我可以在@MappedSuperclasses上创建静态方法吗？

Question

I have an abstract TemporalModel class (annotated with @MappedSuperclass ) that adds created and updated fields to all extending models. I want to add a getLatest() static method to it:

public static TemporalModel getLatest() {
    return find("order by created").first();
}

When I put this method on the base class, and call it through a concrete class ( Transaction.getLatest() ), I get an error:

UnsupportedOperationException occured : Please annotate your JPA model with @javax.persistence.Entity annotation.

I suspect this is because JPA doesn't in fact know I'm calling this method "through" the base class (there is no real static method inheritance in Java).

Is there another way to implement this method once, instead of repeating it on all entity classes?

Update - one way to achieve this (which I'm using in another heavier app) is described here ( gist ). In my current app, however, I wouldn't like to use repositories, and I wondered if there's another, lighter solution.

Answer 1

Constructors and static methods can never be abstract. The idea behind an abstract class is to create blueprints of methods, that have to get worked out in the subclass(es). I suggest trying an interface TemporalModel instead of an abstract class, in which you create the method public static TemporalModel getLatest();

Answer 2

I haven't used this Play framework, so I'm not sure about the details here, but usually, when one does the stuff you want to do, in Java, one simply specifies the concrete class as a parameter to the static method in question. It's kind of ugly, of course, but it is Java.

I assume that this find method is a static method that is added somehow (by annotation processing?) by this framework on every extending class, right? In that case, I think your only recourse is to do something like this:

public static <T extends TemporalModel> T getLatest(Class<T> cl) {
    try {
        /* I don't know what type the find() method returns, so you'll have to fix the casting */
        return(cl.cast(cl.getMethod("find", String.class).invoke("order by created").first()));
    } catch(AllThosePeskyReflectionExceptions e) {
        throw(new Error(e));
    }
}

I think that's the best way available given the premises. I know it's ugly, so I'd be happy to be wrong. :)