C++ Scala JNI - Java Native Interface - Example - Calling C++ from Scala / Java

load.scala

// Author: Caio Rodrigues
// Description: Scala/Java Client Code
//
// On Unix* loads libTest.so in LD_LIBRARY_PATH

object NUtils{
   def getPathToApp(implicit cls: Class[_]): String = {
    import java.io.File 
    val rawPath = cls
      .getProtectionDomain()
      .getCodeSource()
      .getLocation().toExternalForm()
    if(System.getProperty("os.name").toLowerCase().contains("windows"))
      // Windows 
      new File(rawPath.stripPrefix("file:/")).getParent.replace("\\", "/")
    else
      // Unix - Linux, MacOSX, BSD variants and etc.
      new File(rawPath.stripPrefix("file:")).getParent
   }

  def loadDLL(library: String)(implicit cls: Class[_]) =
    System.load(NUtils.getPathToApp + "/" + library)
}

class TestJNI{
  init()
  private def init(){
    //System.loadLibrary("TestJNI")
    NUtils.loadDLL("libTestJNI.so")(getClass())
  }  
  @native def printDisplay(): Unit
  @native def getVersion(): String
  @native def vector3DLen(a: Double, b: Double, c: Double): Double
  @native def average(xs: Array[Double]): Double
}

object Main{
  def main(args: Array[String]) = {
    println("dir = " + System.getProperty("user.dir") + "/" + "libTestJNI.so")
    val t = new TestJNI()
    // val t = new TestJNI()
    t.printDisplay()
    println("Library version        = " + t.getVersion())
    println("vector3DLen(3, 4, 5)   = " + t.vector3DLen(3, 4, 5))    
    println("Average of 1, 2 ... 8  = " + t.average(Array(1, 2, 3, 4, 5, 6, 7, 8)))
  }

}


// val cls = new dummy.com.MyClass()
// cls.printDisplay()

Running.txt

Compile the C++ code creating a shared library (or shared object in UNIX)

$ clang++ TestJNI.cpp -o libTestJNI.so -fPIC -shared -std=c++11 -I$HOME/opt/java/include -I$HOME/opt/java/include/linux

Run the application

$ scala -save load.scala 
dir = /home/archbox/opengl/jni/libTestJNI.so
Hello world java
i = 0
i = 1
i = 2
i = 3
i = 4
Library version        = version 3.4
vector3DLen(3, 4, 5)   = 7.0710678118654755
Average of 1, 2 ... 8  = 4.5

$ scala -save load.jar 
dir = /home/archbox/opengl/jni/libTestJNI.so
Hello world java
i = 0
i = 1
i = 2
i = 3
i = 4
Library version        = version 3.4
vector3DLen(3, 4, 5)   = 7.071

Check compilation output 

$ file libTestJNI.so 
libTestJNI.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=6a21fdf13b652757f1773db2069095eaa050b759, not stripped

Check symbols 

$ nm -D libTestJNI.so | grep " T "
0000000000000f08 T _fini
0000000000000a28 T _init
0000000000000d40 T Java_TestJNI_average
0000000000000cc0 T Java_TestJNI_getVersion
0000000000000c20 T Java_TestJNI_printDisplay
0000000000000cf0 T Java_TestJNI_vector3DLen
0000000000000e00 T main

TestJNI.cpp

// Author: Caio Rodrigues
// Description: Example showing how to call C++ code from Scala/Java
//
#include <iostream>
#include <vector>
#include <jni.h>
#include <cmath>

#define JEXPORT_VOID(name, method, ret) extern "C" JNIEXPORT ret JNICALL \
	Java_ ## name## _ ## method (JNIEnv* env, jobject obj)

// JNI NAME RULE:  Java_[package-name]_[class_name]_[exported function]
//   - JNIEnv* => Pointer to the VM
//   - jobject => Pointer to current object (or this*)
#define JEXPORT(name, method, ret, ...) extern "C" JNIEXPORT ret JNICALL \
	Java_ ## name## _ ## method (JNIEnv* env, jobject obj, ##__VA_ARGS__)

/** Generates: 
   extern "C" JNIEXPORT ret JNICALL Java_TestJNI_printDisplay
 */
JEXPORT_VOID(TestJNI, printDisplay, void){
	std::cout << "Hello world java" << std::endl;
	for (int i = 0; i < 5; i++){
		std::cout << "i = " << i << std::endl;
	}
}

JEXPORT(TestJNI, getVersion, jstring){
	return 	env->NewStringUTF("version 3.4");
}

// extern "C" JNIEXPORT jdouble JNICALL Java_TestJNI_vector3DLen(
// 	JNIEnv*, jobject, jdouble a, jdouble b, jdouble c){
// 	return 	sqrt(a * a + b * b + c * c);
// }
JEXPORT(TestJNI, vector3DLen, jdouble, jdouble a, jdouble b, jdouble c){
	return 	sqrt(a * a + b * b + c * c);
}


// extern "C" JNIEXPORT jdouble JNICALL Java_TestJNI_average(
JEXPORT(TestJNI, average, double, jdoubleArray xs){
	jdouble* ptr = env->GetDoubleArrayElements(xs, nullptr);
	if(ptr == NULL)
		return 0.0;
	jsize n = env->GetArrayLength(xs);
	jdouble sum = 0.0;
	for(int i = 0; i < n; i++){
		sum += ptr[i];
	}
	return sum / n;
}

extern "C" int main(){
	std::cout << "Hello world" << std::endl;
	return 0;
}