JNI binding generation at runtime PoC

hello.c

#include <stdio.h>

void hello() {
    puts("Hello, world!");
}

RuntimeJNI.java

import org.objectweb.asm.*;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Comparator;
import java.util.stream.Stream;

import static org.objectweb.asm.Opcodes.*;

public class Main {

    public static void main(String[] args) throws Throwable {
        ClassWriter cw = new ClassWriter(0);
        cw.visit(49, ACC_PUBLIC + ACC_SUPER, "Test", null, "java/lang/Object", null);

        // public static native void hello();
        cw.visitMethod(ACC_PUBLIC + ACC_STATIC + ACC_NATIVE, "hello", "()V", null, null).visitEnd();

        // public static void loadLibrary(String lib) { System.load(lib); }
        MethodVisitor mv = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "loadLibrary", "(Ljava/lang/String;)V", null, null);
        mv.visitVarInsn(ALOAD, 0);
        mv.visitMethodInsn(INVOKESTATIC, "java/lang/System", "load", "(Ljava/lang/String;)V", false);
        mv.visitInsn(RETURN);
        mv.visitMaxs(1, 1);
        mv.visitEnd();

        cw.visitEnd();

        Path directory = Files.createTempDirectory("test");
        Files.write(directory.resolve("Test.class"), cw.toByteArray());

        String bindingCode = """
                                #include <jni.h>
                                extern void hello();
                                JNIEXPORT void JNICALL Java_Test_hello(JNIEnv* env, jclass class) {
                                    hello();
                                }""";

        String targetLib = new File("hello.so").getAbsolutePath();

        Files.writeString(directory.resolve("glue.c"), bindingCode);
        ProcessBuilder processBuilder = new ProcessBuilder("bash", "-c", "gcc -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux -Os -fPIC -shared glue.c -o binding.so " + targetLib);
        processBuilder.environment().put("JAVA_HOME", System.getProperty("java.home"));
        processBuilder.redirectErrorStream(true);
        processBuilder.redirectOutput(ProcessBuilder.Redirect.INHERIT);
        processBuilder.directory(directory.toFile());
        processBuilder.start().waitFor(); // TODO: Write native library directly in binary form

        try (URLClassLoader classLoader = new URLClassLoader(new URL[]{directory.toUri().toURL()})) {
            Class<?> nativeBinding = classLoader.loadClass("Test");
            nativeBinding.getMethod("loadLibrary", String.class)
                    .invoke(null, directory.resolve("binding.so").toFile().getAbsolutePath());

            nativeBinding.getMethod("hello").invoke(null);
        }

        try (Stream<Path> stream = Files.walk(directory)) {
            stream.sorted(Comparator.reverseOrder())
                    .map(Path::toFile)
                    .forEach(File::deleteOnExit);
        }
    }
}