Android源代码分析之Framework的MediaPlayer

在Android中MediaPlayer用来播放音频和视频文件，在这里分析下在Framework层中MediaPlayer是怎样调用的。MediaPlayer的代码位于：.／frameworks/base/media/java/android/media/MediaPlayer.java   以下用到的代码是基于Android 4.4

打开后有一个静态代码块是载入库文件的，仅仅要这个类被创建就会载入库。

    static {
        System.loadLibrary("media_jni");
        native_init();
    }

libmedia_jni.so的源码位于：./frameworks/base/media/jni

在jni这个目录中有个makefile文件 Android.mk

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)

LOCAL_SRC_FILES:= 
    android_media_ImageReader.cpp 
    android_media_MediaCrypto.cpp 
    android_media_MediaCodec.cpp 
    android_media_MediaCodecList.cpp 
    android_media_MediaDrm.cpp 
    android_media_MediaExtractor.cpp 
    android_media_MediaMuxer.cpp 
    android_media_MediaPlayer.cpp 
    android_media_MediaRecorder.cpp 
    android_media_MediaScanner.cpp 
    android_media_MediaMetadataRetriever.cpp 
    android_media_ResampleInputStream.cpp 
    android_media_MediaProfiles.cpp 
    android_media_AmrInputStream.cpp 
    android_media_Utils.cpp 
    android_mtp_MtpDatabase.cpp 
    android_mtp_MtpDevice.cpp 
    android_mtp_MtpServer.cpp 

LOCAL_SHARED_LIBRARIES := 
    libandroid_runtime 
    libnativehelper 
    libutils 
    libbinder 
    libmedia 
    libskia 
    libui 
    liblog 
    libcutils 
    libgui 
    libstagefright 
    libstagefright_foundation 
    libcamera_client 
    libmtp 
    libusbhost 
    libexif 
    libstagefright_amrnb_common 

LOCAL_REQUIRED_MODULES := 
    libjhead_jni

LOCAL_STATIC_LIBRARIES := 
    libstagefright_amrnbenc

LOCAL_C_INCLUDES += 
    external/libexif/ 
    external/tremor/Tremor 
    frameworks/base/core/jni 
    frameworks/av/media/libmedia 
    frameworks/av/media/libstagefright 
    frameworks/av/media/libstagefright/codecs/amrnb/enc/src 
    frameworks/av/media/libstagefright/codecs/amrnb/common 
    frameworks/av/media/libstagefright/codecs/amrnb/common/include 
    $(TOP)/mediatek/external/amr 
    frameworks/av/media/mtp 
    frameworks/native/include/media/openmax 
    $(call include-path-for, libhardware)/hardware 
    system/media/camera/include 
    $(PV_INCLUDES) 
    $(JNI_H_INCLUDE) 
    $(call include-path-for, corecg graphics)

ifeq ($(strip $(MTK_TB_DEBUG_SUPPORT)),yes)
LOCAL_C_INCLUDES += 
    $(MTK_PATH_SOURCE)/frameworks/base/include 
endif

ifeq ($(strip $(MTK_HIGH_QUALITY_THUMBNAIL)),yes)
LOCAL_CFLAGS += -DMTK_HIGH_QUALITY_THUMBNAIL
endif

ifeq ($(strip $(MTK_USE_ANDROID_MM_DEFAULT_CODE)),yes)
LOCAL_CFLAGS += -DANDROID_DEFAULT_CODE
endif

LOCAL_CFLAGS +=

LOCAL_LDLIBS := -lpthread

LOCAL_MODULE:= libmedia_jni

include $(BUILD_SHARED_LIBRARY)

# build libsoundpool.so
# build libaudioeffect_jni.so
include $(call all-makefiles-under,$(LOCAL_PATH))

LOCAL_MODULE:= libmedia_jni 编译后会生成libmedia_jni库

打开文件有四个重载函数

setDataSource (String path)
setDataSource (FileDescriptor fd)
setDataSource (Context context, Uri uri)
setDataSource (FileDescriptor fd, long offset, long length)

在APP中。如原生的Music

 /**
         * M: add async prepare to aviod ANR, add information and duration update listener
         * 
         * @param player The mediaplayer
         * @param path The data source path
         * @param async M: use async prepare if it's set true .
         * @return If set data source success, return true, otherwise false.
         */
        private boolean setDataSourceImpl(MediaPlayer player, String path, boolean async) {
            MusicLogUtils.d(TAG, "setDataSourceImpl(" + path + ");async = " + async);
            try {
                player.reset();
                if (async) {
                    player.setOnPreparedListener(preparedlistener);
                } else {
                    player.setOnPreparedListener(null);
                }
                if (path.startsWith("content://")) {
                    player.setDataSource(MediaPlaybackService.this, Uri.parse(path));
                } else {
					// / M: add add for DRM secure flag @{
					MediaPlayerEx.setContextForSecureFlag(player,
							MediaPlaybackService.this.getApplicationContext());
					// / @}
					player.setDataSource(path);
                }
                /// M:  Attach auxiliary audio effect only with valid effect id
                if (mAuxEffectId > 0) {
                    player.attachAuxEffect(mAuxEffectId);
                    player.setAuxEffectSendLevel(1.0f);
                    mWhetherAttachWhenPause = false;
                    MusicLogUtils.d(TAG, "setDataSourceImpl: attachAuxEffect mAuxEffectId = " + mAuxEffectId);
                }
                player.setAudioStreamType(AudioManager.STREAM_MUSIC);
                if (async) {
                    player.prepareAsync();
                } else {
                    player.prepare();
                }
            } catch (IOException ex) {
                // TODO: notify the user why the file couldn't be opened
                MusicLogUtils.e(TAG, "setDataSourceImpl: " + ex);
                return false;
            } catch (IllegalArgumentException ex) {
                // TODO: notify the user why the file couldn't be opened
                MusicLogUtils.e(TAG, "setDataSourceImpl: " + ex);
                return false;
            } catch (IllegalStateException ex) {
                MusicLogUtils.e(TAG, "setDataSourceImpl: " + ex);
                return false;
            }
            player.setOnCompletionListener(listener);
            player.setOnErrorListener(errorListener);
            player.setOnInfoListener(infoListener);
            player.setOnDurationUpdateListener(durationListener);
            sendSessionIdToAudioEffect(false);
            return true;
        }

无论在App中调用的是哪个函数，最后在MediaPlayer.java中都是调用

 /**
     * Sets the data source (FileDescriptor) to use.  The FileDescriptor must be
     * seekable (N.B. a LocalSocket is not seekable). It is the caller's responsibility
     * to close the file descriptor. It is safe to do so as soon as this call returns.
     *
     * @param fd the FileDescriptor for the file you want to play
     * @param offset the offset into the file where the data to be played starts, in bytes
     * @param length the length in bytes of the data to be played
     * @throws IllegalStateException if it is called in an invalid state
     */
    public void setDataSource(FileDescriptor fd, long offset, long length)
            throws IOException, IllegalArgumentException, IllegalStateException {
        disableProxyListener();
        _setDataSource(fd, offset, length);
    }

    private native void _setDataSource(FileDescriptor fd, long offset, long length)
            throws IOException, IllegalArgumentException, IllegalStateException;

_setDataSource有native修饰。是libmedia_jni.so中的方法。找到 ./frameworks/base/media/jni/android_media_MediaPlayer.cpp

// ----------------------------------------------------------------------------

static JNINativeMethod gMethods[] = {
    {
        "_setDataSource",
        "(Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/String;)V",
        (void *)android_media_MediaPlayer_setDataSourceAndHeaders
    },

    {"_setDataSource",       "(Ljava/io/FileDescriptor;JJ)V",    (void *)android_media_MediaPlayer_setDataSourceFD},
    {"_setVideoSurface",    "(Landroid/view/Surface;)V",        (void *)android_media_MediaPlayer_setVideoSurface},
    {"prepare",             "()V",                              (void *)android_media_MediaPlayer_prepare},
    {"prepareAsync",        "()V",                              (void *)android_media_MediaPlayer_prepareAsync},
    {"_start",              "()V",                              (void *)android_media_MediaPlayer_start},
    {"_stop",               "()V",                              (void *)android_media_MediaPlayer_stop},
    {"getVideoWidth",       "()I",                              (void *)android_media_MediaPlayer_getVideoWidth},
    {"getVideoHeight",      "()I",                              (void *)android_media_MediaPlayer_getVideoHeight},
    {"seekTo",              "(I)V",                             (void *)android_media_MediaPlayer_seekTo},
    {"_pause",              "()V",                              (void *)android_media_MediaPlayer_pause},
    {"isPlaying",           "()Z",                              (void *)android_media_MediaPlayer_isPlaying},
    {"getCurrentPosition",  "()I",                              (void *)android_media_MediaPlayer_getCurrentPosition},
    {"getDuration",         "()I",                              (void *)android_media_MediaPlayer_getDuration},
    {"_release",            "()V",                              (void *)android_media_MediaPlayer_release},
    {"_reset",              "()V",                              (void *)android_media_MediaPlayer_reset},
    {"setAudioStreamType",  "(I)V",                             (void *)android_media_MediaPlayer_setAudioStreamType},
    {"setLooping",          "(Z)V",                             (void *)android_media_MediaPlayer_setLooping},
    {"isLooping",           "()Z",                              (void *)android_media_MediaPlayer_isLooping},
    {"setVolume",           "(FF)V",                            (void *)android_media_MediaPlayer_setVolume},
    {"native_invoke",       "(Landroid/os/Parcel;Landroid/os/Parcel;)I",(void *)android_media_MediaPlayer_invoke},
    {"native_setMetadataFilter", "(Landroid/os/Parcel;)I",      (void *)android_media_MediaPlayer_setMetadataFilter},
    {"native_getMetadata", "(ZZLandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_getMetadata},
    {"native_init",         "()V",                              (void *)android_media_MediaPlayer_native_init},
    {"native_setup",        "(Ljava/lang/Object;)V",            (void *)android_media_MediaPlayer_native_setup},
    {"native_finalize",     "()V",                              (void *)android_media_MediaPlayer_native_finalize},
    {"getAudioSessionId",   "()I",                              (void *)android_media_MediaPlayer_get_audio_session_id},
    {"setAudioSessionId",   "(I)V",                             (void *)android_media_MediaPlayer_set_audio_session_id},
    {"setAuxEffectSendLevel", "(F)V",                           (void *)android_media_MediaPlayer_setAuxEffectSendLevel},
    {"attachAuxEffect",     "(I)V",                             (void *)android_media_MediaPlayer_attachAuxEffect},
    {"native_pullBatteryData", "(Landroid/os/Parcel;)I",        (void *)android_media_MediaPlayer_pullBatteryData},
    {"setParameter",        "(ILandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_setParameter},
    {"getParameter",        "(ILandroid/os/Parcel;)V",          (void *)android_media_MediaPlayer_getParameter},
    {"native_setRetransmitEndpoint", "(Ljava/lang/String;I)I",  (void *)android_media_MediaPlayer_setRetransmitEndpoint},
    {"setNextMediaPlayer",  "(Landroid/media/MediaPlayer;)V",   (void *)android_media_MediaPlayer_setNextMediaPlayer},
    {"updateProxyConfig", "(Landroid/net/ProxyProperties;)V", (void *)android_media_MediaPlayer_updateProxyConfig},
};

会进入例如以下函数

static void
android_media_MediaPlayer_setDataSourceFD(JNIEnv *env, jobject thiz, jobject fileDescriptor, jlong offset, jlong length)
{
    sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
    if (mp == NULL ) {
        jniThrowException(env, "java/lang/IllegalStateException", NULL);
        return;
    }

    if (fileDescriptor == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }
    int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
    ALOGV("setDataSourceFD: fd %d", fd);
    process_media_player_call( env, thiz, mp->setDataSource(fd, offset, length), "java/io/IOException", "setDataSourceFD failed." );
}

mp->setDataSource(fd, offset, length) //调用打开文件方法

打开 ./frameworks/av/media/libmedia/mediaplayer.cpp   setDataSource有三个重载函数，依据參数

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
    ALOGV("setDataSource(%d, %lld, %lld)", fd, offset, length);
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService>& service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(fd, offset, length))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

打开 ./frameworks/av/media/libmedia/IMediaPlayer.cpp setDataSource也有三个重载函数，依据參数

    status_t setDataSource(int fd, int64_t offset, int64_t length) {
        Parcel data, reply;
        data.writeInterfaceToken(IMediaPlayer::getInterfaceDescriptor());
        data.writeFileDescriptor(fd);
        data.writeInt64(offset);
        data.writeInt64(length);
        remote()->transact(SET_DATA_SOURCE_FD, data, &reply);
        return reply.readInt32();
    }

这个打开文件流程仅仅要是针对本地媒体文件。

process_media_player_call( env, thiz, mp->setDataSource(fd, offset, length), "java/io/IOException", "setDataSourceFD failed." ) //依据返回的结果处理

// If exception is NULL and opStatus is not OK, this method sends an error
// event to the client application; otherwise, if exception is not NULL and
// opStatus is not OK, this method throws the given exception to the client
// application.
static void process_media_player_call(JNIEnv *env, jobject thiz, status_t opStatus, const char* exception, const char *message)
{
    if (exception == NULL) {  // Don't throw exception. Instead, send an event.
        if (opStatus != (status_t) OK) {
            sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
            if (mp != 0) mp->notify(MEDIA_ERROR, opStatus, 0);
        }
    } else {  // Throw exception!
        if ( opStatus == (status_t) INVALID_OPERATION ) {
            jniThrowException(env, "java/lang/IllegalStateException", NULL);
        } else if ( opStatus == (status_t) PERMISSION_DENIED ) {
            jniThrowException(env, "java/lang/SecurityException", NULL);
        } else if ( opStatus != (status_t) OK ) {
            if (strlen(message) > 230) {
               // if the message is too long, don't bother displaying the status code
               jniThrowException( env, exception, message);
            } else {
               char msg[256];
                // append the status code to the message
               sprintf(msg, "%s: status=0x%X", message, opStatus);
               jniThrowException( env, exception, msg);
            }
        }
    }
}
相关文件
Environment.java
MediaScannerReceiver.java
MediaScannerService.java

其他功能（start‘stop‘play‘seek_to）的流程也差点儿相同，难点就是JNI技术和APP中的AIDL。当然以上仅仅是一个简单的调用过程。要更深入了解还是要花些时间的。