Get IMPL_LOOKUP using FFI via JNI

ImplLookupGetter.java

import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.lang.reflect.Field;

public class ImplLookupGetter {
    public static final MethodHandles.Lookup IMPL_LOOKUP;
    public static final int JNI_VERSION_21  = 0x00150000;

    private static final Linker LINKER = Linker.nativeLinker();
    private static final AddressLayout pp = ValueLayout.ADDRESS.withTargetLayout(ValueLayout.ADDRESS);// void**
    private static final AddressLayout JavaVM = ValueLayout.ADDRESS.withTargetLayout(MemoryLayout.sequenceLayout(8, ValueLayout.ADDRESS));
    private static final VarHandle GET_FUNCTION_AT = ValueLayout.ADDRESS.withTargetLayout(
            MemoryLayout.sequenceLayout(235, ValueLayout.ADDRESS)
    ).varHandle(
            MemoryLayout.PathElement.dereferenceElement(),
            MemoryLayout.PathElement.sequenceElement()
    );
    private static final MethodHandle P_PP = LINKER.downcallHandle(FunctionDescriptor.of(
            ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS
    ));
    private static final MethodHandle V_PP = LINKER.downcallHandle(FunctionDescriptor.ofVoid(
            ValueLayout.ADDRESS, ValueLayout.ADDRESS
    ));
    private static final MethodHandle P_PPPP = LINKER.downcallHandle(FunctionDescriptor.of(
            ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS
    ));
    private static final MethodHandle V_PPPP = LINKER.downcallHandle(FunctionDescriptor.ofVoid(
            ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.ADDRESS
    ));

    private static void handleResult(int result) {
        if (result != 0) throw new RuntimeException("JNI error occurred. Error code: " + result);
    }
    private static MemorySegment getFunctionAt(MemorySegment env, int index) {
        return (MemorySegment) GET_FUNCTION_AT.get(env, 0, index);
    }
    /** jclass FindClass(JNIEnv *env, const char *name); */
    private static MemorySegment FindClass(MemorySegment env, MemorySegment name) throws Throwable {
        return (MemorySegment) P_PP.invokeExact(getFunctionAt(env, 6), env, name);
    }
    /** jobject NewGlobalRef(JNIEnv *env, jobject obj); */
    private static MemorySegment NewGlobalRef(MemorySegment env, MemorySegment obj) throws Throwable {
        return (MemorySegment) P_PP.invokeExact(getFunctionAt(env, 21), env, obj);
    }
    /** void DeleteGlobalRef(JNIEnv *env, jobject globalRef); */
    private static void DeleteGlobalRef(MemorySegment env, MemorySegment globalRef) throws Throwable {
        V_PP.invokeExact(getFunctionAt(env, 22), env, globalRef);
    }
    /** jmethodID GetMethodID(JNIEnv *env, jclass clazz, const char *name, const char *sig); */
    private static MemorySegment GetMethodID(MemorySegment env, MemorySegment clazz, MemorySegment name, MemorySegment sig) throws Throwable {
        return (MemorySegment) P_PPPP.invokeExact(getFunctionAt(env, 33), env, clazz, name, sig);
    }
    /** void CallVoidMethodA(JNIEnv *env, jobject obj, jmethodID methodID, const jvalue *args); */
    private static void CallVoidMethodA(MemorySegment env, MemorySegment obj, MemorySegment methodID, MemorySegment args) throws Throwable {
        V_PPPP.invokeExact(getFunctionAt(env, 63), env, obj, methodID, args);
    }
    /** jobject GetModule(JNIEnv *env, jclass clazz); */
    private static MemorySegment GetModule(MemorySegment env, MemorySegment clazz) throws Throwable {
        return (MemorySegment) P_PP.invokeExact(getFunctionAt(env, 233), env, clazz);
    }
    /** jstring NewStringUTF(JNIEnv *env, const char *bytes); */
    private static MemorySegment NewStringUTF(MemorySegment env, MemorySegment bytes) throws Throwable {
        return (MemorySegment) P_PP.invokeExact(getFunctionAt(env, 167), env, bytes);
    }

    static {
        try (Arena arena = Arena.ofConfined()) {
            MethodHandle JNI_GetCreatedJavaVMs = LINKER.downcallHandle(
                    // jint JNI_GetCreatedJavaVMs(JavaVM **vmBuf, jsize bufLen, jsize *nVMs);
                    SymbolLookup.libraryLookup(System.mapLibraryName("jvm"), arena).find("JNI_GetCreatedJavaVMs").orElseThrow(),
                    FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.ADDRESS, ValueLayout.JAVA_INT, ValueLayout.ADDRESS)
            );
            var ppVM = arena.allocate(ValueLayout.ADDRESS);// JavaVM **
            handleResult((int) JNI_GetCreatedJavaVMs.invokeExact(ppVM, 1, MemorySegment.NULL));
            MemorySegment vm = ppVM.get(pp, 0);// JavaVM*
            MethodHandle GetEnv = LINKER.downcallHandle(
                    // jint GetEnv(JavaVM *vm, void **p_env, jint version);
                    vm.get(JavaVM, 0).getAtIndex(ValueLayout.ADDRESS, 6),
                    FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.ADDRESS, ValueLayout.ADDRESS, ValueLayout.JAVA_INT)
            );
            var pEnv = arena.allocate(ValueLayout.ADDRESS);// JNIEnv **
            handleResult((int) GetEnv.invokeExact(vm, pEnv, JNI_VERSION_21));
            var env = pEnv.get(pp, 0);// JNIEnv *

            var moduleClass = NewGlobalRef(env, FindClass(env, arena.allocateFrom("java/lang/Module")));
            var module = NewGlobalRef(env, GetModule(env, moduleClass));
            var implAddOpens = GetMethodID(env, moduleClass, arena.allocateFrom("implAddOpens"), arena.allocateFrom("(Ljava/lang/String;)V"));
            MemorySegment arg0 = NewGlobalRef(env, NewStringUTF(env, arena.allocateFrom("java.lang.invoke")));
            MemorySegment args = arena.allocate(ValueLayout.ADDRESS, 1);// 1 arg
            args.setAtIndex(ValueLayout.ADDRESS, 0, arg0);
            CallVoidMethodA(env, module, implAddOpens, args);
            DeleteGlobalRef(env, arg0);
            DeleteGlobalRef(env, module);
            DeleteGlobalRef(env, moduleClass);
            Field implLookupField = MethodHandles.Lookup.class.getDeclaredField("IMPL_LOOKUP");
            implLookupField.setAccessible(true);
            IMPL_LOOKUP = (MethodHandles.Lookup) implLookupField.get(null);
        } catch (Throwable e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String[] args) {
        System.out.println(IMPL_LOOKUP);
    }
}

cleaner version of https://gist.github.com/burningtnt/c188e65f048c2cf096db095e5858b5af