最新的android studio2.2引入了cmake可以很好地實現ndk的編寫。這裡使用最新的方式，對於以前的android下的ndk編譯什麼的可以參考之前的文章:Android開發學習之路–NDK、JNI之初體驗。

1.ffmpeg編譯

  新建ffmpeg資料夾，然後新建指令碼用來編譯ffmpeg，命名為build.sh，指令碼如下：

#!/bin/bash
cd ffmpeg
export TMPDIR=/Users/jared/Desktop/jared/study/external/ffmpeg/tempdir
NDK=/Users/jared/Desktop/jared/software/sdk/ndk-bundle
SYSROOT=$NDK/platforms/android-16/arch-arm/
TOOLCHAIN=/Users/jared/Desktop/jared/software/sdk/ndk-bundle/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64
CPU=arm
PREFIX=/Users/jared/Desktop/jared/study/external/ffmpeg/output
ADDI_CFLAGS="-marm"
function build_one
{
    ./configure \
        --prefix=$PREFIX
 
 \
        --enable-shared \
        --disable-static \
        --disable-doc \
        --disable-ffmpeg \
        --disable-ffplay \
        --disable-ffprobe \
        --disable-ffserver \
        --disable-doc \
        --disable-symver \
        --enable-small \
        --cross-prefix=$TOOLCHAIN
 
/bin/arm-linux-androideabi- \
        --target-os=linux \
        --arch=arm \
        --enable-cross-compile \
        --sysroot=$SYSROOT \
        --extra-cflags="-Os -fpic $ADDI_CFLAGS" \
        --extra-ldflags="$ADDI_LDFLAGS" \
        $ADDITIONAL_CONFIGURE_FLAG
    make clean
    make
    make install
}
build_one
cd ../
 

  注意的是這裡的路徑是對應的ndk的相關路徑。

  然後chmod 777 build.sh，然後執行source build.sh。開始編譯ffmpeg。小憩片刻之後，編譯完成，在output目錄下就會生成對應的so檔案:

➜  ffmpeg cd output
➜  output ls
include lib
➜  output cd lib
➜  lib ls
libavcodec-57.so   libavfilter-6.so   libavutil-55.so    libswscale-4.so
libavcodec.so      libavfilter.so     libavutil.so       libswscale.so
libavdevice-57.so  libavformat-57.so  libswresample-2.so pkgconfig
libavdevice.so     libavformat.so     libswresample.so
➜  lib

  編譯完成，這裡只是用了arm的平臺，如果需要mips，或者x86需要修改build.sh指令碼的arch和路徑這裡不贅述了。
  既然庫檔案都已經編譯出來了那就可以android開搞了。

2.android環境搭建

  首先需要最新的android studio2.2，並且安裝好cmake和ndk。然後新建工程，可以開始了。
  新建工程取名為helloffmpeg，然後選中include c++ support，然後下一步直到新建完成為止。如下圖：

  然後比一般的工程多了些東西，首先是build.gradle了，

  CMake的cpp的flags

externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }

  CMake的路徑

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

  然後看CMakeLists.txt檔案，這裡主要講解下配置。先看下已經配置好的庫檔案：

# Sets the minimum version of CMake required to build the native
# library. You should either keep the default value or only pass a
# value of 3.4.0 or lower.

cmake_minimum_required(VERSION 3.4.1)

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(distribution_DIR ${CMAKE_SOURCE_DIR}/../../../../libs)

add_library( avutil-55
             SHARED
             IMPORTED )
set_target_properties( avutil-55
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavutil-55.so )

add_library( swresample-2
             SHARED
             IMPORTED )
set_target_properties( swresample-2
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libswresample-2.so )
add_library( avcodec-57
             SHARED
             IMPORTED )
set_target_properties( avcodec-57
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavcodec-57.so )
add_library( avfilter-6
             SHARED
             IMPORTED)
set_target_properties( avfilter-6
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavfilter-6.so )
add_library( swscale-4
             SHARED
             IMPORTED)
set_target_properties( swscale-4
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libswscale-4.so )
add_library( avdevice-57
             SHARED
             IMPORTED)
set_target_properties( avdevice-57
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavdevice-57.so )
add_library( avformat-57
             SHARED
             IMPORTED)
set_target_properties( avformat-57
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavformat-57.so )

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")

add_library( native-lib
             SHARED
             src/main/cpp/native-lib.cpp )

include_directories(libs/include)

#target_include_directories(native-lib PRIVATE libs/include)

target_link_libraries( native-lib swresample-2 avcodec-57 avfilter-6 swscale-4 avdevice-57 avformat-57
                       ${log-lib} )

  cmake_minimum_required(VERSION 3.4.1):表示cmake的最低版本是3.4.1。
  add_library():新增庫，分為兩種，一種是需要編譯為庫的程式碼，一種是已經編譯好的庫檔案。
  比如上面編譯好的ffmpeg的庫，

add_library( avutil-55
             SHARED
             IMPORTED )

  這裡avutil-55表示庫的名稱，SHARED表示是共享庫，一般.so檔案，還有STATIC，一般.a檔案。IMPORTED表示引用的不是生成的。
  接著是這裡用到的需要生產的.so檔案

add_library( native-lib
             SHARED
             src/main/cpp/native-lib.cpp )

  最後的引數是原始碼的路徑，如果有更多的原始碼就接下去寫上。

  set_target_properties：對於已經編譯好的so檔案需要引入，所以需要設定。

set_target_properties( avutil-55
                       PROPERTIES IMPORTED_LOCATION
                       ../../../../libs/armeabi-v7a/libavutil-55.so )

  這裡avutil-55是名字，然後是PROPERTIES IMPORTED_LOCATION加上庫的路徑。
  include_directories：一般外面引入的庫檔案需要標頭檔案，所以可以通過這個來引入：

include_directories(libs/include)

  target_link_libraries：連結，把需要的so檔案連結起來，這裡native-lib需要連結ffmpeg的庫檔案。

  最後看下和java，res同級的cpp目錄下有native-lib.cpp檔案，系統自動生成的。
  首先是native-lib.cpp內容：

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

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

  這裡有一個函式Java_com_jared_helloffmpeg_MainActivity_stringFromJNI，命名方式也可以知道，Java開頭，com_jared_helloffmpeg為包名，MainActivity是呼叫的java類，最後stringFromJNI是方法名。然後返回了hello from c++字元。

  看下java的程式碼：

 // Example of a call to a native method
 TextView tv = (TextView) findViewById(R.id.sample_text);
 tv.setText(avformatinfo());
/**
  * A native method that is implemented by the 'native-lib' native library,
  * which is packaged with this application.
 */
  public native String stringFromJNI();
  // Used to load the 'native-lib' library on application startup.
    static {
        loadLibrary("native-lib");
    }

  也很好理解這裡的stringFromJNI的native方法，具體需要看jni的基本概念了，這裡就不多贅述了。好了基本上最簡單的通了，那麼接下去就是ffmpeg的使用了。

3.修改使用ffmpeg

  接著來修改介面顯示ffmpeg的一些基本資訊。如下：

<?xml version="1.0" encoding="utf-8"?>
<layout>

    <data class="MainBinding"></data>

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/activity_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:paddingBottom="@dimen/activity_vertical_margin"
        android:paddingLeft="@dimen/activity_horizontal_margin"
        android:paddingRight="@dimen/activity_horizontal_margin"
        android:paddingTop="@dimen/activity_vertical_margin"
        tools:context="com.jared.helloffmpeg.MainActivity">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <Button
                android:id="@+id/btn_protocol"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:text="Protocol"
                android:textAllCaps="false" />

            <Button
                android:id="@+id/btn_format"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:text="Format"
                android:textAllCaps="false" />

            <Button
                android:id="@+id/btn_codec"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:text="Codec"
                android:textAllCaps="false" />

            <Button
                android:id="@+id/btn_filter"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_margin="2dp"
                android:text="Filter"
                android:textAllCaps="false" />
        </LinearLayout>

        <ScrollView
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <TextView
                android:id="@+id/tv_info"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:text="Hello World!" />
        </ScrollView>

    </LinearLayout>
</layout>

  主要是實現了四個button，分別是protocol，format，codec和filter。如下圖所示：

  
  修改jni部分的程式碼，也就是native-lib.cpp的程式碼如下：

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

extern "C"
{
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavfilter/avfilter.h>

jstring
Java_com_jared_helloffmpeg_MainActivity_stringFromJNI(
        JNIEnv *env, jobject) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

jstring
Java_com_jared_helloffmpeg_MainActivity_urlprotocolinfo(
        JNIEnv *env, jobject) {
    char info[40000] = {0};
    av_register_all();

    struct URLProtocol *pup = NULL;

    struct URLProtocol **p_temp = &pup;
    avio_enum_protocols((void **) p_temp, 0);

    while ((*p_temp) != NULL) {
        sprintf(info, "%sInput: %s\n", info, avio_enum_protocols((void **) p_temp, 0));
    }
    pup = NULL;
    avio_enum_protocols((void **) p_temp, 1);
    while ((*p_temp) != NULL) {
        sprintf(info, "%sInput: %s\n", info, avio_enum_protocols((void **) p_temp, 1));
    }
    return env->NewStringUTF(info);
}

jstring
Java_com_jared_helloffmpeg_MainActivity_avformatinfo(
        JNIEnv *env, jobject) {
    char info[40000] = {0};

    av_register_all();

    AVInputFormat *if_temp = av_iformat_next(NULL);
    AVOutputFormat *of_temp = av_oformat_next(NULL);
    while (if_temp != NULL) {
        sprintf(info, "%sInput: %s\n", info, if_temp->name);
        if_temp = if_temp->next;
    }
    while (of_temp != NULL) {
        sprintf(info, "%sOutput: %s\n", info, of_temp->name);
        of_temp = of_temp->next;
    }
    return env->NewStringUTF(info);
}

jstring
Java_com_jared_helloffmpeg_MainActivity_avcodecinfo(
        JNIEnv *env, jobject) {
    char info[40000] = {0};

    av_register_all();

    AVCodec *c_temp = av_codec_next(NULL);

    while (c_temp != NULL) {
        if (c_temp->decode != NULL) {
            sprintf(info, "%sdecode:", info);
        } else {
            sprintf(info, "%sencode:", info);
        }
        switch (c_temp->type) {
            case AVMEDIA_TYPE_VIDEO:
                sprintf(info, "%s(video):", info);
                break;
            case AVMEDIA_TYPE_AUDIO:
                sprintf(info, "%s(audio):", info);
                break;
            default:
                sprintf(info, "%s(other):", info);
                break;
        }
        sprintf(info, "%s[%10s]\n", info, c_temp->name);
        c_temp = c_temp->next;
    }

    return env->NewStringUTF(info);
}

jstring
Java_com_jared_helloffmpeg_MainActivity_avfilterinfo(
        JNIEnv *env, jobject) {
    char info[40000] = {0};
    avfilter_register_all();

    AVFilter *f_temp = (AVFilter *)avfilter_next(NULL);
    while(f_temp != NULL) {
        sprintf(info, "%s%s\n", info, f_temp->name);
        f_temp = f_temp->next;
    }
    return env->NewStringUTF(info);
}
}

  這裡要特別注意ffmpeg的標頭檔案已經呼叫的程式碼要extern “C”裡面，要不然會報錯誤的。

  最後就是java呼叫程式碼了。如下所示：

package com.jared.helloffmpeg;

import android.databinding.DataBindingUtil;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;

import com.jared.helloffmpeg.databinding.MainBinding;

import static java.lang.System.loadLibrary;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private MainBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_main);
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        binding.btnProtocol.setOnClickListener(this);
        binding.btnCodec.setOnClickListener(this);
        binding.btnFilter.setOnClickListener(this);
        binding.btnFormat.setOnClickListener(this);
        // Example of a call to a native method
        //TextView tv = (TextView) findViewById(R.id.sample_text);
        //tv.setText(avformatinfo());
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.btn_protocol:
                binding.tvInfo.setText(urlprotocolinfo());
                break;
            case R.id.btn_format:
                binding.tvInfo.setText(avformatinfo());
                break;
            case R.id.btn_codec:
                binding.tvInfo.setText(avcodecinfo());
                break;
            case R.id.btn_filter:
                binding.tvInfo.setText(avfilterinfo());
                break;
            default:
                break;
        }
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    public native String urlprotocolinfo();
    public native String avformatinfo();
    public native String avcodecinfo();
    public native String avfilterinfo();

    // Used to load the 'native-lib' library on application startup.
    static {
        loadLibrary("native-lib");
    }
}

  都準備好了，那麼就繼續執行看看結果了：

  基本上Android Studio下的cmake編譯ffmpeg就到此結束了，那麼接下去就可以在android手機上使用ffmpeg來做一些音視訊相關的東西了。