一. 从C++返回hello world
这一步是android studio自带功能
Screenshot from 2020-12-10 00-41-57.png
做一个简单的记录
- 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']
}
}
}
- 编写jni代码和对应的java调用代码
jni代码
#include
#include
//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();
}
- 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
- 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'
}
}
-
复制头文件和.so文件到工程
Screenshot from 2020-12-10 01-01-49.png
- 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})
- 修改native-lib.cpp,获取ffmpeg的版本号
#include
#include
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());
}