2.6.2. Composite identifiers

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Composite identifiers correspond to one or more persistent attributes.
Here are the rules governing composite identifiers, as defined by the JPA specification.

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• 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. </div>

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  The restriction that a composite identifier has to be represented by a "primary key class" is only JPA specific. Hibernate does allow composite identifiers to be defined without a "primary key class", although that modeling technique is deprecated and therefore omitted from this discussion.

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

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