public class CompositeNestedGeneratedValueGenerator
extends Object
implements IdentifierGenerator, Serializable, IdentifierGeneratorAggregator

For composite identifiers, defines a number of "nested" generations that need to happen to "fill" the identifier property(s).

This generator is used implicitly for all composite identifier scenarios if an explicit generator is not in place. So it make sense to discuss the various potential scenarios:

• "embedded" composite identifier - this is possible only in HBM mappings as <composite-id/> (notice the lack of both a name and class attribute declarations). The term "embedded" here refers to the Hibernate usage which is actually the exact opposite of the JPA meaning of "embedded". Essentially this means that the entity class itself holds the named composite pk properties. This is very similar to the JPA @IdClass usage, though without a separate pk-class for loading.
• pk-class as entity attribute - this is possible in both annotations (@EmbeddedId) and HBM mappings (<composite-id name="idAttributeName" class="PkClassName"/>)
• "embedded" composite identifier with a pk-class - this is the JPA @IdClass use case and is only possible in annotations

Most of the grunt work is done in Component.