Another implementation, way simpler.
This one works with both annotation-based or xml-based configuration: it rely on hibernate meta-data to get the id value for the object. Replace SequenceGenerator by IdentityGenerator (or any other generator) depending on your configuration. (The creation of a decorator instead of subclassing, passing the decorated ID generator as a parameter to this generator, is left as an exercise to the reader).
public class UseExistingOrGenerateIdGenerator extends SequenceGenerator {
@Override
public Serializable generate(SessionImplementor session, Object object)
throws HibernateException {
Serializable id = session.getEntityPersister(null, object)
.getClassMetadata().getIdentifier(object, session);
return id != null ? id : super.generate(session, object);
}
}
Answer to the exercise (using a decorator pattern, as requested), not really tested:
public class UseExistingOrGenerateIdGenerator implements IdentifierGenerator, Configurable {
private IdentifierGenerator defaultGenerator;
@Override
public void configure(Type type, Properties params, Dialect d)
throws MappingException;
// For example: take a class name and create an instance
this.defaultGenerator = buildGeneratorFromParams(
params.getProperty("default"));
}
@Override
public Serializable generate(SessionImplementor session, Object object)
throws HibernateException {
Serializable id = session.getEntityPersister(null, object)
.getClassMetadata().getIdentifier(object, session);
return id != null ? id : defaultGenerator.generate(session, object);
}
}
