Entities and Sequence Generation

Many databases support an internal mechanism for id generation called sequences. You can use a database sequence to generate identifiers when the underlying database supports them.

• @SequenceGenerator—If you use the @GeneratedValue annotation to specify a primary key generator of type SEQUENCE, then you can use the @SequenceGenerator annotation to fine-tune this primary key generator to do the following:

  • change the allocation size to match your application requirements or database performance parameters

  • change the initial value to match an existing data model (for example, if you are building on an existing data set for which a range of primary key values has already been assigned or reserved)

  • use a predefined sequence in an existing data model

• @TableGenerator—If you use the @GeneratedValue annotation to specify a primary key generator of type TABLE, then you can use the @TableGenerator annotation to fine-tune this primary key generator to do the following:

  • change the name of the primary key generator's table, because the name is awkward, a reserved word, incompatible with a preexisting data model, or invalid as a table name in your database

  • change the allocation size to match your application requirements or database performance parameters

  • change the initial value to match an existing data model (for example, if you are building on an existing data set, for which a range of primary key values has already been assigned or reserved)

  • configure the primary key generator's table with a specific catalog or schema

  • configure a unique constraint on one or more columns of the primary key generator's table

For more information and examples of these annotations, see "Metadata for Object/Relational Mapping" in the JPA Specification.