Wednesday, April 25, 2007

Customize IdGenerator in JPA, gap between Hibernate and JPA annotations

JPA annotation is like a subset of Hibernate annotation, this means people will find something available in Hibernate missing in JPA. One of the important missing features in JPA is customized ID generator. JPA doesn't provide an approach for developer to plug in their own IdGenerator. For example, if you want the primary key of a table to be BigInteger coming from sequence, JPA will be out of solution.

Assume you don't mind the mixture of Hibernate and JPA Annotation and your JPA provider is Hibernate, which is mostly the case, a solution before JPA starts introducing new Annotation is, to replace JPA @SequenceGenerator with Hibernate @GenericGenerator. Now, let the code talk.
/**
* Ordinary JPA sequence.
* If the Long is changed into BigInteger,
* there will be runtime error complaining about the type of primary key
*/
@Id
@Column(name = "id", precision = 12)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "XyzIdGenerator")
@SequenceGenerator(name = "XyzIdGenerator", sequenceName = "xyz_id_sequence")
public Long getId() {
   return id;
}

In order to generate BigInteger primary key, a custom BigIntegerSequenceGenerator is created.
package com.mycompany.myapp.id;

import org.hibernate.id.SequenceGenerator;
...

public class BigIntegerSequenceGenerator
    extends SequenceGenerator
{
    @Override
    public Serializable generate(SessionImplementor session, Object obj)
    {
        ...
    }
}
Replace the JPA @SequenceGenerator with Hibernate @GenericGenerator, where the strategy can be the class name of IdGenerator.

@Id
@Column(name="id", precision = 32)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "XyzIdGenerator")
//@SequenceGenerator(name = "XyzIdGenerator", sequenceName = "xyz_id_sequence")
@GenericGenerator(name = "XyzIdGenerator",
        strategy = "com.mycompany.myapp.id.BigIntegerSequenceGenerator",
        parameters = {
            @Parameter(name = "sequence", value = "xyz_id_sequence")
        })
public BigInteger getId()
{
   return id;
}
Customized IdGenerator is one of the features in the gap between JPA and Hibernate persistence Annotations. Similar missing features in JPA include other things like @Generated annotation in Hibernate for non-primarykey generated fields.

4 comments:

Coder said...

Hi Jiaqi's thanks for this post.
I'm trying to implement a custom generator for JPA, (Hibernate is the JPA provider in my case), and this article turns out to be very handy. :)
I have two questions, I would appreciate very much your answers.
Is the "xyz_id_sequence" value for the sequenceName arbitrary ? How about the generator "XyzIdGenerator" value
is it arbitrary?
Also, I tried your sample and got
javax.persistence.PersistenceException: org.hibernate.MappingException: could not instantiate id generator exception.
Any ideas?

Thanks !
Paul

Jiaqi Guo said...

xyz_id_sequence is the sequence name in database. XyzIdGenerator is arbitrary, but the generator in @GeneratedValue must match name of @SequenceGenerator.

Basically it means field is a generated value, generated by a sequence type generator named "XyzIdGenerator" and @SequenceGenerator defines what "XyzIdGenerator" is, and it actually is a database sequence named "xyz_id_sequence"

Rama Konjeti said...

Once can use @prePersist and other JPA events to create their own ids.

mark said...

I think your exception is wrong

public Serializable generate(SessionImplementor session, Object obj)
throws HibernateException {