Re: JPA, Hibernate: Custom primary key generator, before persist 843859 Jun 22, 2009 2:34 PM ( in response to Tolls ) So, you want to generate the barcode on the document, but ensure that the code is unique (ie using the database to keep track of the id possibly). In this Embeddable Composite Primary Key example, we will be declaring the IDs (Primary Key fields) as a separate class annotated with @Embeddable annotation. An Employee is identified by its EmployeeId, which is defined by empId and department. Let’s dig into the code Hibernate Embeddable Composite Primary Key Creating table.

The <generator> class is a sub-element of id. It is used to generate the unique identifier for the objects of persistent class. There are many generator classes defined in the Hibernate Framework.

To represent the composite keys, hibernate provides key-property tag under the composite-id tag. We can represent each primary key with key-property tag, so that all primary keys will be binded with the composite-id tag. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify a element. The assigned generator makes Hibernate use unsaved-value='undefined'.

Hibernate generator element generates the primary key for new record. There are many options provided by the generator method to be used in different situations. Generation strategy of composite primary key can be managed by @IdClass. Annotate the class with @IdClass for which you need composite pk. Annotate the fields that make for composite pk with @Id and @GeneratedValue.

Purpose Of Composite Primary Key

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

1) assigned

/how-is-the-private-key-generated.html. It is the default generator strategy if there is no <generator> element . In this case, application assigns the id. For example:

2) increment

It generates the unique id only if no other process is inserting data into this table. It generates short, int or long type identifier. If a table contains an identifier then the application considers its maximum value else the application consider that the first generated identifier is 1. For each attribute value, the hibernate increment the identifier by 1. Syntax:

3) sequence

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:

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

4) hilo

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

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5) native

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

6) identity

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 is responsibility of database to generate unique identifier.

7) seqhilo

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

8) uuid

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.

9) guid

10) select

11) foreign

12) sequence-identity

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