Shortcut for obtaining a MethodHandle for an anonymous code fragment

[Florian Weimer]

* Vladimir Ivanov:

>> Is it true that there is no shortcut for obtaining a method handle for
>> some code fragment?
>> 
>> That is, there is no way to write something like this?
>> 
>>    MethodHandle repeatIt = (String x) -> x + x;
>> 
>> Instead it's necessary to give the expression a name and put it into a
>> static method somewhere, and obtain a MethodHandle for it using
>> MethodHandles.lookup().
>
> There's no language support for that, but you can write a utility method 
> to convert a lambda into a method handle:
>
>      wrapRunnable(() -> System.out.println(""));
>
>      static MethodHandle wrapRunnable(Runnable r)  {
>          return RUNNABLE_RUN.bindTo(r);
>      }
>
>      static final MethodHandle RUNNABLE_RUN;
>      static {
>          try {
>              RUNNABLE_RUN = 
> MethodHandles.lookup().findVirtual(Runnable.class, "run", 
> MethodType.methodType(void.class));
>          } catch (NoSuchMethodException | IllegalAccessException e) {
>              throw new InternalError(e);
>          }
>      }

I poked at the generated method handle with reflection, and it retains
a reference to the Runnable object with the synthesized lambda class.
Would it be a valid optimization if the lookup cut through to the
method implementing the lambda?

> Or even a generic version:
>
>      wrap(Runnable.class, () -> System.out.println(""));
>
>      static <T> MethodHandle wrap(Class<T> functionalInterface, T lambda) {
>          MethodHandle sam = ... // find SAM in a functional interface
>          return sam.bindTo(lambda);
>      }

This is an interesting use of a comment.  Is there even a well-defined
concept of a functional interface at the JVM level?  I guess it's
possible to ignore JLS-level override equivalence and generics, and
focus on the generic signatures only.

Still it would be nice to have something in the JDK to compute a
JVM-level SAM at run time.