More on the equals()/hashCode() Hack with Hibernate

The last blog I did not address Object Retrival. A reader of the blog mentioned the same. Prior to Hibernate 3, I beleive that custom collections could not be supported. However, as of Hibernate 3, they are. R.J.Lorimer's blog on how to implement the same is super!

The use of a the attribute "collection-type" when defining the mapping of the Set is the key to the solution.

So moving forward, for Hibernate, we define a custom CollectionType that factories the PropertyListenerHashSet as shown below:

  1 public class HibernatePropListenerHashSetType implements UserCollectionType {
  2   public boolean contains(Object collection, Object entity) {
  3     Set set = (Set) collection;
  4     return set.contains(entity);
  5   }
  6
  7   public Iterator getElementsIterator(Object collection) {
  8     return ((Set) collection).iterator();
  9   }
 10
 11   public Object indexOf(Object collection, Object entity) {
 12     return null;
 13   }
 14
 15   public Object instantiate(int anticipatedSize) {
 16     return new PropertyListenerHashSet();
 17   }
 18
 19   public PersistentCollection instantiate(SessionImplementor session, CollectionPersister persister) throws HibernateException {
 20     return new PersistentSet(session);
 21   }
 22
 23   @SuppressWarnings("unchecked") public Object replaceElements(Object original, Object target,
 24     CollectionPersister persister, Object owner, Map copyCache, SessionImplementor session) throws HibernateException {
 25     Set setA = (Set) original;
 26     Set setB = (Set) target;
 27     setB.clear();
 28     setB.addAll(setA);
 29
 30     return null;
 31   }
 32
 33   public PersistentCollection wrap(SessionImplementor session, Object collection) {
 34     return new PersistentSet(session, (Set) collection);
 35   }
 36 }

Pretty simple...now the blocker!! Hibernate does not have an annotation that will support the custom collection-type, for example, something like @CollectionType (name=FooCollection.class). This hibernate forum entry describes the situation. It might be supported at a later date.

In the meanwhile, we can still achieve the custom collection type using hbm mappings :-). So reverting to XML mapping, we defined the Organization object's mapping to use our custom collections as follows:

  1 <set name="applications"  inverse="true" cascade="all-delete-orphan"
  2    collection-type="com.welflex.collection.HibernatePropListenerHashSetType">
  3    <key column="ORG_ID" not-null="true"/>
  4    <one-to-many  class="com.welflex.model.SmartApplication" />
  5 </set>

The unit test has been augmented to test for the same:

  1     try {
  2       session = HibernateUtil.getSessionFactory().getCurrentSession();
  3       tx = session.beginTransaction();
  4       Organization org = (Organization) session.load(Organization.class, orgId);
  5       org.addApplication(app);
  6       assertTrue(org.getApplications().contains(app));
  7
  8       // Lets add a new App to the set
  9       SmartApplication otherApp = new SmartApplication();
 10       otherApp.setName("Foo Bar");
 11       org.addApplication(otherApp);
 12       assertEquals(2, org.getApplications().size());
 13
 14       session.update(org);
 15       session.flush();
 16
 17       // Upon Saving if our collection's contains matches up, our code woiked!
 18       assertTrue(otherApp.getId() != null);
 19       assertTrue("More acid test", org.getApplications().contains(otherApp));
 20       tx.commit();
 21     }

A Maven example of the above is available HERE.  Enjoy!!!