Question

I am learning JPA from this tutorial.

I have some confusions in understanding the following annotations:

• @Basic
• @Embedded

Fields of an embeddable type default to persistent, as if annotated with @Embedded.

If the fields of embeddable types default to persistent, then why would we need the @Embedded annotation

Answer 1

The @Embeddable annotation allows to specify a class whose instances are stored as intrinsic part of the owning entity. This annotation has no attributes.

@Embeddable
public class EmploymentPeriod {
     java.util.Date startDate;
     java.util.Date endDate;
     ...
}

The @Embedded annotation is used to specify a persistent field or property of an entity whose value is an instance of an embeddable class. By default, column definitions specified in the @Embeddable class apply to the table of the owning entity but you can override them using@AttributeOverride:

@Embedded
@AttributeOverrides({
    @AttributeOverride(name="startDate", column=@Column(name="EMP_START")),
    @AttributeOverride(name="endDate", column=@Column(name="EMP_END"))
})
public EmploymentPeriod getEmploymentPeriod() { ... }

---

Regarding the optional @Basic annotation, you may use it to configure the fetch type to LAZY and to configure the mapping to forbid null values (for non primitive types) with the optional attribute.

@Basic(fetch=LAZY)
protected String getName() { return name; }

You can also place it on a field or property to explicitly mark it as persistent (for documentation purpose).