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.
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