Java has the transient
keyword. Why does JPA have @Transient
instead of simply using the already existing java keyword?
views:
1856answers:
3Java's transient keyword is used to denote that a field is not to be serialized, whereas JPA's @Transient annotation is used to indicate that a field is not to be persisted in the database, i.e. their semantics are different.
Because they have different meanings. The @Transient
annotation tells the JPA provider to not persist any (non-transient
) attribute. The other tells the serialization framework to not serialize an attribute. You might want to have a @Transient
property and still serialize it.
As others have said, @Transient
is used to mark fields which shouldn't be persisted. Consider this short example:
public enum Gender { MALE, FEMALE, UNKNOWN }
@Entity
public Person {
private Gender g;
private long id;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
public long getId() { return id; }
public void setId(long id) { this.id = id; }
public Gender getGender() { return g; }
public void setGender(Gender g) { this.g = g; }
@Transient
public boolean isMale() {
return Gender.MALE.equals(g);
}
@Transient
public boolean isFemale() {
return Gender.FEMALE.equals(g);
}
}
When this class is fed to the JPA, it persists the gender
and id
but doesn't try to persist the helper boolean methods - without @Transient
the underlying system would complain that the Entity class Person
is missing setMale()
and setFemale()
methods and thus wouldn't persist Person
at all.