NDK配置

Windows10 64
AndroidStudio Chipmunk | 2021.2.1 Patch 1

创建NDK工程之前，请先保证本地已经搭建好了NDK的相关环境。

依次选择【Tools】->【SDK Manager】下载配置NDK，如下所示。

ndk_1.jpg
新建一个Native C++工程，如下所示。

NDK2.jpg

NDK3.jpg

NDK4.jpg

NDK项目结构

NDK5.jpg

我们接下来看一下，Android的NDK工程和普通的Android应用工程有哪些不一样的地方。首先，我们来看下build.gradle配置。

plugins {
    id 'com.android.application'
    id 'org.jetbrains.kotlin.android'
}

android {
    compileSdk 32

    defaultConfig {
        applicationId "com.example.helloworldndk"
        minSdk 21
        targetSdk 32
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ''
            }
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
    externalNativeBuild {
        cmake {
            path file('src/main/cpp/CMakeLists.txt')
            version '3.18.1'
        }
    }
    buildFeatures {
        viewBinding true
    }
}

dependencies {

    implementation 'androidx.core:core-ktx:1.7.0'
    implementation 'androidx.appcompat:appcompat:1.4.2'
    implementation 'com.google.android.material:material:1.6.1'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.4'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

可以看到，相比普通的Android应用，build.gradle配置中多了两个externalNativeBuild配置项。其中，defaultConfig里面的的externalNativeBuild主要是用于配置Cmake的命令参数，而外部的
externalNativeBuild的主要是定义了CMake的构建脚本CMakeLists.txt的路径。

然后，我们来看一下CMakeLists.txt文件，CMakeLists.txt是CMake的构建脚本，作用相当于ndk-build中的Android.mk，代码如下。

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.18.1)

# Declares and names the project.

project("helloworldndk")

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
        helloworldndk

        # Sets the library as a shared library.
        SHARED

        # Provides a relative path to your source file(s).
        native-lib.cpp)

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
        log-lib

        # Specifies the name of the NDK library that
        # you want CMake to locate.
        log)

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
        helloworldndk

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

关于CMake的更多知识，可以查看CMake官方手册。

官方示例
默认创建Android NDK工程时，Android提供了一个简单的JNI交互示例，返回一个字符串给Java层，方法名的格式为：Java_包名类名方法名。首先，我们看一下native-lib.cpp的代码。

#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_helloworldndk_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

然后，我们在看一下Android的MainActivity.kt 的代码。

package com.example.helloworldndk

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import com.example.helloworldndk.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        // Example of a call to a native method
        binding.sampleText.text = stringFromJNI()
    }

    /**
     * A native method that is implemented by the 'helloworldndk' native library,
     * which is packaged with this application.
     */
    external fun stringFromJNI(): String

    companion object {
        // Used to load the 'helloworldndk' library on application startup.
        init {
            System.loadLibrary("helloworldndk")
        }
    }
}