JPA Objects requires that all entities have an @Id property (or inherit one from a superclass). If this isn't the case then JPA Objects' metamodel validator will throw an exception, and the application won't boot.

JPA Objects currently does not support composite primary keys. Specifically, the @IdClass annotation (which specifies a composite primary key class mapped to multiple properties of the entity) is not allowed. The @EmbeddedId annotation (which is applied to a persistent property of an entity class or mapped superclass to denote a composite primary key that is an embeddable class) is also not allowed.

This restriction is by design. Coupled with discriminators, JPA Objects can guarantee that every entity instance can be identified in a standard fashion. See Section 2.5, “Specify a discriminator” for further discussion.

In addition, the @Id property should be a wrapper class, eg java.lang.Integer rather than int. This allows the JPA provider (Hibernate) to distinguish between transient and persisted objects if no @Version property has been specified; transient objects have a null id.