DefineClass.java

package jni;
import jdk.incubator.foreign.CLinker;
import jdk.incubator.foreign.FunctionDescriptor;
import jdk.incubator.foreign.LibraryLookup;
import jdk.incubator.foreign.MemoryAccess;
import jdk.incubator.foreign.MemoryAddress;
import jdk.incubator.foreign.MemoryLayout;
import jdk.incubator.foreign.MemorySegment;
import jdk.incubator.foreign.NativeScope;

import static java.lang.invoke.MethodType.methodType;
import static jdk.incubator.foreign.CLinker.*;
import static jdk.incubator.foreign.MemoryAddress.NULL;

import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class ClassDefiner {
    public static void main(String[] args) throws Throwable {
        MemoryAddress vm;
        try (var ns = NativeScope.unboundedScope()) {
            var JNI_GetCreatedJavaVMs = CLinker.getInstance().downcallHandle(
                    LibraryLookup.ofDefault().lookup("JNI_GetCreatedJavaVMs").orElseThrow(),
                    methodType(int.class, MemoryAddress.class, int.class, MemoryAddress.class),
                    FunctionDescriptor.of(C_INT, C_POINTER, C_INT, C_POINTER));
            var vmPtr = ns.allocate(C_POINTER);
            int result = (int) JNI_GetCreatedJavaVMs.invokeExact(vmPtr.address(), 1, NULL);
            if (result != 0) throw new Exception("JNI Error: " + result);
            vm = MemoryAccess.getAddress(vmPtr);
        }
        MemoryAddress env;
        try (var ns = NativeScope.unboundedScope()) {
            var vmMethodCount = 8;
            var vmTable = MemoryAccess.getAddress(vm.asSegmentRestricted(C_POINTER.byteSize()));
            var vmTableSeg = vmTable.asSegmentRestricted(MemoryLayout.ofSequence(vmMethodCount, C_POINTER).byteSize());
            var GetEnvIDX = 6;
            var GetEnv = CLinker.getInstance().downcallHandle(MemoryAccess.getAddressAtIndex(vmTableSeg, 6),
                    methodType(int.class, MemoryAddress.class, MemoryAddress.class, int.class),
                    FunctionDescriptor.of(C_INT, C_POINTER, C_POINTER, C_INT));
            var envPtr = ns.allocate(C_POINTER);
            int JNI_VERSION_10 = 0x000a0000; // 10
            int result = (int) GetEnv.invokeExact(vm, envPtr.address(), JNI_VERSION_10);
            if (result != 0) throw new Exception("JNI Error: " + result);
            env = MemoryAccess.getAddress(envPtr);
        }
        try (var ns = NativeScope.unboundedScope()) {
            var envMethodCount = 234;
            var envTable = MemoryAccess.getAddress(env.asSegmentRestricted(C_POINTER.byteSize()));
            var envTableSeg = envTable.asSegmentRestricted(MemoryLayout.ofSequence(envMethodCount, C_POINTER).byteSize());
            var DefineClassIDX = 5;
            var DefineClass = CLinker.getInstance().downcallHandle(MemoryAccess.getAddressAtIndex(envTableSeg, DefineClassIDX),
                    methodType(MemoryAddress.class, MemoryAddress.class, MemoryAddress.class, MemoryAddress.class, MemoryAddress.class, int.class),
                    FunctionDescriptor.of(C_POINTER, C_POINTER, C_POINTER, C_POINTER, C_POINTER, C_INT));
            var name = CLinker.toCString("java/lang/Example", StandardCharsets.UTF_8, ns);
            var bytes = classFile();
            var bytesPtr = ns.allocate(bytes.length);
            bytesPtr.copyFrom(MemorySegment.ofArray(bytes));
            MemoryAddress result = (MemoryAddress) DefineClass.invokeExact(env, name.address(), NULL, bytesPtr.address(), bytes.length);
            if (result.equals(NULL)) throw new Exception("JNI Error: " + result);
        }
        System.out.println(Class.forName("java.lang.Example"));
    }

    private static byte[] classFile() {
        return Base64.getDecoder().decode("yv66vgAAADsABQEAEWphdmEvbGFuZy9FeGFtcGxlBwABAQAQamF2YS9sYW5nL09iamVjdAcAAwAhAAIABAAAAAAAAAAA");
    }
}