Why can’t I insert an entity twice by using Spring Data Jdbc？

Question

my entity:

@Table("user")
public class User {

    @Id
    private Long user_id;
    private String username;
    private String password;

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public User() {
    }

    public Long getUser_id() {
        return user_id;
    }

    public void setUser_id(Long user_id) {
        this.user_id = user_id;
    }

    public String getUsername() {
        return username;
    }

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

    public String getPassword() {
        return password;
    }

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

my repository:

public interface UserRepository extends CrudRepository<User, Long> {

    @Query("select * from user where username = :username")
    User findByUsername(@Param("username") String username);
}

my sql for creating the user table:

CREATE TABLE `user` (
  `user_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(32) NOT NULL,
  `password` text NOT NULL,
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `UINQUE_USERNAME`(`username`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

User userForRegister = new User(username, passwordEncoder.encode(password));
userRepository.save(userForRegister)

If I execute the line of 'userRepository.save(userForRegister)', I will insert an entity successfully the first time.

But, if I want to insert another user entity with different username, i will get an error:

2021-01-08 21:37:38.242  INFO 11180 --- [nio-8080-exec-8] c.k.centre.controller.UserController     : Failed to execute DbAction.InsertRoot(entity=com.***.***.Entity.User@65bc9ea1, generatedId=null)

I can insert it until I delete all the data of user table. Is there any point I missed?

Answer 1

I think that GenerateValue would solve the problem

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "user_id")
private Long user_id;

Also I think you should map your ther fields to database columns using @Column annotation

@Column(name = "user_id")
private Long user_id;
@Column(name = "username")
private String username;
@Column(name = "password")
private String password;