Editing JavaJEEJPA (section)

===Entity Identity===
Using an ORM introduces additional constraints on object identity. Defining the properties that make up an entity’s natural identity can be tricky, but is very important. Using the object’s identity, or the synthetic identity (database generated primary key) identity can introduce unexpected bugs into your application, so you should always ensure you use a natural identity. The identity is defined by [[JavaJEEJPA#javax.persistence.Id|@Id annotation]].  The type should not be native type, but a class, because a null value is by convention used for not stored objects. For instance it could be of type Long.

For composite keys you have three options:
# encapsulate the identifier properties in a separate class and mark it as @Embeddable. Include an attribute of this class in the entity class an mark it with @Id
# encapsulale the identifier properties in a separate class without annotation, include an attribute of this class in the entity class and mark it with @EmbeddedId
# @IdClass