Creating a simple Hello World application Using Android NDK with C++

01.step1.md

Prerequisites

1. I am using GNU/Linux
2. My IDE is IntelliJ IDEA
3. The project is Ant based.
4. My working directory is $JNI
5. My C++ IDE is QtCreator (I'll mention the build steps inline).
6. NDK should be installed and ndk-build should be available in $PATH
7. javah should be available in $PATH

Step 1 - Create The Android Project

1. Create and Empty Project Named JNI (at location $JNI)
2. Create a new Android application module named JNIHelloWorld.
3. Package Name - com.jnisample.JNIHelloWorld
4. Create one activity JNIActivity and define one native function getNativeMessage()
5. Build the project

02.JNIActivity.java

package com.jnisample.JNIHelloWorld;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import org.w3c.dom.Text;

public class JNIActivity extends Activity {

    public native String getNativeMessage();

    static {
        System.loadLibrary("mynative");
    }

    Button btnInvokeJNI;
    TextView tvNativeMsg;

    /**
     * Called when the activity is first created.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        btnInvokeJNI = (Button) findViewById(R.id.btnInvokeJni);
        tvNativeMsg = (TextView) findViewById(R.id.txtJniText);

        btnInvokeJNI.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                tvNativeMsg.setText(getNativeMessage());
            }
        });
    }
}

03.step2.md

Step 2 - Create the JNI Header file

1. Create a folder jni inside $JNI/JNIHelloWorld
2. In Terminal(command line) move to $JNI/out/production/JNIHelloWorld directory (This is where the java files get compiled to class files by IDEA; Verify the directory structure is same as the package name com/jnisample/JNIHelloWorld/JNIActivity.class)
3. Create the header for the native function using javah

$ javah $JNI/JNIHelloWorld/jni com.jnisample.JNIHelloWorld.JNIActivity

The Above command will generate a header file with name com_jnisample_JNIHelloWorld_JNIActivity.h inside the jni directory.

04.com_jnisample_JNIHelloWorld_JNIActivity.h

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

#ifndef _Included_com_jnisample_JNIHelloWorld_JNIActivity
#define _Included_com_jnisample_JNIHelloWorld_JNIActivity
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_jnisample_JNIHelloWorld_JNIActivity
 * Method:    getNativeMessage
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_jnisample_JNIHelloWorld_JNIActivity_getNativeMessage
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

05.step3.md

Step 3 - Add a C++ Class

1. Create a C++ class with name HelloWorld
2. Create define a member function getNativeString() which returns a string

06.helloworld.h

#ifndef HELLOWORLD_H
#define HELLOWORLD_H

#include <string>

using namespace std;

class HelloWorld
{
public:
    HelloWorld();
    string getNativeString();
};

#endif // HELLOWORLD_H

07.helloworld.cpp

#include "helloworld.h"

HelloWorld::HelloWorld()
{
}

string HelloWorld::getNativeString()
{
    return "Hello World from JNI";
}

08.step4.md

Step 4 - Implement JNI Entry (or Entries)

Implement the entry method com_jnisample_JNIHelloWorld_JNIActivity_getNativeMessage defined in header com_jnisample_JNIHelloWorld_JNIActivity.h

09.jni_entry.cpp

#include "com_jnisample_JNIHelloWorld_JNIActivity.h"
#include "helloworld.h"

JNIEXPORT jstring JNICALL Java_com_jnisample_JNIHelloWorld_JNIActivity_getNativeMessage
  (JNIEnv* pEnv, jobject pThis)
{
    HelloWorld hello;
    return pEnv->NewStringUTF(hello.getNativeString().c_str());
}

10.step5.md

Step 5 - Configure make

Define Android.mk and Application.mk

11.Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := mynative
LOCAL_SRC_FILES := helloworld.cpp jni_entry.cpp

include $(BUILD_SHARED_LIBRARY)

12.Application.mk

# This variable enables the inclusion of STL classes
APP_STL := stlport_static

13.step6.md

Step 6 - Build Shared Object

1. In Terminal, move to directory $JNI/JNIHelloWorld
2. Build the Shared Object using the command ndk-build

$ ndk-build

On successful build, the Shared Object file libmynative.so will be copied to $JNI/JNIHelloWorld/libs/armeabi

Step 7 - Build APK

1. Build the android project in IntelliJ IDEA
2. Deploy and Run

nice work

Thank you for sharing, it was really helpful 👍
Just one thing: it seems that you are missing the
#include <jni.h>
declaration on the jni_entry.cpp file.

Hi,
I've a problem with string included.

helloworld.h:3:10: fatal error: 'string' file not found

My Application.mk
APP_STL := stlport_static

Hi,
I've a problem with string included.

helloworld.h:3:10: fatal error: 'string' file not found

My Application.mk
APP_STL := stlport_static

Solved. I make a directory "jni" in application folder and put the *.mk. Also, I downloaded the new stable Android NDK.

Thanks