JNI Hello World on macOS

README.md

javac -h . NavitiveMethodsClass.java  # generates header
javac NavitiveMethodsClass.java Main.java
gcc -shared -I /opt/homebrew/Cellar/openjdk/19.0.2/include -o libNativeMethodsClass.jnilib NativeMethodsClass.c
java Main

The shared library must have a lib prefix and .jnilib extension on macOS.

Main.java

public class Main {

  public static void main(String[] args) {
    NativeMethodsClass cl = new NativeMethodsClass();
    cl.nonNative();
    cl.nativeMethod();
  }
}

NativeMethodsClass.c

#include <stdio.h>
#include "NativeMethodsClass.h"

JNIEXPORT void JNICALL
Java_NativeMethodsClass_nativeMethod(
  JNIEnv* env,
  jobject thisObj
) {
  printf("Hello from C, now get rekt son.\n");
}

NativeMethodsClass.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class NativeMethodsClass */

#ifndef _Included_NativeMethodsClass
#define _Included_NativeMethodsClass
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     NativeMethodsClass
 * Method:    nativeMethod
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_NativeMethodsClass_nativeMethod
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

NavitiveMethodsClass.java

class NativeMethodsClass {
  static {
    System.loadLibrary("NativeMethodsClass");
  }

  public void nonNative() {
    System.out.println("Non native method");
  }

  public native void nativeMethod();
}