Review Request: JDK-8235521: Replacement API for Unsafe::ensureClassInitialized

[John Rose]

P.S.C. (post-send clarification)

> The workflow would be:
> 
> static final MethodHandle MH_ensureInit
>   = publicLookup().findVirtual(L…, “ensureInitialized”…);
> 

By that I mean the workflow of the dynamic language
implementor.  And after hitting “send” I realized that optimizing
that one case (of a findVirtual -> bindTo -> bindTo) is harder
than I thought.  The entire workflow below could instead be
a call to a wrapping function that takes an arbitrary “mh”
(as below) and wraps an init-barrier onto the front *if needed*.
It would have to take a Lookup and a Class, plus the mh.
It would handle all the internal stuff.  If the user wanted
a bare init MH, just pass in an empty MH as a “seed”.
And maybe take a short-circuit action if the “seed” comes
back unchanged; this is an alternative to isFullyInitialized,
but it seems sneaky.

class Lookup { …
/**
 * To the given method handle, prepend an action, if necessary, to trigger
 * initialization of the given class the first time the method handle is called.
 * If it would be illegal to call ensureInitialized on the given class from this lookup,
 * immediately report an error.  Otherwise, if the class is already fully initialized
 * return the method handle unchanged.  Otherwise, return a new method handle
 * (of the same type) which incorporates the necessary initialization action,
 * as if by a call to ensureInitialized.
 */
MethodHandle ensureInitialized(Class target, MethodHandle mh) {
  checkAccess(target);
  if (isFullyInitialized(target))   return mh;
  // we need to tack a “before action” onto mh to trigger inits:
 MethodHandle before = MH_ensureInit.bindTo(target);
 return collectArguments(mh, 0, before);
}

// note: IMPL_LOOKUP requires previous checkAccess call
private static final MethodHandle MH_ensureInit
  = publicLookup().findVirtual(L…, “ensureInitialized”…).bindTo(IMPL_LOOKUP);