9.2 Entity Mapping and Strategies

The first step in using JPA is making declarations about which Table in the database a given Java Class corresponds to. A mapped class is termed an Entity.

Basic Mapping Annotations

To map an entity, the most essential annotations are @Entity and @Id.

import jakarta.persistence.*;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.AccessLevel;

@Entity
@Table(name = "users") // Used to explicitly define plural or specific table names
@Getter
@NoArgsConstructor(access = AccessLevel.PROTECTED) // A default constructor is required for JPA.
public class User {

    @Id // Indicates that this field is the Primary Key (PK) of the table
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // The CamelCase 'userEmail' automatically maps to a snake_case 'user_email' column.
    @Column(nullable = false, length = 100, unique = true)
    private String userEmail;

    @Column(nullable = false)
    private String password;

    @Column(length = 50)
    private String nickname;
    
    // Convenience method for business logic (e.g. changing nickname)
    public void changeNickname(String newNickname) {
        this.nickname = newNickname;
    }
}

• @Entity: Indicates to JPA that this class is a managed entity.
• @Id: Marks the field as the primary key (PK).
• @Column: Maps an object's field to a table column. You can set constraints (nullable, unique, length) for the DB schema here. (Even if omitted, JPA recognizes fields as columns automatically.)

Primary Key Generation Strategies (@GeneratedValue)

Different databases adopt different methods to auto-generate identifiers. To mitigate this disparity, varied strategy values are offered:

1. GenerationType.IDENTITY (Highly Recommended): Delegates ID generation to the database. Equivalent to AUTO_INCREMENT in MySQL. You can comprehend the ID's value only post-save.
2. GenerationType.SEQUENCE: Creates keys using a Sequence object in the database. Predominantly used in Oracle or PostgreSQL.
3. GenerationType.UUID (Modern JPA option): Automatically computes a 32-character UUID string, optimal for distributed systems or where unpredictable keys are prioritized. (The ID type must be a UUID or String.)

By wrapping up your entity design and launching the application with the ddl-auto: update option in application.yml, you'll notice JPA analyzes your entity structure and automatically executes CREATE statements to construct tables inside the database.