Domain classes that are persistent entities - which is most of them - should be annotated using javax.persistence.Entity. In addition - as described in Chapter 2, JPA Objects / Naked Objects Restrictions - JPA Objects requires that all entities are annotated using javax.persistence.@DiscriminatorValue and must define a single property to act as the identifier, annotated using @javax.persistence.Id. The type of this Id property will depend on the number of instances expected, but will generally be one of java.lang.Integer, java.lang.Long.

JPA Objects itself does not care how the values for the property are generated. It's usually easiest to get the RDBMS to generate the IDs, using a sequence or identity (or perhaps a table). This is specified using the @javax.persistence.GeneratedValue annotation.

@Entity
@DiscriminatorValue("CUS")
public class Customer {

    private Integer id;
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    public Integer getId() { ... }
    private void setId(Integer id) { ... }

    ...
}

Other generation strategies include SEQUENCE, IDENTITY and TABLE. Note that it isn't necessary for the setter to have public visibility.

For very large ranges it is also possible to use java.lang.BigInteger. This isn't a datatype supported out-of-the-box by JPA, but Hibernate can be made to support it. See for example this blog post for details on how.

Alternatively, you can take control yourself, and hook into JPA's @PrePersist lifecycle method.