Java 内置音乐播放器

音乐播放器在没有外部存储器的智能手机上工作，应用程序崩溃，而在有外部存储器的智能手机上工作。 你能告诉我为什么吗

Java类

    import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;

import java.io.IOException;
import java.math.BigDecimal;

public class mediaplayer extends ListActivity {
    private static final int UPDATE_FREQUENCY = 500;
    private static final int STEP_VALUE = 4000;

    private MediaCursorAdapter mediaAdapter = null;
    private TextView selelctedFile = null;
    private SeekBar seekbar = null;
    private MediaPlayer player = null;
    private ImageButton playButton = null;
    private ImageButton prevButton = null;
    private ImageButton nextButton = null;

    private boolean isStarted = true;
    private String currentFile = "";
    private boolean isMoveingSeekBar = false;

    private final Handler handler = new Handler();

    private final Runnable updatePositionRunnable = new Runnable() {
        public void run() {
            updatePosition();
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mediaplayer);
        AdView mAdView = (AdView) findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();
        mAdView.loadAd(adRequest);
        selelctedFile = (TextView) findViewById(R.id.selectedfile);
        seekbar = (SeekBar) findViewById(R.id.seekbar);
        playButton = (ImageButton) findViewById(R.id.play);
        prevButton = (ImageButton) findViewById(R.id.prev);
        nextButton = (ImageButton) findViewById(R.id.next);

        player = new MediaPlayer();

        player.setOnCompletionListener(onCompletion);
        player.setOnErrorListener(onError);
        seekbar.setOnSeekBarChangeListener(seekBarChanged);

        Cursor cursor = getContentResolver().query(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, null, null, null, null);

        if (null != cursor) {
            cursor.moveToFirst();

            mediaAdapter = new MediaCursorAdapter(this, R.layout.listitem, cursor);

            setListAdapter(mediaAdapter);

            playButton.setOnClickListener(onButtonClick);
            nextButton.setOnClickListener(onButtonClick);
            prevButton.setOnClickListener(onButtonClick);
        }
    }

    @Override
    protected void onListItemClick(ListView list, View view, int position, long id) {
        super.onListItemClick(list, view, position, id);

        currentFile = (String) view.getTag();

        startPlay(currentFile);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        handler.removeCallbacks(updatePositionRunnable);
        player.stop();
        player.reset();
        player.release();

        player = null;
    }

    private void startPlay(String file) {
        Log.i("Selected: ", file);

        selelctedFile.setText(file);
        seekbar.setProgress(0);

        player.stop();
        player.reset();

        try {
            player.setDataSource(file);
            player.prepare();
            player.start();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        seekbar.setMax(player.getDuration());
        playButton.setImageResource(android.R.drawable.ic_media_pause);

        updatePosition();

        isStarted = true;
    }

    private void stopPlay() {
        player.stop();
        player.reset();
        playButton.setImageResource(android.R.drawable.ic_media_play);
        handler.removeCallbacks(updatePositionRunnable);
        seekbar.setProgress(0);

        isStarted = false;
    }

    private void updatePosition() {
        handler.removeCallbacks(updatePositionRunnable);

        seekbar.setProgress(player.getCurrentPosition());

        handler.postDelayed(updatePositionRunnable, UPDATE_FREQUENCY);
    }


    private class MediaCursorAdapter extends SimpleCursorAdapter {

        public MediaCursorAdapter(Context context, int layout, Cursor c) {

            super(context, layout, c,
                    new String[]{MediaStore.MediaColumns.DISPLAY_NAME, MediaStore.MediaColumns.TITLE, MediaStore.Audio.AudioColumns.DURATION},
                    new int[]{R.id.displayname, R.id.title, R.id.duration});

        }

        @Override
        public void bindView(View view, Context context, Cursor cursor) {
            TextView title = (TextView) view.findViewById(R.id.title);
            TextView name = (TextView) view.findViewById(R.id.displayname);
            TextView duration = (TextView) view.findViewById(R.id.duration);

            name.setText(cursor.getString(
                    cursor.getColumnIndex(MediaStore.MediaColumns.DISPLAY_NAME)));

            title.setText(cursor.getString(
                    cursor.getColumnIndex(MediaStore.MediaColumns.TITLE)));

            long durationInMs = Long.parseLong(cursor.getString(
                    cursor.getColumnIndex(MediaStore.Audio.AudioColumns.DURATION)));

            double durationInMin = ((double) durationInMs / 1000.0) / 60.0;

            durationInMin = new BigDecimal(Double.toString(durationInMin)).setScale(2, BigDecimal.ROUND_UP).doubleValue();

            duration.setText("" + durationInMin);

            view.setTag(cursor.getString(cursor.getColumnIndex(MediaStore.MediaColumns.DATA)));
        }

        @Override
        public View newView(Context context, Cursor cursor, ViewGroup parent) {
            LayoutInflater inflater = LayoutInflater.from(context);
            View v = inflater.inflate(R.layout.listitem, parent, false);

            bindView(v, context, cursor);

            return v;
        }
    }

    private View.OnClickListener onButtonClick = new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            switch (v.getId()) {
                case R.id.play: {
                    if (player.isPlaying()) {
                        handler.removeCallbacks(updatePositionRunnable);
                        player.pause();
                        playButton.setImageResource(android.R.drawable.ic_media_play);
                    } else {
                        if (isStarted) {
                            player.start();
                            playButton.setImageResource(android.R.drawable.ic_media_pause);

                            updatePosition();
                        } else {
                            startPlay(currentFile);
                        }
                    }

                    break;
                }
                case R.id.next: {
                    int seekto = player.getCurrentPosition() + STEP_VALUE;

                    if (seekto > player.getDuration())
                        seekto = player.getDuration();

                    player.pause();
                    player.seekTo(seekto);
                    player.start();

                    break;
                }
                case R.id.prev: {
                    int seekto = player.getCurrentPosition() - STEP_VALUE;

                    if (seekto < 0)
                        seekto = 0;

                    player.pause();
                    player.seekTo(seekto);
                    player.start();

                    break;
                }
            }
        }
    };

    private MediaPlayer.OnCompletionListener onCompletion = new MediaPlayer.OnCompletionListener() {

        @Override
        public void onCompletion(MediaPlayer mp) {
            stopPlay();
        }
    };

    private MediaPlayer.OnErrorListener onError = new MediaPlayer.OnErrorListener() {

        @Override
        public boolean onError(MediaPlayer mp, int what, int extra) {
            // returning false will call the OnCompletionListener
            return false;
        }
    };

    private SeekBar.OnSeekBarChangeListener seekBarChanged = new SeekBar.OnSeekBarChangeListener() {
        @Override
        public void onStopTrackingTouch(SeekBar seekBar) {
            isMoveingSeekBar = false;
        }

        @Override
        public void onStartTrackingTouch(SeekBar seekBar) {
            isMoveingSeekBar = true;
        }

        @Override
        public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
            if (isMoveingSeekBar) {
                player.seekTo(progress);

                Log.i("OnSeekBarChangeListener", "onProgressChanged");
            }
        }
    };}

---

感谢您的帮助

因为您需要外部存储（

MediaStore.Audio.Media.external\u CONTENT\u URI

）上的文件，所以没有外部存储的手机不会返回任何查询结果

要正确处理空结果，请尝试更改：

  //...
  if (null != cursor) {
      cursor.moveToFirst();
      //...
  }

为此：

  //...
  if (null != cursor &&  cursor.moveToFirst()) {
      //...
  }

更新

如果要从内部存储器中提取文件，只需将查询更改为：

Cursor cursor = getContentResolver().query(MediaStore.Audio.Media.INTERNAL_CONTENT_URI, null, null, null, null);

---

请添加您的堆栈跟踪。我添加了日志，谢谢您的帮助。如果我没有解释清楚，很抱歉，但我不会说英语。我需要一个内部存储的方法。谢谢您的帮助。@MichelePio我已经更新了我的答案，尝试根据我的建议更改您的查询谢谢您的回答，但我只找到了系统的歌曲。@MichelePio我已经回答了您的问题。如果您需要更多帮助，您需要澄清您的最终目标。使用此方法，我可以获得系统声音，但我需要歌曲（存档中的歌曲，例如您下载的歌曲）

  //...
  if (null != cursor) {
      cursor.moveToFirst();
      //...
  }

  //...
  if (null != cursor &&  cursor.moveToFirst()) {
      //...
  }

Cursor cursor = getContentResolver().query(MediaStore.Audio.Media.INTERNAL_CONTENT_URI, null, null, null, null);