@Entity Annotation in JPA Explained

Direct Answer

@Entity marks a class as a Jakarta Persistence entity. It needs an identifier and a public or protected no-argument constructor, which may be supplied by the class. The specification requires the entity class and persistent instance variables/methods not to be final; some providers support final classes with reduced proxy/lazy behavior, but portable code follows the specification. Discovery depends on the persistence unit or Spring entity scanning.

Code Example

import jakarta.persistence.*;
import java.time.LocalDateTime;

@Entity // managed by JPA
public class Product {
    @Id // mandatory primary key
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;
    private Double price;
    private LocalDateTime createdAt;

    protected Product() { }            // no-arg ctor — required by JPA

    public Product(String name, Double price) {
        this.name = name;
        this.price = price;
        this.createdAt = LocalDateTime.now();
    }
    // getters/setters
}

Common Mistakes & Pitfalls

• Missing no-arg constructor — JPA throws InstantiationException when loading; add one (can be protected).
• final class — JPA can't subclass it for proxies; lazy loading and tracking break.
• No @Id — startup fails with "No identifier specified for entity".
• Using javax.persistence — Spring Boot 3.2 uses jakarta.persistence.