Hibernate

The subelement of id used to generate the unique identifier for the objects of persistent class. There are many generator classes defined in the Hibernate Framework.

All the generator classes implements the org.hibernate.id.IdentifierGenerator interface. The application programmer may create one's own generator classes by implementing the IdentifierGenerator interface. Hibernate framework provides many 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
12. sequence-identity

It is the default generator strategy if there is no element . In this case, application assigns the id. For example:

....  
 <hibernate-mapping>  
  <class ...>  
    <id ...>  
     <generator class="assigned"></generator>  
    </id>  
            
    .....  
            
  </class>  
 </hibernate-mapping> 

It generates the unique id only if no other process is inserting data into this table. It generates short, int or long type identifier. The first generated identifier is 1 normally and incremented as 1. Syntax:

....  
 <hibernate-mapping>  
  <class ...>  
    <id ...>  
     <generator class="increment"></generator>  
    </id>  
            
    .....  
            
  </class>  
 </hibernate-mapping> 

It uses the sequence of the database. if there is no sequence defined, it creates a sequence automatically e.g. in case of Oracle database, it creates a sequence named HIBERNATE_SEQUENCE. In case of Oracle, DB2, SAP DB, Postgre SQL or McKoi, it uses sequence but it uses generator in interbase. Syntax:

 .....  
 <id ...>  
  <generator class="sequence"></generator>  
 </id>  
 .....  

For defining your own sequence, use the param subelement of generator.

.....  
 <id ...>  
  <generator class="sequence">  
      <param name="sequence">your_sequence_name</param>  
  </generator>  
 </id>  
 .....

It uses high and low algorithm to generate the id of type short, int and long. Syntax:

.....  
 <id ...>  
  <generator class="hilo"></generator>  
 </id>  
 .....  

It uses identity, sequence or hilo depending on the database vendor. Syntax:

.....  
 <id ...>  
  <generator class="native"></generator>  
 </id>  
 ..... 

It is used in Sybase, My SQL, MS SQL Server, DB2 and HypersonicSQL to support the id column. The returned id is of type short, int or long.

It uses high and low algorithm on the specified sequence name. The returned id is of type short, int or long.

It uses 128-bit UUID algorithm to generate the id. The returned id is of type String, unique within a network (because IP is used). The UUID is represented in hexadecimal digits, 32 in length.

It uses GUID generated by database of type string. It works on MS SQL Server and MySQL

It uses the primary key returned by the database trigger.

It uses the id of another associated object, mostly used with association.

It uses a special sequence generation strategy. It is supported in Oracle 10g drivers only.