BackgroundAudioService.java

import android.app.Service;
import android.content.Intent;
import android.media.MediaCodec;
import android.net.Uri;
import android.os.Binder;
import android.os.IBinder;
import android.util.Log;
import android.view.Surface;
import android.widget.MediaController;

import com.google.android.exoplayer.ExoPlaybackException;
import com.google.android.exoplayer.ExoPlayer;
import com.google.android.exoplayer.MediaCodecAudioTrackRenderer;
import com.google.android.exoplayer.MediaCodecSelector;
import com.google.android.exoplayer.MediaCodecVideoTrackRenderer;
import com.google.android.exoplayer.extractor.ExtractorSampleSource;
import com.google.android.exoplayer.upstream.Allocator;
import com.google.android.exoplayer.upstream.DataSource;
import com.google.android.exoplayer.upstream.DefaultAllocator;
import com.google.android.exoplayer.upstream.DefaultUriDataSource;
import com.google.android.exoplayer.util.Util;

public class BackgroundAudioService extends Service implements ExoPlayer.Listener, MediaController.MediaPlayerControl {

  private static final int BUFFER_SEGMENT_SIZE = 64 * 1024;
  private static final int BUFFER_SEGMENT_COUNT = 256;
  private static final int BUFFER_SIZE = BUFFER_SEGMENT_SIZE * BUFFER_SEGMENT_COUNT;

  private MediaCodecVideoTrackRenderer mVideoRenderer;
  private MediaCodecAudioTrackRenderer mAudioRenderer;

  private ExoPlayer mExoPlayer;
  private Surface mSurface;

  private final IBinder mBackgroundAudioServiceBinder = new BackgroundAudioServiceBinder();

  public class BackgroundAudioServiceBinder extends Binder {
    BackgroundAudioService getService() {
      return BackgroundAudioService.this;
    }
  }

  @Override
  public IBinder onBind(Intent intent) {
    return mBackgroundAudioServiceBinder;
  }

  public void playUri(Uri uri){
    mExoPlayer = ExoPlayer.Factory.newInstance(2);
    mExoPlayer.addListener(this);
    Allocator allocator = new DefaultAllocator(BUFFER_SEGMENT_SIZE);
    String userAgent = Util.getUserAgent(this, BackgroundAudioService.class.getSimpleName());
    DataSource dataSource = new DefaultUriDataSource(this, null, userAgent);
    ExtractorSampleSource sampleSource = new ExtractorSampleSource(uri, dataSource, allocator, BUFFER_SIZE);
    mAudioRenderer = new MediaCodecAudioTrackRenderer(sampleSource, MediaCodecSelector.DEFAULT);
    mVideoRenderer = new MediaCodecVideoTrackRenderer(getApplicationContext(), sampleSource, MediaCodecSelector.DEFAULT, MediaCodec.VIDEO_SCALING_MODE_SCALE_TO_FIT);
    mExoPlayer.sendMessage(mVideoRenderer, MediaCodecVideoTrackRenderer.MSG_SET_SURFACE, mSurface);
    mExoPlayer.prepare(mVideoRenderer, mAudioRenderer);
    mExoPlayer.setPlayWhenReady(true);
  }

  public ExoPlayer getExoPlayer(){
    return mExoPlayer;
  }

  public void setSurface(Surface surface, boolean shouldBlock) {
    mSurface = surface;
    if(mExoPlayer != null) {
      if(shouldBlock) {
        mExoPlayer.blockingSendMessage(mVideoRenderer, MediaCodecVideoTrackRenderer.MSG_SET_SURFACE, surface);
      } else {
        mExoPlayer.sendMessage(mVideoRenderer, MediaCodecVideoTrackRenderer.MSG_SET_SURFACE, surface);
      }
    }
  }

  @Override
  public void onPlayerStateChanged(boolean playWhenReady, int playbackState) {
    Log.d(BackgroundAudioService.class.getSimpleName(), String.format(">>>>> Player state changed"));
    Log.d(BackgroundAudioService.class.getSimpleName(), String.format(">>>>> %s", playbackState));
  }

  @Override
  public void onPlayWhenReadyCommitted() {

  }

  @Override
  public void onPlayerError(ExoPlaybackException error) {

  }

  /**
   * MediaController
   */

  @Override
  public int getAudioSessionId() {
    throw new UnsupportedOperationException();
  }

  @Override
  public int getBufferPercentage() {
    if(mExoPlayer == null){
      return 0;
    }
    return mExoPlayer.getBufferedPercentage();
  }

  @Override
  public int getCurrentPosition() {
    if(mExoPlayer == null){
      return 0;
    }
    return (int) mExoPlayer.getCurrentPosition();
  }

  @Override
  public int getDuration() {
    if(mExoPlayer == null){
      return 0;
    }
    int duration = (int) mExoPlayer.getDuration();
    return duration == ExoPlayer.UNKNOWN_TIME ? 0 : duration;
  }

  @Override
  public boolean isPlaying() {
    return mExoPlayer != null && mExoPlayer.getPlayWhenReady();
  }

  @Override
  public void start() {
    if(mExoPlayer != null) {
      mExoPlayer.setPlayWhenReady(true);
    }
  }

  @Override
  public void pause() {
    if(mExoPlayer != null) {
      mExoPlayer.setPlayWhenReady(false);
    }
  }

  @Override
  public void seekTo(int timeMillis) {
    if(mExoPlayer != null) {
      long seekPosition = Math.min(Math.max(0, timeMillis), getDuration());
      mExoPlayer.seekTo(seekPosition);
    }
  }

  @Override
  public boolean canPause() {
    return true;
  }

  @Override
  public boolean canSeekBackward() {
    return true;
  }

  @Override
  public boolean canSeekForward() {
    return true;
  }


}

Did you manage to make it work with a forgroundService mate?
If so, it'll be nice if you can share the code or at least give us some tips.
Thanks.