红橙Darren视频引申第一次写NDK项目（Android studio 4.1.1）

1.第一次创建NDK项目

使用Android studio创建NDK的步骤如下：
打开Android Studio中的File->Settings->Android SDK
打开SDK Tools tab页选择NDK和CMake下载

下载完毕之后看看项目的NDK路径有没有识别如果识别即可创建C++项目
下载完毕之后点击 invalidate Caches/Restart 重启Android Studio

创建完毕后等待sync结束即可运行项目可以从c++层获取到一个string 如果编译不成功，去项目的local.properties查看ndk的路径是否指定如果没有指定则指定为下载好的ndk路径
Android Studio不同的版本下载的ndk路径不一样较老的版本似乎下载到
C:\Users<username>\AppData\Local\Android\Sdk
android 4.1.1的话则下载到指定sdk的路径去了
我的ndk路径如下

2.将已有的普通项目转变为NDK project

如果一个项目已经创建完毕那么我们怎么让他变成支持NDK的项目呢

2.1.指定ndk路径

打开项目的local.properties文件加入ndk的路径

sdk.dir=C\:\\codebase\\android-sdk_60_windows
ndk.dir=C\:\\codebase\\android-sdk_60_windows\\ndk\\23.0.7272597

然后使用as中的invalid/restart 选项重启

重启完了进入项目中查看是否修改成功

如图所示即修改成功

2.2 修改moudle的build.gradle文件

加入两个节点（externalNativeBuild和externalNativeBuild）如下注释部分

plugins {id 'com.android.application'
}android {compileSdkVersion 29buildToolsVersion "30.0.3"defaultConfig {applicationId "com.example.testrecyclerview"minSdkVersion 24targetSdkVersion 29versionCode 1versionName "1.0"testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"//1 加入该节点externalNativeBuild {cmake {cppFlags ""}}}buildTypes {release {minifyEnabled falseproguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'}}//2 加入该节点externalNativeBuild {cmake {path "src/main/cpp/CMakeLists.txt"version "3.10.2"}}compileOptions {sourceCompatibility JavaVersion.VERSION_1_8targetCompatibility JavaVersion.VERSION_1_8}
}dependencies {implementation 'androidx.appcompat:appcompat:1.2.0'implementation 'com.google.android.material:material:1.3.0'implementation 'androidx.constraintlayout:constraintlayout:2.0.4'testImplementation 'junit:junit:4.+'androidTestImplementation 'androidx.test.ext:junit:1.1.2'androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'implementation 'com.github.bumptech.glide:glide:4.12.0'annotationProcessor 'com.github.bumptech.glide:compiler:4.12.0'
}

2.3 创建编译文件

在src/main下创建cpp/CMakeLists.txt
内容如下

# 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 3.10.2)# Declares and names the project.project("testRecyclerView")# 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.# so文件与c++文件的关联 如下这段则是native-lib.so 关联 native-lib.cpp
add_library( # Sets the name of the library.native-lib# Sets the library as a shared library.SHARED# Provides a relative path to your source file(s).native-lib.cpp )# 以下add_library是另外加的 为了测试添加一个新的c++文件
add_library( # Sets the name of the library.native-mylib# Sets the library as a shared library.SHARED# Provides a relative path to your source file(s).native-mylib.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.# 最终用到哪些so文件 既可以是自己编译的 也可以是第三方so库 也可以是系统的so库
# 这里我指定了两个so库
target_link_libraries( # Specifies the target library.native-libnative-mylib# Links the target library to the log library# included in the NDK.${log-lib} )

2.4 创建c++文件

native-lib.cpp

#include <jni.h>
#include <string>extern "C" JNIEXPORT jstring JNICALL
Java_com_example_testrecyclerview_MainActivity_stringFromJNI(JNIEnv *env,jobject /* this */) {std::string hello = "Hello from C++";return env->NewStringUTF(hello.c_str());
}extern "C" JNIEXPORT jstring JNICALL
Java_com_example_testrecyclerview_MainActivity_myStringFromJNI(JNIEnv *env,jobject /* this */) {std::string hello = "My String from C++";return env->NewStringUTF(hello.c_str());
}

native-mylib.cpp

#include <jni.h>
#include <string>extern "C" JNIEXPORT jstring JNICALL
Java_com_example_testrecyclerview_MainActivity_testString1(JNIEnv *env,jobject /* this */) {std::string hello = "testString1 C++";return env->NewStringUTF(hello.c_str());
}extern "C" JNIEXPORT jstring JNICALL
Java_com_example_testrecyclerview_MainActivity_testString2(JNIEnv *env,jobject /* this */) {std::string hello = "testString2 C++";return env->NewStringUTF(hello.c_str());
}

c++中的方法名取自Java文件的完整包名+类名+方法名，用下划线代替分隔符

2.5 在java文件中调用native方法

    // Used to load the 'native-lib' library on application startup.static {System.loadLibrary("native-lib");System.loadLibrary("native-mylib");}public native String myStringFromJNI();public native String stringFromJNI();public native String testString1();public native String testString2();TextView tv = findViewById(R.id.test);tv.setText(myStringFromJNI()+stringFromJNI()+testString1()+testString2());

最终能够看到textView上的文字是从c++获取的
另外打开app 也能看到apk内部编译了两个so文件