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3. avformat_open_input函数详解并示例打开mp4文件~1

发布时间：2021-05-30 10:37:10编辑：雪饮阅读（）

Ffmpeg的avformat_open_input函数用于打开多媒体数据并且获取一些信息。

它的声明位于libavformat/avformat.h

这里就以这个函数为例，实现打开一个视频文件

那么我们就需要用到头文件libavformat/avformat.h，另外就是我们希望不要有太多其它代码，这里就将native-lib.cpp中之前的open方法干掉，只用默认的给我们生成的stringFromJNI方法来实验，于是乎cpp/native-lib.cpp:

#include <jni.h>
#include <string>
#include <android/log.h>
#define LOGW(...) __android_log_print(ANDROID_LOG_WARN,"testff",__VA_ARGS__)
extern "C"{
    #include <libavcodec/avcodec.h>
   #include <libavformat/avformat.h>
}
extern "C" JNIEXPORT jstring JNICALL
Java_com_example_ndk_1and_141_MainActivity_stringFromJNI(JNIEnv* env,jobject) {
    std::string hello = "Hello from C++";
    hello+=avcodec_configuration();
    //初始化解封裝
    av_register_all();
    //初始化網絡
    avformat_network_init();
    AVFormatContext *ic=NULL;
    char path[]="/sdcard/1080.mp4";
    int re=avformat_open_input(&ic,path,0,0);
    if(re==0){
        LOGW("avformat_open_input %s success!");
    }
    else{
        LOGW("avformat_open_input failed!:%s",av_err2str(re));
    }
    return env->NewStringUTF(hello.c_str());
}

那么同样的MainActivity.java中也要清除掉之前的open的native方法，并做了些稍微调整后的MainActivity.java:如：

package com.example.ndk_and_41;
 import androidx.appcompat.app.AppCompatActivity;
 import android.os.Bundle;
 import android.widget.TextView;
 public class MainActivity extends AppCompatActivity {
 
     // Used to load the 'native-lib' library on application startup.
     static {
         System.loadLibrary("native-lib");
     }
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
 
         // Example of a call to a native method
         TextView tv = findViewById(R.id.sample_text);
         tv.setText(stringFromJNI());
     }
 
     /**
      * A native method that is implemented by the 'native-lib' native library,
      * which is packaged with this application.
      */
     public native String stringFromJNI();
 }

那么对于本次实例，我们还需要用到两个动态链接库libavformat.so、libavutil.so于是乎

cpp/CMakeLists.txt调整如：

# 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 3.10.2)

# Declares and names the project.

project("ndk_and_41")

# 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.
#添加頭文件路徑(括號中的include是相對於本文件路徑)
include_directories(../../../include)
#設置ffmpeg庫所在路徑的變量，這裏的FF是自定義的一個名字
set(FF ${CMAKE_CURRENT_SOURCE_DIR}/../../../libs/${ANDROID_ABI})
#avcodec這個是自定義的一個名字
add_library(avcodec SHARED IMPORTED)
set_target_properties(avcodec PROPERTIES IMPORTED_LOCATION ${FF}/libavcodec.so)

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

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

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
avcodec avformat avutil
#這裏將avcodec庫鏈接進來
# Links the target library to the log library
# included in the NDK.
${log-lib} )

那么此时可以编译并部署到设备了吗？这里以魅族16T为例

可以发现在build过程中都已经出现了问题，这里就更不必等着去看install之后的效果了，肯定是失败输出。

那么如何解决这个问题呢？

这里就需要引入昨天咱们新写的那个build_android_64.sh脚本所编译出来的动态链接库。

那么这里我们在我们这个安卓项目中app/libs下面建立arm64-v8a目录

然后将昨天编译输出的64位动态链接库目录\\192.168.1.101\root\ff\ffmpeg-3.4\android\arm64-neon-hard\lib中的所有.so文件（即排除该目录中的pkgconfig文件夹以外的文件）都拷贝到刚刚新建的这个arm64-v8a目录中。

然后和前面一样，在我们这个安卓项目app/build.gradle中还需要对arm64-v8a架构的cpu对应动态链接库进行相关处理，则build.gradle：

plugins {
     id 'com.android.application'
 }
 
 android {
     compileSdkVersion 30
     buildToolsVersion "30.0.3"
 
     defaultConfig {
         applicationId "com.example.ndk_and_41"
         minSdkVersion 16
         targetSdkVersion 30
         versionCode 1
         versionName "1.0"
 
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         externalNativeBuild {
             cmake {
                 cppFlags "-std=c++11"
             }
             ndk{
                 abiFilters "armeabi-v7a"
                 abiFilters "arm64-v8a"
             }
         }
         sourceSets{
             main{
                 jniLibs.srcDirs=['libs']
             }
         }
     }
     packagingOptions {
         pickFirst 'lib/armeabi-v7a/libavcodec.so'
         pickFirst 'lib/armeabi-v7a/libavutil.so'
         pickFirst 'lib/armeabi-v7a/libavformat.so'
         pickFirst 'lib/arm64-v8a/libavcodec.so'
         pickFirst 'lib/arm64-v8a/libavutil.so'
         pickFirst 'lib/arm64-v8a/libavformat.so'
     }
 
     buildTypes {
         release {
             minifyEnabled false
             proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
         }
     }
     externalNativeBuild {
         cmake {
             path "src/main/cpp/CMakeLists.txt"
             version "3.10.2"
         }
     }
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }
 }
 
 dependencies {
 
     implementation 'androidx.appcompat:appcompat:1.2.0'
     implementation 'com.google.android.material:material:1.3.0'
     implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
     testImplementation 'junit:junit:4.+'
     androidTestImplementation 'androidx.test.ext:junit:1.1.2'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
 }

最后编译部署运行到魅族16T中，则可以看到成功的打开多媒体数据并且获取一些信息。

日志猫中的信息如：

2021-05-30 10:15:38.181 15696-15696/com.example.ndk_and_41 W/testff: avformat_open_input  success!

关键字词：avformat_open_input,64位,动态链接库

上一篇：10. JNI和c传递文件路径并设置app的读写权限~1

下一篇：4. AVFormatContext结构分析和清理代码示例~1

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