userList.add(user);
        }
        return userList;
    }

@RequestMapping("getUserMap")
    public Map<String, User> getUserMap() {
        Map<String, User> userMap = new HashMap<>();
        for (int i = 0; i < 5; i++) {
            User user = new User();
            user.setId(i);
            user.setUsername("hello spring boot" + i);
            user.setPassword("123456");
            userMap.put(i + "", user);
        }
        return userMap;
    }