Calling C methods from Java

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Static and member methods in Java can be marked as native to indicate that their implementation is to be found in a shared library file. Upon execution of a native method, the JVM looks for a corresponding function in loaded libraries (see Loading native libraries), using a simple name mangling scheme, performs argument conversion and stack setup, then hands over control to native code.

Java code

/*** com/example/jni/JNIJava.java **/

package com.example.jni;

public class JNIJava {
    static {
        System.loadLibrary("libJNI_CPP");
    }

    // Obviously, native methods may not have a body defined in Java
    public native void printString(String name);
    public static native double average(int[] nums);

    public static void main(final String[] args) {
        JNIJava jniJava = new JNIJava();
        jniJava.printString("Invoked C++ 'printString' from Java");

        double d = average(new int[]{1, 2, 3, 4, 7});
        System.out.println("Got result from C++ 'average': " + d);
    }
}

C++ code

Header files containing native function declarations should be generated using the javah tool on target classes. Running the following command at the build directory :

javah -o com_example_jni_JNIJava.hpp com.example.jni.JNIJava

… produces the following header file (comments stripped for brevity) :

// com_example_jni_JNIJava.hpp

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h> // The JNI API declarations

#ifndef _Included_com_example_jni_JNIJava
#define _Included_com_example_jni_JNIJava
#ifdef __cplusplus
extern "C" { // This is absolutely required if using a C++ compiler
#endif

JNIEXPORT void JNICALL Java_com_example_jni_JNIJava_printString
  (JNIEnv *, jobject, jstring);

JNIEXPORT jdouble JNICALL Java_com_example_jni_JNIJava_average
  (JNIEnv *, jclass, jintArray);

#ifdef __cplusplus
}
#endif
#endif

Here is an example implementation :

// com_example_jni_JNIJava.cpp

#include <iostream>
#include "com_example_jni_JNIJava.hpp"

using namespace std;

JNIEXPORT void JNICALL Java_com_example_jni_JNIJava_printString(JNIEnv *env, jobject jthis, jstring string) {
    const char *stringInC = env->GetStringUTFChars(string, NULL);
    if (NULL == stringInC)
        return;
    cout << stringInC << endl;
    env->ReleaseStringUTFChars(string, stringInC);
}

JNIEXPORT jdouble JNICALL Java_com_example_jni_JNIJava_average(JNIEnv *env, jclass jthis, jintArray intArray) {
    jint *intArrayInC = env->GetIntArrayElements(intArray, NULL);
    if (NULL == intArrayInC)
        return -1;
    jsize length = env->GetArrayLength(intArray);
    int sum = 0;
    for (int i = 0; i < length; i++) {
        sum += intArrayInC[i];
    }
    env->ReleaseIntArrayElements(intArray, intArrayInC, 0);
    return (double) sum / length;
}

Output

Running the example class above yields the following output :

Invoked C++ ‘printString’ from Java

Got result from C++ ‘average’: 3.4

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