JPAアノテーションによるエンティティマッピング

JPAエンティティマッピングとは

@Entityと@Table

@Entity
@Table(name = "users", uniqueConstraints = @UniqueConstraint(columnNames = "email"))
public class User {
    // ...
    protected User() {} // required by JPA
}

@Idと@GeneratedValue

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
}

@Column: カラムのカスタマイズ

@Column(name = "full_name", nullable = false, length = 100)
private String name;
@Column(name = "email", unique = true, nullable = false)
private String email;
@Column(name = "created_at", updatable = false)
private LocalDateTime createdAt;

@Embeddedと@Embeddable

@Embeddable
public class Address {
    private String street, city, country;
    @Column(name = "zip_code") private String zipCode;
}
@Entity
public class User {
    @Embedded private Address address;
}

@Enumerated: enumの永続化

@Enumerated(EnumType.STRING)
@Column(nullable = false)
private UserStatus status; // stored as "ACTIVE", "INACTIVE", etc.

@CreationTimestampと@UpdateTimestamp

@CreationTimestamp
@Column(updatable = false)
private LocalDateTime createdAt;
@UpdateTimestamp
private LocalDateTime updatedAt;

@Transient: フィールドの除外

@Transient
public String getFullName() { return firstName + " " + lastName; }

@Lob: ラージオブジェクト

@Lob
@Column(name = "content")
private String content; // maps to TEXT in PostgreSQL

equalsとhashCodeの契約

@Override public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof User u)) return false;
    return email != null && email.equals(u.email);
}
@Override public int hashCode() { return getClass().hashCode(); }

@Versionによる楽観的ロック

@Version
private Long version; // auto-managed by Hibernate

クイックチェック

まとめ