Constable interface should die

Remi Forax forax at
Sat Sep 2 14:51:42 UTC 2017

Brian ask me to explain my concerns about the Constable interface.

The whole constant folding story is like a macro system, it's a limited macro system, but still a macro system.
I've developed several macro systems, all have limitations, some limitation that i have introduced voluntarily, some that have appear after being being used, all the macro systems have always evolved after the first release.
So the first lesson of designing a macro system seems to be, because it will evolve, it should provide the minimal API so it can be refactored easily.

In the case of constant-folding mechanism, it's not a mechanism that target end users but JDK maintainers, so end users should not be able to see the implementation of such mecanism.
It's my main concern with the Constable interface, it's a public visible type with a public visible API.

We have already introduced in the past a mechanism that requires a specific interaction between the user code, the JDK and the compiler, it's the polymorphic methods signature and it was solved by using a private annotation.

I think constant folding should use the same trick. Mark constant foldable type with a hidden annotation (@Constable ?) and mark methods (private) that can be called by the compiler with another hidden annotation (@TrackableConstant ?) and i will be happy.

Compared to using an interface, there is a loss of discover-ability from the end user, but their is no loss of compiler checking because the compiler can check if a type is annotated by an annotation the same way it can check if it implements an interface.

Now, we can discuss if @Constable should be a public annotation or not because once a type can be constant folded, removing the annotation is a non backward compatible change. So having the @Constable public is perhaps better than having to have a sentence in the middle of the javadoc saying that this is a constant foladable type.

Note that constant folding things is also a form of serialization, the first Java serialization API have made that mistake to make the implementation of the part that serialize each object too visible. I think we can do better here.
You can also think that like Serializable, Constable could be an empty interface and ldc will take a Constable. But int constant-foldable and i do not see why it should be boxed to an Integer to becomes Constable (The full implication of that is that ldc should be a method with a polymorphic signature but we are moving in that direction anyway).

Long live to @Constable !



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