Static method access dilemma and proposal: @NotInherited

[Remi Forax]

On 06/27/2013 08:53 PM, Jonathan Gibbons wrote:
> It would be reasonable to add a lint warning in the [static] category
> in situations where a static method is invoked through a subclass.
> We already warn in cases where a static method is invoked through
> an instance of a class.
>
> -- Jon

yes,
I know that Eclipse, at least, already proposes this warning.

Rémi

>
> On 06/27/2013 11:36 AM, Brian Goetz wrote:
>> I got what you were saying.  What I was telling you is that our
>> compatibility goals are higher than that, and that at the very least it
>> would take probably 2-3 major versions, even if we were willing to do
>> this.  We have a very limited budget for this kind of incompatibility;
>> we want to spend it where it will provide the most payoff.  Here, our
>> excuse is pretty lame: "we think Java should have been designed the
>> other way, so we're changing it."  That's just a silly thing to spend
>> our incompatibility budget on.  (And I agree with you that this language
>> feature was a mistake.)
>>
>>
>>
>> On 6/27/2013 2:32 PM, Paul Benedict wrote:
>>> Brian, if you're focus is 99/100% compatibility, I guess it won't work.
>>> However, if you loosen the requirements, it should be possible to
>>> overcome the compatibility reasons with this plan:
>>>
>>> For class files <= 52.0, the JVM will resolve static method inheritance.
>>> Otherwise, when code is recompiled for JDK 9+ (>= 53.0), static method
>>> inheritence will be rejected. This should be made even easier with the
>>> deprecation of --source/--target (JEP 182). If absolute source
>>> compatibility is necessary, the source compatibility could be controlled
>>> by another JVM option.
>>>
>>>
>>> On Thu, Jun 27, 2013 at 1:14 PM, Brian Goetz <brian.goetz at oracle.com
>>> <mailto:brian.goetz at oracle.com>> wrote:
>>>
>>>       Total agreement that it was a mistake to have them be inheritable.
>>>       Static methods in interfaces don't have this defect.  (And we took a
>>>       hit for that inconsistency.)  Unfortunately what you suggest for 9
>>>       is impractical for compatibility reasons.
>>>
>>>       Overall I like Stephen's proposal here (except I am not convinced
>>>       that it is suitable for an annotation, but that's a pretty
>>>       superficial aspect of it.)  Anything that moves us towards being
>>>       able to fix this problem over time is good.
>>>
>>>
>>>       On 6/27/2013 1:50 PM, Paul Benedict wrote:
>>>
>>>           Stephen, it's an interesting idea. Bikeshed moment.... At the
>>>           cost of
>>>           having to recompile future code, I'd rather wish to have static
>>>           methods on
>>>           classes not be inheritable in JDK 9 and onward. Then the same
>>>           behavior can
>>>           be predictable across classes or interfaces. Fixing a bad
>>>           language design
>>>           is probably better than patching things up with @NotInherited.
>>>
>>>
>>>           On Thu, Jun 27, 2013 at 12:20 PM, Stephen Colebourne
>>>           <scolebourne at joda.org <mailto:scolebourne at joda.org>>wrote:
>>>
>>>               The addition of static methods on interfaces has given
>>>               developers a
>>>               new very useful tool. However, it has also produced a dliemma.
>>>
>>>               A static method on a class is accessible/"inherited" by
>>>               subclasses.
>>>               Thus SubClass.foo() is just as good as SuperClass.foo() for
>>>               accessing
>>>               the static method defined on SuperClass. By contrast, with
>>>               static
>>>               methods on interfaces, this is not possible - a static
>>>               method on an
>>>               interface is not "inherited" (a Good Thing).
>>>
>>>               The dliemma, from JSR-310
>>>               (https://github.com/ThreeTen/__threeten/issues/321
>>>               <https://github.com/ThreeTen/threeten/issues/321>), is that
>>>               I _really_
>>>               want to avoid the inheritance of static methods from one
>>>               abstract
>>>               class (Chronology), as the methods make no sense at all to
>>>               be called
>>>               on the subclasses, and in fact they may cause bugs. Thus,
>>>               the new
>>>               language feature pushes me to change the abstract class to be an
>>>               interface *just to get the new static method behaviour*. In
>>>               essence I
>>>               have to make a new trade off between the right tool for the job
>>>               (abstract class) and the right tool to avoid static method bugs
>>>               (interface).
>>>
>>>               It occured to me that as this was a new dliemma, I should
>>>               report it
>>>               here on lambda-dev. And propose a possible solution.
>>>
>>>               Consider a new annotation @NotInherited that is only
>>>               applicable to
>>>               static methods. If a developer places it on a static method,
>>>               then the
>>>               method cannot be invoked by subclasses:
>>>
>>>               public class A {
>>>                   @NotInherited
>>>                   public static void foo() {..}
>>>               }
>>>               public class B extends A {
>>>               }
>>>               {  // other code
>>>                   A.foo();  // compiles
>>>                   B.foo();  // would not compile with this proposal
>>>               }
>>>
>>>               - Many IDEs support something similar today, but this would be
>>>               enforced at the compiler level.
>>>               - It is similar to @Override in conceptual scope, and is
>>>               therefore
>>>               suitable for an annotation.
>>>               - It would not change the compiled bytecode at all, other
>>>               than the
>>>               standard storage for the additional annotation.
>>>               - There are no reflection, security or backwards
>>>               compatibility issues
>>>               that I can see.
>>>               - Adding the annotation to a previously published API would be
>>>               backwards incompatible at the source level, but not the
>>>               binary level
>>>               - When compiling B against a previously compiled A, the
>>>               annotation
>>>               would be read from A's bytecode.
>>>               - The annotation would not be processed by the JVM or
>>>               verifier, thus
>>>               B.foo() would be valid if it could be compiled (such as in a
>>>               separate
>>>               compilation scenario).
>>>               - The change appears to be small, and thus not require a
>>>               large effort
>>>               to implement
>>>
>>>               The annotation could be added to the JDK without enforcing it in
>>>               javac, but that would seem to be a change not worth doing as
>>>               it would
>>>               rely on IDEs.
>>>
>>>               Its easy to say that this is an old problem and so nothing
>>>               needs to be
>>>               done. But I hope I've tried to indicate that I think
>>>               lambda's static
>>>               methods on interfaces has changed the nature of the old
>>>               problem and
>>>               made it potentially more problematic in code design terms.
>>>
>>>               Thoughts?
>>>               Stephen
>>>
>>>
>>>
>>>
>