Hibernate supports the three basic inheritance mapping strategies: table per class hierarchy table per subclass table per concrete class In addition, Hibernate supports a fourth, slightly different kind of polymorphism: implicit polymorphism It is possible to use different mapping strategies for different branches of the same inheritance hierarchy, and then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However, Hibernate does not support mixing <subclass>, and <joined-subclass> and <union-subclass> mappings under the same root <class> element. It is possible to mix together the table per hierarchy and table per subclass strategies, under the the same <class> element, by combining the <subclass> and <join> elements (see below). Suppose we have an interface Payment, with implementors CreditCardPayment, CashPayment, ChequePayment. The table per hierarchy mapping would look like: ... ... ... ... ]]> Exactly one table is required. There is one big limitation of this mapping strategy: columns declared by the subclasses, such as CCTYPE, may not have NOT NULL constraints. A table per subclass mapping would look like: ... ... ... ... ]]> Four tables are required. The three subclass tables have primary key associations to the superclass table (so the relational model is actually a one-to-one association). Note that Hibernate's implementation of table per subclass requires no discriminator column. Other object/relational mappers use a different implementation of table per subclass which requires a type discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to implement but arguably more correct from a relational point of view. If you would like to use a discriminator column with the table per subclass strategy, you may combine the use of <subclass> and <join>, as follow: ... ... ... ... ]]> The optional fetch="select" declaration tells Hibernate not to fetch the ChequePayment subclass data using an outer join when querying the superclass. You may even mix the table per hierarchy and table per subclass strategies using this approach: ... ... ... ... ]]> For any of these mapping strategies, a polymorphic association to the root Payment class is mapped using <many-to-one>. ]]> There are two ways we could go about mapping the table per concrete class strategy. The first is to use <union-subclass>. ... ... ... ... ]]> Three tables are involved. Each table defines columns for all properties of the class, including inherited properties. The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. (We might relax this in a future release of Hibernate.) The identity generator strategy is not allowed in union subclass inheritance, indeed the primary key seed has to be shared accross all unioned subclasses of a hierarchy. An alternative approach is to make use of implicit polymorphism: ... ... ... ]]> Notice that nowhere do we mention the Payment interface explicitly. Also notice that properties of Payment are mapped in each of the subclasses. If you want to avoid duplication, consider using XML entities (e.g. [ <!ENTITY allproperties SYSTEM "allproperties.xml"> ] in the DOCTYPE declartion and &allproperties; in the mapping). The disadvantage of this approach is that Hibernate does not generate SQL UNIONs when performing polymorphic queries. For this mapping strategy, a polymorphic association to Payment is usually mapped using <any>. ]]> There is one further thing to notice about this mapping. Since the subclasses are each mapped in their own <class> element (and since Payment is just an interface), each of the subclasses could easily be part of another inheritance hierarchy! (And you can still use polymorphic queries against the Payment interface.) ... ... ... ... ]]> Once again, we don't mention Payment explicitly. If we execute a query against the Payment interface - for example, from Payment - Hibernate automatically returns instances of CreditCardPayment (and its subclasses, since they also implement Payment), CashPayment and ChequePayment but not instances of NonelectronicTransaction. There are certain limitations to the "implicit polymorphism" approach to the table per concrete-class mapping strategy. There are somewhat less restrictive limitations to <union-subclass> mappings. The following table shows the limitations of table per concrete-class mappings, and of implicit polymorphism, in Hibernate. Inheritance strategy Polymorphic many-to-one Polymorphic one-to-one Polymorphic one-to-many Polymorphic many-to-many Polymorphic load()/get() Polymorphic queries Polymorphic joins Outer join fetching table per class-hierarchy <many-to-one> <one-to-one> <one-to-many> <many-to-many> s.get(Payment.class, id) from Payment p from Order o join o.payment p supported table per subclass <many-to-one> <one-to-one> <one-to-many> <many-to-many> s.get(Payment.class, id) from Payment p from Order o join o.payment p supported table per concrete-class (union-subclass) <many-to-one> <one-to-one> <one-to-many> (for inverse="true" only) <many-to-many> s.get(Payment.class, id) from Payment p from Order o join o.payment p supported table per concrete class (implicit polymorphism) <any> not supported not supported <many-to-any> s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult() from Payment p not supported not supported