How to wrap Method handle behind a class?

[Remi Forax]

> From: "tison" <wander4096 at gmail.com>
> To: "panama-dev" <panama-dev at openjdk.org>
> Sent: Wednesday, September 27, 2023 11:59:08 AM
> Subject: Re: How to wrap Method handle behind a class?

> OT - perhaps I should start a new thread, but the first things come to me when I
> talk to the FFM APIs:

> 1. Manually defining MemoryLayout can be clumsy. C# has a structure layout
> attribute for generating struct/class marshalling[1]
> 2. I wonder if the new FFI calls can be JIT-ed. One of the major performance
> impact factor of JNI is that they treat the foreign method as black box and
> nothing optimize can be done =。=

> These are only rough ideas and I'm glad to know where I can start to learn the
> related work and previous discussions.

Did you know that you can use jextract [1], that generates all the MethodHandle / structure layout for you ? 

> Best,
> tison.

regards, 
Rémi Forax 

[1] https://github.com/openjdk/jextract 

> [1] [
> https://learn.microsoft.com/en-us/dotnet/standard/native-interop/type-marshalling
> |
> https://learn.microsoft.com/en-us/dotnet/standard/native-interop/type-marshalling
> ]

> tison < [ mailto:wander4096 at gmail.com | wander4096 at gmail.com ] > 于2023年9月27日周三
> 17:53写道：

>> Thanks for your quick response and suggestions! I realize that using static
>> final MethodHandle and call invoke/invokeExact inside the wrapper class should
>> be better. While the downside is I may have to duplicate some code like:

>> private static final MethodHandle xxxNativeMethod = ...;
>> public static /* native */ ReturnType xxx(T1 t1, T2 t2, ...) {
>> // validate
>> return xxxNativeMethod.invoke(t1, t2, ...); // with return type coercion and
>> exception handling
>> }

>> I'll try to do it and share it openly when I make an MVP.

>> FWIW, in the past months, I built a Java binding for a Rust lib named OpenDAL[1]
>> using JNI since it requires JDK 8 compatibility. But this time I'd like to try
>> out Datafusion with the new FFM APIs for fun :D

>> Best,
>> tison.

>> [ 1] [ https://github.com/apache/incubator-opendal/tree/main/bindings/java |
>> https://github.com/apache/incubator-opendal/tree/main/bindings/java ]

>> Maurizio Cimadamore < [ mailto:maurizio.cimadamore at oracle.com |
>> maurizio.cimadamore at oracle.com ] > 于2023年9月27日周三 17:43写道：

>>> 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 < [ mailto:wander4096 at gmail.com |
>>>> 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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