Android项目集成NDK开发步骤

1. App内build.gradle文件添加

   defaultConfig {
           applicationId "com.damao.ndkdevdemo"
           minSdkVersion 16
           targetSdkVersion 30
           versionCode 1
           versionName "1.0"
   
           testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
           
           //添加下面这句
           externalNativeBuild {
               cmake {
                   cppFlags ""
               }
           }
       }
       
   在defaultConfig同级添加：
   externalNativeBuild {
           cmake {
               path "src/main/cpp/CMakeLists.txt"
               version "3.10.2"
           }
       }
       ndkVersion '21.1.6352462'
   

2. 在java同级目录新建文件夹cpp，然后右键，选择MarkDirectory as，再选择Sources Root

   # 在cpp目录下新建CMakeLists.txt文件，粘贴下面的内容：
   # CMakeLists.txt文件主要是用来编写编译的内容
   # 注意：project("ndkdevdemo")中的内容要改为当前工程的名称
   
   # 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("ndkdevdemo")
   
   # 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.
                native-lib
   
                # Sets the library as a shared library.
                SHARED
               
               # 在这里添加新建的cpp文件名称，这样就加入了编译
                # 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.
                          native-lib
   
                          # Links the target library to the log library
                          # included in the NDK.
                          ${log-lib} )
   

3. 在cpp目录下新建native-lib.cpp文件

   //粘贴以下内容，这样一个C++函数就编写好了
   //注意：com_damao_ndkdevdemo需要改为项目的包名，其中.用_表示
   
   #include <jni.h>
   #include <string>
   
   extern "C" JNIEXPORT jstring JNICALL
   Java_com_damao_ndkdevdemo_MainActivity_stringFromJNI(
           JNIEnv* env,
           jobject /* this */) {
       std::string hello = "Hello from C++";
       return env->NewStringUTF(hello.c_str());
   }
   

4. 在MainActivvity添加以下代码，表示导入C++开发库

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

5. 定义native函数

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

6. 调用定义的native函数

   override fun onCreate(savedInstanceState: Bundle?) {
           super.onCreate(savedInstanceState)
           setContentView(R.layout.activity_main)
   
           // Example of a call to a native method
           findViewById<TextView>(R.id.sample_text).text = stringFromJNI()
       }
   

7. 如果集成之后，MainActivity中的Native函数编译通过了，也能跑起来，但是在AS中不识别，点击不会自动跳转到对应的C++文件，可以在build.gradle中的externalNativeBuild里面的cmake内的path路径，把cpp删除，同步一下，再添加回去，然后同步就可以了。