Android – Invoke JNI based methods (Bridging C/C++ and Java)

Here’s a screen shot

Step By Step Instructions below:

Step #1: Compile the project using ant. Some snippets are shown below.

NativeAdd.java class (with Native method)

package org.apache;

import android.util.Log;

public class NativeAdd {
    static {
        try {
            Log.i("JNI", "Trying to load libNativeAdd.so");
            System.loadLibrary("NativeAdd");
        }
        catch (UnsatisfiedLinkError ule) {
            Log.e("JNI", "WARNING: Could not load libNativeAdd.so");
        }
    }

    public static native long add(long a, long b);
}

Snippet that shows how this class/method is invoked from the main activity

    public void onClick(View view) {
        Log.i(LOG_TAG, "onClick");

        EditText a = (EditText) findViewById(R.id.a);
        EditText b = (EditText) findViewById(R.id.b);
        EditText c = (EditText) findViewById(R.id.c);
        Log.i(LOG_TAG, "calling native method");
        long sum = NativeAdd.add(Long.parseLong(a.getText().toString()),
                Long.parseLong(b.getText().toString()));
        Log.i(LOG_TAG, "back from native method");
        String text = Long.toString(sum);
        c.setText("Native add returns = " + text.subSequence(0, text.length()));
    }

Step #2: Generate header files using java by running the following command.

javah -classpath ../../android.jar;../bin/classes; org.apache.NativeAdd

Here’s how the header file (org_apache_CallNative.h) looks like

/* DO NOT EDIT THIS FILE - it is machine generated */
#include
/* Header for class org_apache_NativeAdd */

#ifndef _Included_org_apache_NativeAdd
#define _Included_org_apache_NativeAdd
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_apache_NativeAdd
 * Method:    add
 * Signature: (JJ)J
 */
JNIEXPORT jlong JNICALL Java_org_apache_NativeAdd_add
  (JNIEnv *, jclass, jlong, jlong);

#ifdef __cplusplus
}
#endif
#endif

Step #3: Code a tiny C file (org_apache_NativeAdd.c) as shown below:

#include "org_apache_NativeAdd.h"

JNIEXPORT jlong JNICALL Java_org_apache_NativeAdd_add
  (JNIEnv *env, jclass c, jlong a, jlong b)
{
    return a + b;
}

Step #4: Please read the following article before you try the next step.
Shared library “Hello World!” for Android

Step #5: Compile and link the org_apache_NativeAdd.c/org_apache_NativeAdd.h into a shared library.

arm-none-linux-gnueabi-gcc  -I/usr/lib/jvm/java-1.5.0-sun/include -I/usr/lib/jvm/java-1.5.0-sun/include/linux  -fpic -c org_apache_NativeAdd.c
arm-none-linux-gnueabi-ld -T armelf_linux_eabi.xsc -shared -o libNativeAdd.so org_apache_NativeAdd.o

NOTE: details on armelf_linux_eabi.xsc are in the Step #4.

Step #6: copy the library to the emulator. Copy the Application as well.
adb push native/libNativeAdd.so /system/lib
adb install bin/CallNative.apk

Step #7: Run the app to try some values. That's it!
Download the sources and Application - CallNative.zip