Retrieval of the generated value might require an additional SQLselect statement, but this not necessary if:

• the SQL dialect of the databasesupportsreturning orselect from final table, or
• the JDBC driverprovides a similar capability viagetGeneratedKeys().

@Generated relieves the program of the need to explicitly callrefresh() to synchronize state held in memory with state generated by the database when a SQLinsert orupdate is executed.

This is most useful when:

• a database table has a column value populated by a database trigger,
• a mapped column has a default value defined in DDL, in which case@Generated is used in conjunction with the@ColumnDefault annotation,
• aSQL expression is used to compute the value of a mapped column,
• a custom SQLinsert orupdate statement specified by an entity assigns a value to the annotated property of the entity, ortransforms the value currently assigned to the annotated property, or
• there is no mapped column, and the value of the field is determined by evaluating a SQL@Formula.

On the other hand:

• for identity/autoincrement columns mapped to an identifier property, useGeneratedValue, and
• for columns with agenerated always as clause, prefer theGeneratedColumn annotation, so that Hibernate automatically generates the correct DDL.

A@Generated field may be generated oninserts, onupdates, or on both inserts and updates, as specified by theevent() member. By default,@Generated fields are not immutable, and so a field which is generated on insert may later be explicitly assigned a new value by the application program, resulting in its value being updated in the database. If this is not desired, the@Immutable annotation may be used in conjunction with@Generated to specify that the field may never be updated after initial generation of its value.