Generator classes in Hibernate

• Generator classes are used to generate the ‘identifier or primary key value for a persistent object while saving an object in database.
• Hibernate provides different primary key generator algorithms.
• All hibernate generator classes implements hibernate.id.IdentifierGenerator interface, and overrides thegenerate(SessionImplementor,Object) method to generate the ‘identifier or primary key value‘.
• If we want our own user defined generator, then we should implement IdentiferGenerator interface and override the generate()
•  tag (which is sub element of  tag) is used to configure generator class in mapping file.

Hibernate built-in generator classes

1. assigned
2. increment
3. sequence
4. hilo
5. native
6. identity
7. seqhilo
8. uuid
9. guid
10. select
11. foreign

1) Assigned

• Assigned generator class is the default generator if there is no tag and supports in all the databases.
• Developer should assign the identifier value to entity object before saving into the database.

2) Increment 

• Increment generator supports in all databases and generates identifier value for new records by using below formula.

Max of Id value in Database + 1

• For first record it assigns 1 to the identifier. For second record it assigns based on above formula. i.e. (Max of Id value in Database + 1) = (1+1) = 2.

3) Sequence

• Sequence generator does not support MySql database and it is database dependent. i.e. Before using this generator, we should know whether this generator supports in the database or not.

• If there is no sequence in database, it uses default sequence. For ex for oracle database it uses HIBERNATE_SEQUENCE

4) hilo

• Database independent.
• Uses hi/lo algorithms to generate identifiers.
• It generates the identifiers based on table and column values in tag.
• Default table is ‘hibernate_unique_key’ and column is ‘next_hi‘.

5) native

• It uses internally identity or sequence or hilo generator classes.
• native picks up identity or sequence or hilo generator class depending upon the capabilities of the underlying database.

6) identity

• Identity columns are support by DB2, MYSQL, SQL SERVER  and SYBASE databases.

7) seqhilo

• It is like hilo generator class, but hilo generator stores its high value in table where as seqhilo generator class stores its high value in sequence.

8) uuid

• It uses 128-bit uuid algorithm to generate identifiers of type string.
• Generated identifier is unique within a network.
• Generates identifier using IP address.
• Encodes identifier as a string ( hexadecimal digits) of length 32.
• It is used to generate passwords.

9) guid

• Uses a database generated guid string on MS-SQL and MY-SQL

10) select 

• select retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.

11) Foreign

• foreign uses the identifier of another associated object. Usually uses in conjunction with a primary key association.