Android APP JNI 编写

• Android 的普通的 APP 编写好了之后。如果想调用 C++ 代码，步骤如下.

• 在官网，下载 NDK， 一般最新的就可以， 下载完毕之后配置，配置如下 File -> Project Structure 里面：

  ![](https://img2020.cnblogs.com/blog/991711/202005/991711-20200512094150200-1509732925.png)
  * 这个路径不能有空格，有空格会报错，下载在官网上有下载。
  

• 配置完毕之后，就可以添加 C++ 代码了。

• 在源代码的 app/src 目录添加 CMakeLists.txt 文件，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.
# 设置cmake 的最小版本  一般系统自动生成
cmake_minimum_required(VERSION 3.4.1)

# 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.
             # 设置生成.so 的文件名
             my_lib

             # Sets the library as a shared library.
             #设置库的类型  一种静态文件  STATIC .a   一种动态文件  SHARED .so
             SHARED

             # Provides a relative path to your source file(s).
             # 需要编译的c/c++ 文件
             src/main/cpp/main.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.
                       # 指定链接的目标库
                       my_lib

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

• 在 app/src/build.gradle 里面添加如下内容：

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"

    defaultConfig {
        applicationId "com.example.cmi_at154_test"
        minSdkVersion 25
        targetSdkVersion 29
        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'
        }
    }

    externalNativeBuild {
        cmake {
            path 'CMakeLists.txt'
        }
    }
}

• 在 app/src/main 里面添加 C++ 文件夹，如下：

• main.cpp 内容如下：

#include "jni.h"
#include "android/log.h"

extern "C"
JNIEXPORT jint JNICALL
        Java_com_example_cmi_1at154_1test_MainActivity_ReturnCOM1Result
        (JNIEnv *env, jobject thiz) {
    return 1;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnCOM2Result
        (JNIEnv *env, jobject thiz) {
    return 0;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnCOM3Result
        (JNIEnv *env, jobject thiz) {
    return 1;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnCOM4Result
        (JNIEnv *env, jobject thiz) {
    return 0;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnUSBResult
        (JNIEnv *env, jobject thiz) {
    return 1;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnGPSResult
        (JNIEnv *env, jobject thiz) {
    return 1;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_ReturnWIFIResult
        (JNIEnv *env, jobject thiz) {
    return 0;
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_cmi_1at154_1test_MainActivity_Return4GResult
        (JNIEnv *env, jobject thiz) {
    return 0;
}

• 现在在 Java 文件里面就可以调用这些 C++ 代码.

    static {
        System.loadLibrary("my_lib");
    }

    public native int ReturnCOM1Result();
    public native int ReturnCOM2Result();
    public native int ReturnCOM3Result();
    public native int ReturnCOM4Result();
    public native int ReturnGPSResult();
    public native int ReturnWIFIResult();
    public native int ReturnUSBResult();
    public native int Return4GResult();