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• Android using JNI without static variables

  21 février 2013, par William Seemann

  I created my own version of Android's MediaMetadataRetriever using the source code for MediaMetadataRetriever.java as the basis. My version uses FFmpeg to retrieve the metadata. This approach works however it relies on static variables in the C code to retain state in between JNI calls. This means I can only use one instance of this class at a time or the state can get corrupted. The two Java functions are defined as follows :

  public class MediaMetadataRetriever
  
  {
  
      static {
  
          System.loadLibrary("metadata_retriever_jni");
  
      }
  
  
  
      public MediaMetadataRetriever() {
  
  
  
      }
  
  
  
      public native void setDataSource(String path) throws IllegalArgumentException;
  
      public native String extractMetadata(String key);

  }

  The corresponding C (JNI) code code is :

  const char *TAG = "Java_com_example_metadataexample_MediaMetadataRetriever";
  
  static AVFormatContext *pFormatCtx = NULL;
  
  
  
  JNIEXPORT void JNICALL
  
  Java_com_example_metadataexample_MediaMetadataRetriever_setDataSource(JNIEnv *env, jclass obj, jstring jpath) {
  
  
  
      if (pFormatCtx) {
  
          avformat_close_input(&pFormatCtx);
  
      }
  
  
  
      char duration[30] = "0";
  
      const char *uri;
  
  
  
      uri = (*env)->GetStringUTFChars(env, jpath, NULL);
  
  
  
      if (avformat_open_input(&pFormatCtx, uri, NULL, NULL) != 0) {
  
          __android_log_write(ANDROID_LOG_INFO, TAG, "Metadata could not be retrieved");
  
          (*env)->ReleaseStringUTFChars(env, jpath, uri);
  
          jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
  
          return;
  
      }
  
  
  
      if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
  
          __android_log_write(ANDROID_LOG_INFO, TAG, "Metadata could not be retrieved");
  
          avformat_close_input(&pFormatCtx);
  
          (*env)->ReleaseStringUTFChars(env, jpath, uri);
  
          jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
  
          return;
  
      }
  
  
  
      (*env)->ReleaseStringUTFChars(env, jpath, uri);
  
  }
  
  
  
  JNIEXPORT jstring JNICALL
  
  Java_com_example_metadataexample_MediaMetadataRetriever_extractMetadata(JNIEnv *env, jclass obj, jstring jkey) {
  
  
  
      const char *key;
  
      jstring value = NULL;
  
  
  
      key = (*env)->GetStringUTFChars(env, jkey, NULL) ;
  
  
  
      if (!pFormatCtx) {
  
          goto fail;
  
      }
  
  
  
      if (key) {
  
          if (av_dict_get(pFormatCtx->metadata, key, NULL, AV_DICT_IGNORE_SUFFIX)) {
  
              value = (*env)->NewStringUTF(env, av_dict_get(pFormatCtx->metadata, key, NULL, AV_DICT_IGNORE_SUFFIX)->value);
  
          }
  
      }
  
  
  
      fail:
  
      (*env)->ReleaseStringUTFChars(env, jkey, key);
  
  
  
      return value;
  
  }

  Sample usage that outlines my issue would be :

  MediaMetadataRetriever mmr = new MediaMetadataRetriever();
  
  mmr.setDataSource("one.mp3");
  
  
  
  MediaMetadataRetriever mmr2 = new MediaMetadataRetriever();
  
  // This line resets the data source to two.mp3
  
  mmr2.setDataSource("two.mp3");
  
  
  
  // should retrieve the artist from one.mp3 but retrieves it from two.mp3 due to the static
  
  // variable being reset in the previous statement       
  
  String artist = mmr.extractMetadata(MediaMetadataRetriever.ARTIST);

  Can someone explain how I would structure this code so I could use multiple instances of MediaMetadataRetriever without them interfering with one another ? I don't want to switch the code to C++ and I'm fairly certain I don't need to modify MediaMetadataRetriever.java since this code is taken line-for-line from the Android framework (which allows multiple instances, see example below). It appears I need to re-structure the C code but I'm unsure how to retain state across JNI calls without using a static variable. Thanks in advance.

  File file1 = new File(Environment.getExternalStorageDirectory(), "Music/one.mp3");
  
  File file2 = new File(Environment.getExternalStorageDirectory(), "Music/two.mp3");
  
  
  
  android.media.MediaMetadataRetriever mmr = new android.media.MediaMetadataRetriever();
  
  mmr.setDataSource(file1.toString());
  
  
  
  android.media.MediaMetadataRetriever mmr2 = new android.media.MediaMetadataRetriever();
  
  mmr2.setDataSource(file2.toString());
  
  
  
  // Returns the artist of one.mp3, not two.mp3, as expected. This is the expected behavior
  
  // and confirms that multiple instances of MediaMetadataRetriever can be used simultaneously
  
  mmr.extractMetadata(android.media.MediaMetadataRetriever.METADATA_KEY_ARTIST));

• Upgrading Transcooding system, Ideal database to store chunk metadata [closed]

  11 mai 2022, par Vitoo

  I try to update my transcoding system, to make it more efficient and failure resistant. Right now service transcode the whole video at once generating multiply resolution it consumes a lot of resources and when transcoding crash it has to start from beginning.

  


  I think the better method is to chop a video into small 5-10 sec pieces and encode it separately and at the end merge it, but the main problem is to delegate pieces to different encoder workers. I need to store chunks in some database and every worker should lock that it is already working on it and if successful change state to true.

  


  What do you think about this ?
What kind of database should I use ?
Do you have some additional ideas or propositions ?

  


• interrupt and resume long running ffmpeg encoding

  17 octobre 2022, par simon

  When i have a longer high res video (full-hd or 4k) which are not compressed well, encoding to webm/vp9 can take very long even with a recent system due to the lack of hardware acceleration with webm/vp9.
Are there any options to interrupt an encoding with ffmpeg and resume which survives a reboot.
So i don't want to pause encoding using bg but save & stop encoding state and resume from there.

  


  The next best thing i could think of is to split the source into smaller subsets (by time) and process them and stitch results together again. But this only allows predefined interruption points and requires the split & merge to be done in addition.
On the pro side it could allow parallel processing if multithreading is not using all available cores yet.