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

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

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 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

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 02.JNIActivity.java

	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());
	}
	});
	}
	}

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 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 jnidirectory.

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 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

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 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

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 06.helloworld.h

	#ifndef HELLOWORLD_H
	#define HELLOWORLD_H
	#include <string>
	using namespace std;
	class HelloWorld
	{
	public:
	HelloWorld();
	string getNativeString();
	};
	#endif // HELLOWORLD_H

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 07.helloworld.cpp

	#include "helloworld.h"
	HelloWorld::HelloWorld()
	{
	}
	string HelloWorld::getNativeString()
	{
	return "Hello World from JNI";
	}

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 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

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 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());
	}

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 10.step5.md

Step 5 - Configure make

Define Android.mk and Application.mk

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 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)

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 12.Application.mk

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

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 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

   I Am Not The Owner Of These Code .I Merely Have Copied Them From Various Sources. The Only Thing I Did Is That I Am Going To Present Them In More Easy Way To Understand.