First, I would like to say that I find the proposal to be quite complex compared to the gains. And I agree with Bernd and Michael.
However, I have to say that I did not understand the following comment:
"Having said that, if we think that aliasing is one of those things that could stop large companies from using JPA we might decide to close our eyes, hold our noses and go ahead and add some kind of support."
What do you mean by aliasing?
IMO, the biggest problem with JPA is that it is seen as being slow, and to encourage bad DB schema designs.
/Nicolas
-----Original Message-----
From: michael keith [mailto:michael.keith_at_oracle.com]
Sent: Sunday, March 11, 2012 6:27 PM
To: jsr338-experts_at_jpa-spec.java.net
Cc: Bernd Müller
Subject: [jsr338-experts] Re: proposal : @Entity on interfaces
Let me first say that I don't like the model of interfaces on entities.
It is quite rarely requested and when it is, often the user wants multiple entities to be able to implement the same interface (e.g.
polymorphism w/o inheritance) to do something like this:
public interface Employee { ... }
@Entity public class BusEmployee implements Employee { ... }
@Entity public class TrainEmployee implements Employee { ... }
and then in some entity:
@Entity public class TranportationBureau {
...
@ManyToOne
Employee empOfTheMonth;
}
The feature being proposed, if I understood it correctly, would not support this.
Having said that, if we think that aliasing is one of those things that could stop large companies from using JPA we might decide to close our eyes, hold our noses and go ahead and add some kind of support.
While I understood and sympathized with the reasons why Steve first suggested that the annotation be on the interface (to facilitate interface-to-single-entity enforcement) I tend to agree with Oliver and Bernd that it would make more sense to just add an "alias" element to the @Entity annotation and continue to apply it to the impl class. It would be the responsibility of the provider to disallow two entities from creating an alias to the same class.
Example:
@Entity(alias=Employee.class)
public class EmployeeImpl implements Employee {
// mappings ...
}
Where does one draw the line? Would we assume that a single alias is enough and that an entity would not be allowed to specify a Set? For example, that we would not need to support something like:
@Entity(aliases={Employee.class, Worker.class}) ... or ...
@Entity(aliasInterfaces=true)
public class EmployeeImpl implements Employee, Worker {
// mappings ...
}
-Mike
On 11/03/2012 12:23 PM, Bernd Müller wrote:
> I think, this would make many things more complex to specify in a
> consistent way.
>
> In general, interfaces are a way to describe contracts in an
> implementation independent way and therefore should be as abstract as
> possible. JPA is a mapping between VM-objects and databases and
> therefore as close as possible to the implementation level.
> Here I see some problems from a conceptual point of view.
>
> I also see many problems in practice. How to map fields in classes,
> which are eventually named different than the annotated getter in the
> interface? How to map interface hierarchies with multiple super
> interfaces to eventually DIFFERENT class hierarchies with single
> inheritance? There are more, I think.
>
> We have to balance if it's worth to get such new problems which have
> to be resolved in the spec and on the other hand the benefit is less
> typing (how much?).
>
>
> Bernd
>
>
> Am 09.03.2012 19:04, schrieb Steve Ebersole:
>> I'd like to propose that JPA 2.1 allow @Entity on Java interfaces not
>> just classes. The main reason is typing in spec contracts. For
>> domain models that leverage interfaces, it is usually desirable to
>> deal with the interfaces over the implementation classes. For
>> example, such applications would generally prefer to attempt to load
>> or get a reference to an instance based on the interface name as
>> opposed to the class name. E.g.
>>
>> public interface Person {
>> ...
>> }
>>
>> @Entity
>> public class PersonImpl implements Person {
>> ...
>> }
>>
>> EntityManager em = ...;
>> Person p = em.find( Person.class, theKey );
>>
>> But this does not work today in a portable manner. To work in the
>> most portable manner, I think the @Entity annotated interface also
>> would need to name the "persistent implementation class":
>>
>> @Entity( impl = PersonImpl.class )
>> public interface Person {
>> ...
>> }
>>
>> public class PersonImpl implements Person {
>> ...
>> }
>>
>> It could be up to each provider whether or not to support @Entity on
>> an interface that did not specify a "persistent implementation class".
>>
>> Another way to look at this is as basically "aliasing" the entity
>> type metadata using the interface name instead of the implementation class name.
>>
>> -Steve
>
>