(Recovered from my old Blog).
Over time, Hibernate classes can get very large – especially when modelling a complicated object (for example a financial transaction).
At this point, the @Embedded annotation comes to the rescue. The class annotated with @Embedded will be linked into the parent class as if it were one large class.
This even works with collections – the collection can be defined in the child (@Embeddable) class.
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@Entity public class PretendClass { @Id private Long transactionId; @Embedded private PretendChildClass pretendChildClass = new PretendChildClass(); } @Embeddable public class PretendChildClass { @OneToMany(mappedBy = “transaction”, targetEntity = TransactionAttributeImpl.class, cascade = CascadeType.ALL) @MapKey(name = “transactionAttributeType”) @Cascade(org.hibernate.annotations.CascadeType.DELETE_ORPHAN) private Map<TransactionAttributeType, TransactionAttribute> transactionAttributeMap = new HashMap<TransactionAttributeType, TransactionAttribute>(); } |
One caveat – we ought to be programming to interfaces rather than implementations. Assuming we marked an interface as @Embedded, Hibernate will scan the interface and not import any of the fields defined in the class. In this case, one simply adds the @Target annotation:
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@Embedded @Target(value = PretendChildClassImpl.class) private PretendChildClass pretendChildClass = new PretendChildClassImpl(); |
If using Criteria, this extra ‘level’ in the class hierarchy is simply referenced via a full-stop:
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List<Transaction> transactionList; Criteria crit = getHibernateTemplate().getSessionFactory().getCurrentSession().createCriteria(TransactionImpl.class); crit.createAlias(“embeddable_child_class.variable_we_want”, “transactionExtRef”); // THE IMPORTANT LINE crit.add(Restrictions.eq(“transactionExtRef.transactionExtRefType”, transactionExtRefType)) .add(Restrictions.eq(“transactionExtRef.transactionExtRefValue”, transactionExtRefValue)); transactionList = crit.list(); return transactionList; |