How does composition work in Hibernate?

Question

I'm trying to use composition in hibernate with annotations.

I have:

@Entity
@Table(name = "Foo")
public class Foo {
    private Bar bar;

    public void setBar(Bar bar){...}
    public Bar getBar() {...)
}

public class Bar {
  private double x;

  public void setX(double x) {...}
  public double getX() {...}
}

And when trying to save Foo, I'm getting

Could not determine type for entity org.bla.Bar at table Foo for columns: [org.hibernate.mapping.Column(bar)]

I tried putting an @Entity annotation on Bar, but this gets me:

No identifier specified for entity org.bla.Bar

Answer 1

The mechanism is described in this section of the reference docs:

5.1.5. Embedded objects (aka components)

Apparently hibernate uses JPA annotations for this purpose, so the solution referred to by Ralph is correct

In a nutshell:

if you mark a class Address as @Embeddable and add a property of type Address to class User, marking the property as @Embedded, then the resulting database table User will have all fields specified by Address.

See Ralph's answer for the code.

Answer 2

You need to specifiy the relationship between Foo and Bar (with something like @ManyToOne or @OneToOne).

Alternatively, if Bar is not an Entity, then mark it with @Embeddable, and add @Embedded to the variable declaration in Foo.

@Entity
@Table(name = "Foo")
public class Foo {
    @Embedded
    private Bar bar;

    public void setBar(Bar bar){...}
    public Bar getBar() {...)
}

@Embeddable
public class Bar {
  private double x;

  public void setX(double x) {...}
  public double getX() {...}
}

See: https://www.baeldung.com/jpa-embedded-embeddable -- The example expains the @Embeddable and @Embedded Composite way, where Foo and Bar (Company and ContactPerson in the example) are mapped in the same Table.