一. 从C++返回hello world

这一步是android studio自带功能

新建工程时选择native c++即可 Screenshot from 2020-12-10 00-41-57.png

做一个简单的记录

1. build.gradle

3处修改

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"

    defaultConfig {
        minSdkVersion 19
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        consumerProguardFiles "consumer-rules.pro"

        // 1) 设置cppFlags
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }

// 2) 指定 CMakeLists.txt的路径和版本 
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }
// 3）as 3.6.3上需要加上这段代码，告诉gradle本地库的路径，4.0上不需要
 sourceSets {
        main {
            jniLibs.srcDirs = ['src/main/ffmpegLibs/lib']
        }
    }
}

2. 编写jni代码和对应的java调用代码

jni代码

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


//这里的方法名字要和调用层 FFmpegWrapper的方法签名保持一致,
//并且如果源文件是C++代码,要加上extern "C",不然会影响链接器链接,导致找不到方法

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

java调用代码

public class FFmpegWrapper {

    //加载.so
    static {
        System.loadLibrary("native-lib");
    }
  

    public native static String ffmpegVersion();
}

3. 编写CmakeFile

# 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 3.4.1)

# 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.
             native-lib

             # 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.
                       native-lib

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

二. 使用编译好的.so

1. 设置支持的CPU架构

 defaultConfig {
        applicationId "com.lv.media_demo"
        minSdkVersion 19
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
      
      //由于编译的.so是架构类型是arm64-v8a,所以这里限制只支持'arm64-v8a
        ndk {
            //abiFilters 'armeabi-v7a' //, 'arm64-v8a', 'x86', 'x86_64', 'mips', 'mips64'
            abiFilters 'arm64-v8a' //, 'arm64-v8a', 'x86', 'x86_64', 'mips', 'mips64'
        }
    }

2. 复制头文件和.so文件到工程

   
   Screenshot from 2020-12-10 01-01-49.png

3. 修改cmakefile.txt

有4出修改

# 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 3.4.1)

# 1) 设置.so和头文件存放的位置
set(jni_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../ffmpegLibs)


# 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

        # Provides a relative path to your source file(s).
        native-lib.cpp)

# 2) 设置头文件的查找路径
target_include_directories(native-lib PRIVATE
        ${jni_DIR}/include)


# 3)添加一个.so依赖
add_library(avcodec SHARED IMPORTED)
set_target_properties(avcodec PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libavcodec.so)


add_library(avformat SHARED IMPORTED)
set_target_properties(avformat PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libavformat.so)


add_library(avfilter SHARED IMPORTED)
set_target_properties(avfilter PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libavfilter.so)


add_library(avutil SHARED IMPORTED)
set_target_properties(avutil PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libavutil.so)

add_library(swscale SHARED IMPORTED)
set_target_properties(swscale PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libswscale.so)

add_library(swresample SHARED IMPORTED)
set_target_properties(swresample PROPERTIES IMPORTED_LOCATION
        ${jni_DIR}/lib/${ANDROID_ABI}/libswresample.so)


# 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.

# 4) 添加到链接库
target_link_libraries( # Specifies the target library.
        native-lib
        avcodec
        avformat
        avfilter
        avutil
        swscale
        swresample
        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

4. 修改native-lib.cpp,获取ffmpeg的版本号

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

extern "C" {
#include "libavutil/avutil.h"
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_lv_ffmpeg_FFmpegWrapper_ffmpegVersion(JNIEnv *env, jclass clazz) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(av_version_info());
}