composite identifiers

• The composite identifier must be represented by a "primary key class". The primary key class may be defined using the javax.persistence.EmbeddedId annotation (see Composite identifiers with @EmbeddedId), or defined using the javax.persistence.IdClass annotation (see Composite identifiers with @IdClass).
• The primary key class must be public and must have a public no-arg constructor.
• The primary key class must be serializable.
• The primary key class must define equals and hashCode methods, consistent with equality for the underlying database types to which the primary key is mapped.

Hibernate does allow composite identifiers to be defined without a "primary key class", 但已经 deprecated .

The attributes making up the composition can be either basic, composite, ManyToOne. Note especially that collections and one-to-ones are never appropriate.