SpringBoot返回Json对象报错(返回对象为空{})

package com.example.springboot02.entity;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import java.io.Serializable;

@Entity //表示为实体类
public class User implements Serializable {

    @Id //Jpa 注解可以不写
    private Long id;
    //Jpa 注解可以不写，下边一样
    @Column(nullable = false, unique = true)
    private String userName;
    @Column(nullable = false)
    private String passWord;
    @Column(nullable = false, unique = true)
    private String email;
    @Column(nullable = true, unique = true)
    private String nickName;
    @Column(nullable = false)
    private String regTime;

	// 有参构造函数
    public User(Long id, String userName, String passWord, String email, String nickName, String regTime) {
        this.id = id;
        this.userName = userName;
        this.passWord = passWord;
        this.email = email;
        this.nickName = nickName;
        this.regTime = regTime;
    }
	// 无参构造函数
    public User() {

    }

}

package com.example.springboot02.controller;

import com.example.springboot02.entity.User;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;



@RestController
public class UserController {

    @RequestMapping(value = "/getUser")
    public User getUser() {
        return new User(0L,"zxd", "12345", "zxd@thu.edu.cn", "zxd","123");
    }
}

package com.example.springboot02.entity;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import java.io.Serializable;

@Entity
public class User implements Serializable {

    @Id
    private Long id;
    @Column(nullable = false, unique = true)
    private String userName;
    @Column(nullable = false)
    private String passWord;
    @Column(nullable = false, unique = true)
    private String email;
    @Column(nullable = true, unique = true)
    private String nickName;
    @Column(nullable = false)
    private String regTime;


    public User(Long id, String userName, String passWord, String email, String nickName, String regTime) {
        this.id = id;
        this.userName = userName;
        this.passWord = passWord;
        this.email = email;
        this.nickName = nickName;
        this.regTime = regTime;
    }

    public User() {

    }

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

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public void setPassWord(String passWord) {
        this.passWord = passWord;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }

    public void setRegTime(String regTime) {
        this.regTime = regTime;
    }

    public Long getId() {
        return id;
    }

    public String getUserName() {
        return userName;
    }

    public String getPassWord() {
        return passWord;
    }

    public String getEmail() {
        return email;
    }

    public String getNickName() {
        return nickName;
    }

    public String getRegTime() {
        return regTime;
    }
}

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