The JPA (Java Persistence API) specification has 2 different ways to specify entity composite keys: @IdClass and @EmbeddedId.
I'm using both annotations on my mapped entities, but it turns out to be a big mess to people who aren't very familiar with JPA.
I want to adopt only one way to specify composite keys. Which one is really the best? Why?
I consider that @EmbeddedId is probably more verbose because with @IdClass you cannot access the entire primary key object using any field access operator. Using the @EmbeddedId you can do like this:
@Embeddable class EmployeeId { name, dataOfBirth }
@Entity class Employee {
@EmbeddedId EmployeeId employeeId;
...
}
This gives a clear notion of the fields that make the composite key because they are all aggregated in a class that is accessed trough a field access operator.
Another difference with @IdClass and @EmbeddedId is when it comes to write HQL :
With @IdClass you write:
select e.name from Employee e
and with @EmbeddedId you have to write:
select e.employeeId.name from Employee e
You have to write more text for the same query. Some may argue that this differs from a more natural language like the one promoted by IdClass. But most of the times understanding right from the query that a given field is part of the composite key is of invaluable help.
I discovered an instance where I had to use EmbeddedId instead of IdClass. In this scenario there is a join table that has additional columns defined. I attempted to solve this problem using IdClass to represent the key of an entity that explicitly represents rows in the join table. I couldn't get it working this way. Thankfully "Java Persistence With Hibernate" has a section dedicated to this topic. One proposed solution was very similar to mine but it used EmbeddedId instead. I modeled my objects after those in the book it now behaves correctly.
I think the main advantage is that we could use @GeneratedValue for the id when using the @IdClass? I'm sure we can't use @GeneratedValue for @EmbeddedId.