Why does JPA have a @Transient annotation?

Java'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 }@Entitypublic 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.