构建音频播放器时出错

Question

我正在使用来自http://www.androidhive.info/2012/03/android-building-audio-player-tutorial/的音频播放器现有代码构建。 但是我在模拟器中运行此代码，但出现以下错误。

Logcat：

10-11 17:17:56.707: E/AndroidRuntime(303): FATAL EXCEPTION: main
10-11 17:17:56.707: E/AndroidRuntime(303): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidbuildingmusicplayeractivity/com.example.androidbuildingmusicplayeractivity.AndroidBuildingMusicPlayerActivity}: java.lang.NullPointerException
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.os.Looper.loop(Looper.java:123)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread.main(ActivityThread.java:4627)
10-11 17:17:56.707: E/AndroidRuntime(303):  at java.lang.reflect.Method.invokeNative(Native Method)
10-11 17:17:56.707: E/AndroidRuntime(303):  at java.lang.reflect.Method.invoke(Method.java:521)
10-11 17:17:56.707: E/AndroidRuntime(303):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
10-11 17:17:56.707: E/AndroidRuntime(303):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
10-11 17:17:56.707: E/AndroidRuntime(303):  at dalvik.system.NativeStart.main(Native Method)
10-11 17:17:56.707: E/AndroidRuntime(303): Caused by: java.lang.NullPointerException
10-11 17:17:56.707: E/AndroidRuntime(303):  at com.example.androidbuildingmusicplayeractivity.AndroidBuildingMusicPlayerActivity.onCreate(AndroidBuildingMusicPlayerActivity.java:76)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-11 17:17:56.707: E/AndroidRuntime(303):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
10-11 17:17:56.707: E/AndroidRuntime(303):  ... 11 more

SongsManager类：

public class SongsManager {

    // SDCard Path
    final String MEDIA_PATH = new String("/sdcard/");
    private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

    // Constructor
    public SongsManager(){

    }

    /**
     * Function to read all mp3 files from sdcard
     * and store the details in ArrayList
     * */
    public ArrayList<HashMap<String, String>> getPlayList(){
        File home = new File(MEDIA_PATH);

        if (home.listFiles(new FileExtensionFilter()).length > 0) {
            for (File file : home.listFiles(new FileExtensionFilter())) {
                HashMap<String, String> song = new HashMap<String, String>();
                song.put("songTitle", file.getName().substring(0, (file.getName().length() - 4)));
                song.put("songPath", file.getPath());

                // Adding each song to SongList
                songsList.add(song);
            }
        }
        // return songs list array
        return songsList;
    }

    /**
     * Class to filter files which are having .mp3 extension
     * */
    class FileExtensionFilter implements FilenameFilter {
        public boolean accept(File dir, String name) {
            return (name.endsWith(".mp3") || name.endsWith(".MP3"));
        }
    }

}

AndroidBuildingMusicPlayerActivity.java：

public class AndroidBuildingMusicPlayerActivity extends Activity implements OnCompletionListener, SeekBar.OnSeekBarChangeListener{

    private ImageButton btnPlay;
    private ImageButton btnForward;
    private ImageButton btnBackward;
    private ImageButton btnNext;
    private ImageButton btnPrevious;
    private ImageButton btnPlaylist;
    private ImageButton btnRepeat;
    private ImageButton btnShuffle;
    private SeekBar songProgressBar;
    private TextView songTitleLabel;
    private TextView songCurrentDurationLabel;
    private TextView songTotalDurationLabel;
    // Media Player
    private  MediaPlayer mp;
    // Handler to update UI timer, progress bar etc,.
    private Handler mHandler = new Handler();;
    private SongsManager songManager;
    private Utilities utils;
    private int seekForwardTime = 5000; // 5000 milliseconds
    private int seekBackwardTime = 5000; // 5000 milliseconds
    private int currentSongIndex = 0; 
    private boolean isShuffle = false;
    private boolean isRepeat = false;
    private ArrayList<HashMap<String, String>> songsList = new ArrayList<HashMap<String, String>>();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);     
        // All player buttons
        btnPlay = (ImageButton) findViewById(R.id.btnPlay);
        btnForward = (ImageButton) findViewById(R.id.btnForward);
        btnBackward = (ImageButton) findViewById(R.id.btnBackward);
        btnNext = (ImageButton) findViewById(R.id.btnNext);
        btnPrevious = (ImageButton) findViewById(R.id.btnPrevious);
        btnPlaylist = (ImageButton) findViewById(R.id.btnPlaylist);
        btnRepeat = (ImageButton) findViewById(R.id.btnRepeat);
        btnShuffle = (ImageButton) findViewById(R.id.btnShuffle);
        songProgressBar = (SeekBar) findViewById(R.id.songProgressBar);
        songTitleLabel = (TextView) findViewById(R.id.songTitle);
        songCurrentDurationLabel = (TextView) findViewById(R.id.songCurrentDurationLabel);
        songTotalDurationLabel = (TextView) findViewById(R.id.songTotalDurationLabel);

        // Mediaplayer
        mp = new MediaPlayer();
        songManager = new SongsManager();
        utils = new Utilities();        
        // Listeners
        songProgressBar.setOnSeekBarChangeListener(this); // Important
        mp.setOnCompletionListener(this); // Important

        // Getting all songs list
        songsList = songManager.getPlayList();

        // By default play first song
        playSong(0);

        /**
         * Play button click event
         * plays a song and changes button to pause image
         * pauses a song and changes button to play image
         * */
        btnPlay.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // check for already playing
                if(mp.isPlaying()){
                    if(mp!=null){
                        mp.pause();
                        // Changing button image to play button
                        btnPlay.setImageResource(R.drawable.btn_play);
                    }
                }else{
                    // Resume song
                    if(mp!=null){
                        mp.start();
                        // Changing button image to pause button
                        btnPlay.setImageResource(R.drawable.btn_pause);
                    }
                }

            }
        });

        /**
         * Forward button click event
         * Forwards song specified seconds
         * */
        btnForward.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // get current song position                
                int currentPosition = mp.getCurrentPosition();
                // check if seekForward time is lesser than song duration
                if(currentPosition + seekForwardTime <= mp.getDuration()){
                    // forward song
                    mp.seekTo(currentPosition + seekForwardTime);
                }else{
                    // forward to end position
                    mp.seekTo(mp.getDuration());
                }
            }
        });

        /**
         * Backward button click event
         * Backward song to specified seconds
         * */
        btnBackward.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // get current song position                
                int currentPosition = mp.getCurrentPosition();
                // check if seekBackward time is greater than 0 sec
                if(currentPosition - seekBackwardTime >= 0){
                    // forward song
                    mp.seekTo(currentPosition - seekBackwardTime);
                }else{
                    // backward to starting position
                    mp.seekTo(0);
                }

            }
        });

        /**
         * Next button click event
         * Plays next song by taking currentSongIndex + 1
         * */
        btnNext.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // check if next song is there or not
                if(currentSongIndex < (songsList.size() - 1)){
                    playSong(currentSongIndex + 1);
                    currentSongIndex = currentSongIndex + 1;
                }else{
                    // play first song
                    playSong(0);
                    currentSongIndex = 0;
                }

            }
        });

        /**
         * Back button click event
         * Plays previous song by currentSongIndex - 1
         * */
        btnPrevious.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                if(currentSongIndex > 0){
                    playSong(currentSongIndex - 1);
                    currentSongIndex = currentSongIndex - 1;
                }else{
                    // play last song
                    playSong(songsList.size() - 1);
                    currentSongIndex = songsList.size() - 1;
                }

            }
        });

        /**
         * Button Click event for Repeat button
         * Enables repeat flag to true
         * */
        btnRepeat.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                if(isRepeat){
                    isRepeat = false;
                    Toast.makeText(getApplicationContext(), "Repeat is OFF", Toast.LENGTH_SHORT).show();
                    btnRepeat.setImageResource(R.drawable.btn_repeat);
                }else{
                    // make repeat to true
                    isRepeat = true;
                    Toast.makeText(getApplicationContext(), "Repeat is ON", Toast.LENGTH_SHORT).show();
                    // make shuffle to false
                    isShuffle = false;
                    btnRepeat.setImageResource(R.drawable.btn_repeat_focused);
                    btnShuffle.setImageResource(R.drawable.btn_shuffle);
                }   
            }
        });

        /**
         * Button Click event for Shuffle button
         * Enables shuffle flag to true
         * */
        btnShuffle.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                if(isShuffle){
                    isShuffle = false;
                    Toast.makeText(getApplicationContext(), "Shuffle is OFF", Toast.LENGTH_SHORT).show();
                    btnShuffle.setImageResource(R.drawable.btn_shuffle);
                }else{
                    // make repeat to true
                    isShuffle= true;
                    Toast.makeText(getApplicationContext(), "Shuffle is ON", Toast.LENGTH_SHORT).show();
                    // make shuffle to false
                    isRepeat = false;
                    btnShuffle.setImageResource(R.drawable.btn_shuffle_focused);
                    btnRepeat.setImageResource(R.drawable.btn_repeat);
                }   
            }
        });

        /**
         * Button Click event for Play list click event
         * Launches list activity which displays list of songs
         * */
        btnPlaylist.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                Intent i = new Intent(AndroidBuildingMusicPlayerActivity.this, PlayListActivity.class);
                startActivityForResult(i, 100);         
            }
        });

    }

    /**
     * Receiving song index from playlist view
     * and play the song
     * */
    @Override
    protected void onActivityResult(int requestCode,
                                     int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(resultCode == 100){
             currentSongIndex = data.getExtras().getInt("songIndex");
             // play selected song
             playSong(currentSongIndex);
        }

    }

    /**
     * Function to play a song
     * @param songIndex - index of song
     * */
    public void  playSong(int songIndex){
        // Play song
        try {
            mp.reset();
            mp.setDataSource(songsList.get(songIndex).get("songPath"));
            mp.prepare();
            mp.start();
            // Displaying Song title
            String songTitle = songsList.get(songIndex).get("songTitle");
            songTitleLabel.setText(songTitle);

            // Changing Button Image to pause image
            btnPlay.setImageResource(R.drawable.btn_pause);

            // set Progress bar values
            songProgressBar.setProgress(0);
            songProgressBar.setMax(100);

            // Updating progress bar
            updateProgressBar();            
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * Update timer on seekbar
     * */
    public void updateProgressBar() {
        mHandler.postDelayed(mUpdateTimeTask, 100);        
    }   

    /**
     * Background Runnable thread
     * */
    private Runnable mUpdateTimeTask = new Runnable() {
           public void run() {
               long totalDuration = mp.getDuration();
               long currentDuration = mp.getCurrentPosition();

               // Displaying Total Duration time
               songTotalDurationLabel.setText(""+utils.milliSecondsToTimer(totalDuration));
               // Displaying time completed playing
               songCurrentDurationLabel.setText(""+utils.milliSecondsToTimer(currentDuration));

               // Updating progress bar
               int progress = (int)(utils.getProgressPercentage(currentDuration, totalDuration));
               //Log.d("Progress", ""+progress);
               songProgressBar.setProgress(progress);

               // Running this thread after 100 milliseconds
               mHandler.postDelayed(this, 100);
           }
        };

    /**
     * 
     * */
    @Override
    public void onProgressChanged(SeekBar seekBar, int progress, boolean fromTouch) {

    }

    /**
     * When user starts moving the progress handler
     * */
    @Override
    public void onStartTrackingTouch(SeekBar seekBar) {
        // remove message Handler from updating progress bar
        mHandler.removeCallbacks(mUpdateTimeTask);
    }

    /**
     * When user stops moving the progress hanlder
     * */
    @Override
    public void onStopTrackingTouch(SeekBar seekBar) {
        mHandler.removeCallbacks(mUpdateTimeTask);
        int totalDuration = mp.getDuration();
        int currentPosition = utils.progressToTimer(seekBar.getProgress(), totalDuration);

        // forward or backward to certain seconds
        mp.seekTo(currentPosition);

        // update timer progress again
        updateProgressBar();
    }

    /**
     * On Song Playing completed
     * if repeat is ON play same song again
     * if shuffle is ON play random song
     * */
    @Override
    public void onCompletion(MediaPlayer arg0) {

        // check for repeat is ON or OFF
        if(isRepeat){
            // repeat is on play same song again
            playSong(currentSongIndex);
        } else if(isShuffle){
            // shuffle is on - play a random song
            Random rand = new Random();
            currentSongIndex = rand.nextInt((songsList.size() - 1) - 0 + 1) + 0;
            playSong(currentSongIndex);
        } else{
            // no repeat or shuffle ON - play next song
            if(currentSongIndex < (songsList.size() - 1)){
                playSong(currentSongIndex + 1);
                currentSongIndex = currentSongIndex + 1;
            }else{
                // play first song
                playSong(0);
                currentSongIndex = 0;
            }
        }

    }
    @Override
     public void onDestroy(){
     super.onDestroy();
        mp.release();
     }

Answer 1

我尝试了另一个类似的初学者mediaplayer代码，我花了一段时间才意识到，崩溃的原因是它无法处理空文件夹。

由于sdcard根目录中没有mp3，因此将其更改为sdcard/Music ，在这里我保持音乐，一切正常。

我没有仔细查看您的代码，但可能与尝试获取空文件列表的原因相同。

Answer 2

这部分不正确，但这不是造成问题的原因：

原始代码：

        public void onClick(View arg0) {
           // check for already playing
            if(mp.isPlaying()){
                if(mp!=null){
                    mp.pause();
                    // Changing button image to play button
                    btnPlay.setImageResource(R.drawable.btn_play);
                }

应该：

        public void onClick(View arg0) {
           // check for already playing
            if(mp!=null){
                if(mp.isPlaying()){
                    mp.pause();
                    // Changing button image to play button
                    btnPlay.setImageResource(R.drawable.btn_play);
                }

问题是您的堆栈跟踪清楚地表明问题不是此问题（尽管这也会引起NullPointerException，但仅在单击按钮后才会浮出水面...）

我会尝试进一步研究。