Entity Mapping Annotations

Hibernate uses annotations to define how Java objects are mapped to database tables.
These annotations provide metadata about the object-relational mapping, including table names, column mappings, relationships, and constraints.

classDiagram
    class User {
        -Long id
        -String username
        -String email
        -Date createdAt
        -UserStatus status
        +getId() Long
        +setId(Long id)
        +getUsername() String
        +setUsername(String username)
    }
    
    class UserStatus {
        <>
        ACTIVE
        INACTIVE
        PENDING
    }
    
    User --> UserStatus : status
    User : @Entity
    User : @Table(name="users")
    User : @Id id
    User : @GeneratedValue(strategy=IDENTITY)
    User : @Column(name="username", nullable=false)
    User : @Column(name="email", unique=true)
    User : @Temporal(TemporalType.TIMESTAMP) createdAt
    User : @Enumerated(EnumType.STRING) status

Basic Entity

@Entity
@Table(name = "users")
public class User {

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

    @Column(name = "username", nullable = false, unique = true, length = 50)
    private String username;

    @Column(name = "email", nullable = false, unique = true)
    private String email;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "created_at")
    private Date createdAt;

    @Enumerated(EnumType.STRING)
    @Column(name = "status")
    private UserStatus status;

    // Constructors, getters, setters
}

Common Annotations

Class Level:

@Entity: Marks class as entity
@Table: Specifies table details
@SecondaryTable: Maps entity to multiple tables

Field Level:

@Id: Primary key
@GeneratedValue: Auto-generation strategy
@Column: Column mapping
@Transient: Field not persisted
@Temporal: Date/time mapping
@Enumerated: Enum mapping
@Lob: Large object mapping