Android使用ffmpeg（二）

一、开发环境

Android Studio: 3.3.3
FFMPEG: 3.4
NDK: android-ndk-r16b

二、新建项目

C++支持选择默认就行

AS 3.3.3中CMakeLists.txt在main下cpp文件夹内，建议移动到app目录下，main下新建ffmpeg文件夹，ffmpeg文件夹下新建armeabi-v7a文件夹，将编译好的ffmpeg文件赋值到相应文件夹下

include: FFmpeg的方法的头文件
lib: 生成的so动态链接库
share: 一些FFmpeg的示例程序

image.png

app层次build.gradle中进行修改

 defaultConfig {
       
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
            ndk {
                abiFilters "armeabi-v7a"
            }
        }
        //打包apk将ffmpeg的so库打包进libs目录里面
        sourceSets{
            main{
                jniLibs.srcDirs = ['src/main/ffmpeg']
            }
        }
    }
  
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }

编写CMakeLists.txt

# 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).
        src/main/cpp/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)

# set 设置变量  //${CMAKE_SOURCE_DIR} 表示CMakeLists.txt所在的目录
set(JNI_LIBS_DIR ${CMAKE_SOURCE_DIR}/src/main/ffmpeg)
message("JNI_LIBS_DIR:" ${CMAKE_SOURCE_DIR})

# ${ANDROID_ABI}表示各种平台，armeabi或x86
# add_library( xx ) “配置加载动态库的方式 avcodec”动态库的名称，“SHARED ”表示加载的是动态库
# set_target_properties(xx）配置加载动态库的路径
add_library(avutil
        SHARED
        IMPORTED )
set_target_properties(avutil
        PROPERTIES IMPORTED_LOCATION
        ${JNI_LIBS_DIR}/${ANDROID_ABI}/libavutil.so )

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

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

add_library(avcodec
        SHARED
        IMPORTED )
set_target_properties(avcodec
        PROPERTIES IMPORTED_LOCATION
        ${JNI_LIBS_DIR}/${ANDROID_ABI}/libavcodec.so )

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

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


# 指定ffmpeg的头文件的路径 ，如果这里不指定的话，我们在编写代码时include头文件需要写很长的路径
# ffmpeg本身的头文件之间互相引用就是默认这个路径作为根目录的
# 如果不指定这个目录，编译的时候会在ffmpeg的头文件里面报错，说找不到其他头文件
include_directories(src/main/ffmpeg/include)




# 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.
# 链接so库。这里有个要注意的地方就是：如果我们在编写代码时使用了ANativeWindow相关的方法的话，
# 需要在链接库文件的时候加入android这个库文件，不然会报undefined reference to的错

target_link_libraries( # Specifies the target library.
        native-lib

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib}
        avutil
        swresample
        swscale
        avcodec
        avformat
        avfilter
        android
        )

开始调用，新建类，定义native方法，通过System.loadLibrary("库名");先看一下怎么通过java调用C/C++中方法和C/C++调用java方法

/**
 * @author : create by kirito
 * Time: 2019/4/15
 */
public class Test {

    static {
        //将默认的MainActivity中的删除
        System.loadLibrary("native-lib");
        System.loadLibrary("avcodec");
        System.loadLibrary("avfilter");
        System.loadLibrary("avformat");
        System.loadLibrary("avutil");
        System.loadLibrary("swresample");
        System.loadLibrary("swscale");
    }

    /**
     * java层调用native层方法
     */
    public static native int CalculateSum(int a, int b);

}

AS通过下面的控制台进入到项目app/src/main/java/ 目录下，通过命令生成头文件

javah 包名.类名

执行成功后在java目录下会看到.h头文件

image.png

复制这个方法到新建项目默认生成的cpp文件中，我这是native-lib.cpp

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

extern "C" JNIEXPORT jstring JNICALL
Java_com_tp_learning_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}
/*
 * 如果本地代码是C++（.ccp或.cc）,需要添加extern "C"
 * JNIEXPORT 和JNICALL是固定格式不能省，其中jint是返回类型
 * 方法命名规则：Java_包名_类名_方法名，其中Java_必须大写，包名中的.替换成_,包名中的_替换成_1
 * 使用javah生成Native方法对应的Native函数声明，
 * 几乎所有的Native函数的第一个参数永远是JNIEnv指针，而第二个参数永远是jobject或jclass中的一个
 * JNIEnv *env:JNI接口指针，通过这个JNIEnv* 指针，就可以对Java端的代码进行操作
 * jclass jclass1：当所声明Native方法是静态方法时，对应参数jclass，因为静态方法不依赖对象实例，而依赖于类，所以参数中传递的是一个jclass类型。
 *                 如果声明的Native方法时非静态方法时，那么对应参数是jobject
 * 剩下的就是java层方法参数
 *
*/
extern "C"
JNIEXPORT jint JNICALL Java_com_tp_learning_Test_CalculateSum
        (JNIEnv *env, jclass jclass1, jint a, jint b){
    jint sum = a + b;
    return sum;
}

编写完成之后，build->make,编译错误一般都是路径问题,没有错误编写测试代码

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity:";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        int sum = Test.CalculateSum(2, 3);
        Log.d(TAG, String.valueOf(sum));
    }
}

04-15 16:43:20.835 18477-18477/com.tp.learning D/MainActivity:: sum = 5

native调用java中方法

java中定义一个简单方法 getHello（）

public class Test {

    static {
        System.loadLibrary("native-lib");
        System.loadLibrary("avcodec");
        System.loadLibrary("avfilter");
        System.loadLibrary("avformat");
        System.loadLibrary("avutil");
        System.loadLibrary("swresample");
        System.loadLibrary("swscale");
    }

    /**
     * java call native
     */
    public static native int CalculateSum(int a, int b);

    /**
     * native call java
     */

    public String getHello(){
        return "Hello ---";
    }

    /**
     * get string "Hello"
     */
    public static native String getHelloStr();
}

native中定义方法callGetHello用来调用java中的方法

jstring callGetHello(JNIEnv *env,jobject jobject1){
    //加载要调用的方法所在类的class对象,FindClass参数是方法所在类的包名/类名
    jclass jc = env->FindClass("com/tp/learning/Test");
    //得到对应的方法对象,第二个参数是要调用的方法名，第三个是要调用方法的参数类型和返回类型
    jmethodID jmethodID1 = env->GetMethodID(jc, "getHello", "()Ljava/lang/String;");
    //得到类对象
    jobject obj = env->AllocObject(jc);
    //调用方法
    jstring str = (jstring) env->CallObjectMethod(obj, jmethodID1);
    return str;
}

extern "C"
JNIEXPORT jstring JNICALL Java_com_tp_learning_Test_getHelloStr
        (JNIEnv *env, jclass jclass1){
    return callGetHello(env,jclass1);
}

MainActivity中添加代码测试

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity:";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        int sum = Test.CalculateSum(2, 3);
        Log.d(TAG, "sum = "+String.valueOf(sum));
        String str = Test.getHelloStr();
        Log.d(TAG,"get string hello :"+str);
    }
}

运行结果

04-15 17:01:10.445 21646-21646/com.tp.learning D/MainActivity:: sum = 5
04-15 17:01:10.445 21646-21646/com.tp.learning D/MainActivity:: get string hello :Hello ---

未完待续