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

[Mandy Chung]

I create https://bugs.openjdk.java.net/browse/JDK-8246634 to track this.

thanks
Mandy

On 6/4/20 4:35 PM, John Rose wrote:
> 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);