Android进阶-NDK技术

# 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)   //cmake的最低版本是3.18.1

# Declares and names the project.

project("ndkstudy")

# 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.
        ndkstudy         //生成的库的名称

        # Sets the library as a shared library.
        SHARED     //设置生成的库为共享库.so

        # Provides a relative path to your source file(s).
        native-lib.cpp     //c++源文件的相对路径
)

# 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)   //使用find_library来查找log库，并把找到的log库存储在变量log-lib中

# 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.
        ndkstudy

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})  //将ndkstudy库和log库进行链接

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

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_ndkstudy_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}//函数的功能就是返回一个"Hello from C++"字符串

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

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

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        ndk {
            // 设置支持的SO库架构（开发者可以根据需要，选择一个或多个平台的so）
            abiFilters "armeabi", "armeabi-v7a", "arm64-v8a", "x86","x86_64"
        }//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
    }
}

dependencies {
    implementation 'androidx.appcompat:appcompat:1.4.1'
    implementation 'com.google.android.material:material:1.5.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.3'
    testImplementation 'junit:junit:4.13.2'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

public class MainActivity extends AppCompatActivity {
    private TextView tv_display;
    {
        System.loadLibrary("ndkstudy");//第一步，加载动态库，放到静态代码块里就行
    }

    @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());//第三步，调用本地函数，实际上调用的函数是Java_com_example_ndkstudy_MainActivity_stringFromJNI();

extern "C" JNIEXPORT void JNICALL
Java_com_example_ndkstudy_MainActivity_javaToJni(JNIEnv *env, jobject thiz, jbyte a, jboolean b,
                                                 jint c, jshort d, jlong e, jchar f, jfloat g,
                                                 jdouble h) {

    //将jni类型转换成c类型
    int8_t c_a=a;
    uint8_t c_b=b;
    int32_t c_c=c;
    int16_t c_d=d;
    int64_t c_e=e;
    uint16_t c_f=f;
    float c_g=g;
    double c_h=h;

extern "C" JNIEXPORT jstring JNICALL   //jstring表示返回值为jstring类型
Java_com_example_ndkstudy_MainActivity_test(JNIEnv *env, jobject thiz, jstring str) {
    // TODO: implement test()
    const char *p=env->GetStringUTFChars(str, nullptr);//将jstring类型转换成c中的const char *类型
    return env->NewStringUTF(p);//将const char *类型转换成jstring类型
}//所以这个函数的功能就是返回传进来的字符串