JNI Example For Mac

JNI-Example-Mac.md

JNI (Java Native Interface) allows implementing methods in C/C++, and use them in Java.

0. Prepare

I use hombrew install java, java version is java 10.

$ java --version
java 10.0.2 2018-07-17
Java(TM) SE Runtime Environment 18.3 (build 10.0.2+13)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10.0.2+13, mixed mode)

1. Create JNIExample.java file

class JNIExample {

  // Native method, no body.
  public native void sayHello(int length);

  public static void main (String args[]) {
    String str = "Hello, world!";

    (new JNIExample()).sayHello(str.length());
  }

  // This loads the library at runtime. NOTICE: on *nix/Mac the extension of the
  // lib should exactly be `.jnilib`, not `.so`, and have `lib` prefix, i.e.
  // the library file should be `libjniexample.jnilib`.
  static {
    System.loadLibrary("jniexample");
  }
}

2. Compile the JNIExample.java file

This gives you JNIExample.class:

javac JNIExample.java

3. Codegen .h file for JNI

javac -h ./ JNIExample

NOTICE: Java 10 removed javah, we can use javac generate header file.

javah -jni JNIExample # java 8

This gives you JNIExample.h (do not edit it manually!):

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

#ifndef _Included_JNIExample
#define _Included_JNIExample
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     JNIExample
 * Method:    sayHello
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_JNIExample_sayHello
  (JNIEnv *, jobject, jint);

#ifdef __cplusplus
}
#endif
#endif

4. Create JNIExample.c

Native implementation:

#include <stdio.h>
#include <jni.h>
#include "JNIExample.h"

JNIEXPORT void JNICALL Java_JNIExample_sayHello
  (JNIEnv *env, jobject object, jint len) {
  printf("\nThe length of your string is %d.\n\n", len);
}

5. Compile C library

IMPORTANT: library extension should exactly be .jnilib (not .so!), and should have lib prefix, i.e. libjniexample.jnilib:

gcc -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/darwin/" -o libjniexample.jnilib -shared JNIExample.c

NOTICE: must set $JAVA_HOME :

export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-10.0.2.jdk/Contents/Home

6. Execute the java class

Notice, we set java.library.path to current directory, where the libjniexample.jnilib was compiled to:

java -Djava.library.path=. JNIExample

Result:

The length of your string is 13.