如何向已有的项目中添加C/C++代码？

第一步：

　　我们需要在src/main下面建立一个cpp目录，然后在其中写一个CMakeLists.txt文件和一个cpp文件，直接给出代码：

#CMakeLists.txt文件
# 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("hellondk")

# 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.
        hellondk

        # 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.
        hellondk

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

　　native-lib.cpp文件的代码如下：

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

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

　　然后在app/build.gradle文件中添加以下代码：

android {
    namespace 'com.example.myapplication'
    compileSdk 32

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

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }//新增代码
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"    //指定CMakeLists.txt文件的路径
        }
    }//新增代码
}

　　接下来就可以调用c++代码了：

public class MainActivity extends AppCompatActivity {
    private TextView tv_display;
    static{
        System.loadLibrary("hellondk");
    }
    @SuppressLint("MissingInflatedId")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv_display = findViewById(R.id.tv_display);
        tv_display.setText(stringFromJNI());
    }
    public native String stringFromJNI();
}

　　需要注意的是：本地函数声明public native String stringFromJNI()一定要在Java_com_example_myapplication_MainActivity_stringFromJNI函数名指定的包和类下，要不然程序会出错。