How to wrap Method handle behind a class?

[Maurizio Cimadamore]

Glavo's suggestion is correct. If you want the method handle call to 
apply the maximum set of conversions, you should use `invokeWithArguments`.

The price to pay is that, the further you move away from `invokeExact` 
the slower the method call will go (because of the conversion code than 
needs to surround the code itself).

Also notice that, with your approach, the method handle wrapping the 
function is not static final (it is final, but not static). This means 
it is not a true JVM constant. This means your method handle call will 
probably not be inlined very well. Wrapping with a record instead of a 
plain class might be better because records fields are trusted. So if 
you wrap a method handle in a NativeMethod _record_ and then you stick 
the record instance in a static final field somewhere, that _should_ 
work as expected (waving hands furiously).

Anyway, of course all the above doesn't mean you shouldn't do what you 
are trying to do - if performance is not a concern for you then your 
approach is totally reasonable.

Cheers
Maurizio

On 27/09/2023 10:29, Glavo wrote:
> See MethodHandle::invokeWithArguments.
>
> Glavo
>
> On Wed, Sep 27, 2023 at 5:14 PM tison <wander4096 at gmail.com> wrote:
>
>     I'm using OpenJDK 21 and prototyping FFM APIs.
>
>     Instead of passing the MethodHandle returned
>     by linker.downcallHandle here and there, I'm trying to wrap it
>     into a NativeMethod class:
>
>     public final class NativeMethod {
>         private final String name;
>         private final FunctionDescriptor descriptor;
>         private final MethodHandle handle;
>         public Object invoke(Object... args) throws Throwable {
>             return this.handle.invoke(args);
>         }
>     }
>
>     But it seems the varargs pass through is not quit fluent:
>
>     java.lang.invoke.WrongMethodTypeException: cannot convert
>     MethodHandle(int)int to (Object[])Object
>
>     at
>     java.base/java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:903)
>     at
>     java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:870)
>     at
>     java.base/java.lang.invoke.Invokers.checkGenericType(Invokers.java:541)
>     at
>     io.montumi.datafusion.core.NativeMethod.invoke(NativeMethod.java:22)
>
>     While if I public the handle and call invoke from the handle
>     directly, all the behavior is expected.
>
>     Best,
>     tison.
>
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