Extracted from Hibernate online official tutorials
- In Hibernate the optional <generator> child element names a Java class used to generate unique identifiers for instances of the persistent class.
generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster.
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.
uses a database-generated GUID string on MS SQL Server and MySQL.
picks identity, sequence or hilo depending upon the capabilities of the underlying database.
lets the application to assign an identifier to the object before save() is called. This is the default strategy if no <generator> element is specified.
retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.
uses the identifier of another associated object. Usually used in conjunction with a <one-to-one> primary key association.
a specialized sequence generation strategy which utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to actually return the generated identifier value as part of the insert statement execution. This strategy is only known to be supported on Oracle 10g drivers targetted for JDK 1.4. Note comments on these insert statements are disabled due to a bug in the Oracle drivers.
If any parameters are required to configure or initialize the generator instance, they are passed using the <param> element.
<id name="id" type="long" column="cat_id"> <generator class="org.hibernate.id.TableHiLoGenerator"> <param name="table">uid_table</param> <param name="column">next_hi_value_column</param> </generator> </id>
All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface; some applications may choose to provide their own specialized implementations. However, Hibernate provides a range of built-in implementations. There are shortcut names for the built-in generators: