问题
what's better for JPA/Hibernate composite primary keys, @IdClass or @EmbeddedId implementations and why?
This is an intentionally naive question. I decided to use @EmbeddedId (for whatever reason) and I feel like I made the wrong choice. Dereferencing the embeddedId that contains the column properties is redundant and quite error-prone when coding.
Are there any more reasons for and/or against the other? Is the a recommendation by the JPA (spec)?
回答1:
First, if possible, avoid composite ids at all costs. But if you really need, I would recommend @EmbeddedId.
@IdClass is basically a leftover from EJB 2.1 times to make it easier from migrating from BMP. In some other rare corner cases it may be better than @EmbeddedId too. However, generally @EmbeddedId is better and more OO since it encapsulates the concept os the key much better in the object.
You may want to use @AttributeOverride(s) if you need in the key field.
I don't see why do you think that dereferencing the embedded id is redundant and error-prone.
回答2:
As Pascal wrote here's part of the answer:
Which annotation should I use: @IdClass or @EmbeddedId
In the end, I believe using @IdClass is much easier in practice, because you have to add the embeddedId property name to dereference PK properties, while these aren't written for all non-PK properties.
You always have to remember exactly which properties are part of a PK and those which are not. That complicates writing JPQL queries unneccessarily.
Also, AFAIK the JPA 2.0 spec allows you to put @Id onto @XToX/@JoinColumn/s properties and it introduces the @MapsId annotation, so that mapping identifying relationships (a.k.a. derived identifiers in JPA) are more natural to implement.
回答3:
I. Remember using idclass to do it. But I'd recommend doing everything you can to avoid multi field keys. They just create extra work.
来源：https://stackoverflow.com/questions/4024800/jpa-hibernate-whats-better-for-composite-primary-keys-idclass-or-embeddedid