Ndk 初步開發 In Android

Android NDK 簡單來說就是可以讓你的應用使用natice-code語言, （比如C ,C++)的工具集

一：what is the ndk

Ndk 是允許你在你的工程中可以使用native-code (比如C or c++)的工具, 你可以利用ndk來構建你的natice-app
正如我們知道的那樣, Java 是構建android app 的預設語言, 然而, java 程式碼很容易被反編譯, 但是這對C++ code 來說這並不簡單, 並且c++ 程式碼更加高效

二：why is ndk

C++ 程式碼反編譯困難

對於計算複雜量大的程式碼, C or C++ code can increase the performance of your app

三：buliding native app

如果你使用android studio 開發, 你需要下載安裝ndk包, More info: NDK

1, 首先在程式碼中宣告要在C++ code中要實現的native方法

/**
 * auth:huanjulu
 */
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ((TextView) findViewById(R.id.text)).setText(nativeMethod());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
    /**
     * native code
     *
     * @return
     */
    public native String nativeMethod();

}複製程式碼

2, 對包含native 關鍵字方法的類, 生成.h 標頭檔案

進入到本地Java資料夾下 ,在Terminal input the follow commond

 javah -d ../jni jniproject.ndkproject.MainActivity複製程式碼

first , jniproject.ndkproject is your package name and MainActivity is the class name that contains native code

and after that , you can see the .h file of MainActivity class , it looks like :

and we can looks what contents it contains

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

#ifndef _Included_jniproject_ndkproject_MainActivity
#define _Included_jniproject_ndkproject_MainActivity
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     jniproject_ndkproject_MainActivity
 * Method:    nativeMethod
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_jniproject_ndkproject_MainActivity_nativeMethod
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif複製程式碼

3, android.mk``application.mk file

在你的jni directory, 新建android.mk,

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)

LOCAL_MODULE := hellojni
LOCAL_SRC_FILES := hellojni.cpp
include $(BUILD_SHARED_LIBRARY)複製程式碼

application.mk

APP_MODULES := hellojni  //生成的.so name

APP_ABI := all  //生成的.so 目標cpu平臺複製程式碼

if you want to know more information of syntax about them, here is the dox
android.mk/#intro

4, 編寫你的natie-code 實現檔案, (ig .cpp or .c)

#include "jniproject_ndkproject_MainActivity.h"

JNIEXPORT jstring JNICALL Java_jniproject_ndkproject_MainActivity_nativeMethod
       (JNIEnv *env, jclass type) {

    return env->NewStringUTF("this code from jni cpp");
}複製程式碼

as wo can see that in above c++ code

first , you need to include you .hfile

#include "jniproject_ndkproject_MainActivity.h"複製程式碼

接下來你需要按照c++ 程式碼的規範編寫實現方法, 請注意方法實現的命名規範 (JNIEXPORT 返回值型別 JNICALL 包名類名方法名)

5, ndk-build

under the jni dictionary, generate .so file

luhuanju:jni huanjulu$ ndk-build複製程式碼

接下來你需要把生成的app/src/main/libs 下的平臺的.so 檔案複製到 app/libs 下

6, app/build.gradle config seeting

apply plugin: `com.android.application`

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "jniproject.ndkproject"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"


        ndk {//指定生成的lib，比如此時生成native.so
            moduleName   "hellojni"
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile(`proguard-android.txt`), `proguard-rules.pro`
        }
    }
}

dependencies {
    compile fileTree(dir: `libs`, include: [`*.jar`])
    androidTestCompile(`com.android.support.test.espresso:espresso-core:2.2.2`, {
        exclude group: `com.android.support`, module: `support-annotations`
    })
    compile `com.android.support:appcompat-v7:25.2.0`
    compile `com.android.support.constraint:constraint-layout:1.0.0-beta4`
    compile `com.android.support:design:25.2.0`
    testCompile `junit:junit:4.12`
}複製程式碼

the the last step is that you just need call you natice-code library

package jniproject.ndkproject;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.widget.TextView;


/**
 * auth:huanjulu
 */

public class MainActivity extends AppCompatActivity {


    static {
        System.loadLibrary("hellojni");
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ((TextView) findViewById(R.id.text)).setText(nativeMethod());
    }
    /**
     * native code
     *
     * @return
     */
    public native String nativeMethod();

}複製程式碼

here is source code

github.com/isicout/JNI…