有些表没有定义id或主键，只有一个unique约束，你又无权修表。
但Hibernate说，一定要id，怎么办？
这时，可以把unique约束相关的field定义为id，一个类里，可以有多个@Id 。
这样可以省去写@EmbeddedId的麻烦。
下面是hibernate文档摘录：
http://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/
---------------------2.2.3.2.2. Multiple @Id properties
Another, arguably more natural, approach is to place @Id on multiple properties of my entity. This approach is only supported by Hibernate but does not require an extra embeddable component.
@Entity
class Customer implements Serializable {
@Id @OneToOne
@JoinColumns({
@JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
@JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
})
User user;
@Id String customerNumber;
boolean preferredCustomer;
}
In this case Customer being it's own identifier representation, it must implement Serializable.
------------------------------------------------------------------
守护