@Entity - JPA Annotation

In this post, we will learn the usage of @Entity JPA annotation with an example.

@Entity - This annotation specifies that the class is an entity. This annotation can be applied to Class, Interface of Enums.


@Entity - JPA Annotation Example

Creating the JPA Entity Class(Persistent class)

Let's create a Student persistent class that is mapped to a student database table:
Create a Student entity class with @Entity annotation:
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "student")
public class Student {

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

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "email")
    private String email;

    public Student() {

    }

    public Student(String firstName, String lastName, String email) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + ", email=" + email + "]";
    }
}

Rules or Requirements to define Entity Class


The Entity Class of the JPA 2.1 specification defines its requirements for an entity class. Applications that wish to remain portable across JPA providers should adhere to these requirements.

  • The entity class must be annotated with the javax.persistence.Entity annotation (or be denoted as such in XML mapping)
  • The entity class must have a public or protected no-argument constructor. It may define additional constructors as well.
  • The entity class must be a top-level class.
  • An enum or interface may not be designated as an entity.
  • The entity class must not be final. No methods or persistent instance variables of the entity class may be final.
  • If an entity instance is to be used remotely as a detached object, the entity class must implement the Serializable interface.
  • Both abstract and concrete classes can be entities. Entities may extend non-entity classes as well as entity classes, and non-entity classes may extend entity classes.
  • The persistent state of an entity is represented by instance variables, which may correspond to JavaBean-style properties. An instance variable must be directly accessed only from within the methods of the entity by the entity instance itself. The state of the entity is available to clients only through the entity’s accessor methods (getter/setter methods) or other business methods.

References

https://javaee.github.io/javaee-spec/javadocs/javax/persistence/Entity.html

https://www.javaguides.net/2018/11/hibernate-5-java-configuration-example.html

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