我正在尝试开发音乐播放器，但在从服务调用方法（skipToNext）时遇到问题... 我正在尝试从PlayerActivity中的服务调用skipToNext函数（单击前进的图像按钮），但是发生nullPointer异常。
PlayerActivity：
public class PlayerActivity extends AppCompatActivity {
private MediaPlayerService service = new MediaPlayerService();
private SeekBar seekBar;
private TextView currentDurationTextView;
MusicFragment fragment = new MusicFragment();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_player);
setupViews();
}
private void setupViews() {
//Setup Play Button
final ImageView playImageView = findViewById(R.id.iv_play);
if (MediaPlayerService.mediaPlayer.isPlaying()) {
playImageView.setImageDrawable(ResourcesCompat.getDrawable(getResources(), R.drawable.ic_stat_pause, null));
}
playImageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (MediaPlayerService.mediaPlayer.isPlaying()) {
service.pauseMedia();
playImageView.setImageDrawable(ResourcesCompat.getDrawable(getResources(), R.drawable.ic_stat_play, null));
} else if (!MediaPlayerService.mediaPlayer.isPlaying()) {
service.resumeMedia();
playImageView.setImageDrawable(ResourcesCompat.getDrawable(getResources(), R.drawable.ic_stat_pause, null));
}
}
});
//Setup Forward Button
ImageView forwardImageView = findViewById(R.id.iv_forward);
forwardImageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
service.skipToNext();
}
});
// Setup Rewind Button
ImageView rewindImageView = findViewById(R.id.iv_rewind);
rewindImageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
service.skipToPrevious();
}
});
//setup title TextView
TextView titleTextView = findViewById(R.id.txt_song_title);
titleTextView.setText(HomeActivity.getInstance().titleAudioList.get(fragment.audioIndex).getTitle());
//Setup Duration TextView
TextView totalDurationTextView = findViewById(R.id.txt_total_time);
totalDurationTextView.setText(formatDuration(MediaPlayerService.mediaPlayer.getDuration()));
currentDurationTextView = findViewById(R.id.txt_current_time);
// Setup SeekBar
seekBar = findViewById(R.id.seek_bar);
seekBar.setMax(MediaPlayerService.mediaPlayer.getDuration());
seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
@Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
if (fromUser) {
MediaPlayerService.mediaPlayer.seekTo(progress);
}
}
@Override
public void onStartTrackingTouch(SeekBar seekBar) {
}
@Override
public void onStopTrackingTouch(SeekBar seekBar) {
}
});
Timer timer = new Timer();
timer.schedule(new MainTimer(), 0, 1000);
}
private String formatDuration(long duration) {
int second = (int) duration / 1000;
int minute = second / 60;
second %= 60;
return String.format(Locale.ENGLISH, "%02d", minute) + ":" + String.format(Locale.ENGLISH, "%02d", second);
}
private class MainTimer extends TimerTask {
@Override
public void run() {
runOnUiThread(new Runnable() {
@Override
public void run() {
seekBar.setProgress(MediaPlayerService.mediaPlayer.getCurrentPosition());
currentDurationTextView.setText(formatDuration(MediaPlayerService.mediaPlayer.getCurrentPosition()));
}
});
}
}
}
和我的服务：
public class MediaPlayerService extends Service implements MediaPlayer.OnCompletionListener, MediaPlayer.OnPreparedListener,
MediaPlayer.OnErrorListener, MediaPlayer.OnSeekCompleteListener, MediaPlayer.OnInfoListener
, AudioManager.OnAudioFocusChangeListener {
public void skipToNext() {
if (audioIndex == audioList.size() - 1) {
audioIndex = 0;
activeAudio = audioList.get(audioIndex);
} else {
activeAudio = audioList.get(++audioIndex);
}
new StorageUtil(getApplicationContext()).storeAudioIndex(audioIndex);
stopMedia();
mediaPlayer.reset();
initMediaPlayer();
}
}
com.example.developer.musicplayer E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.developer.musicplayer, PID: 7461
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:107)
at com.example.developer.musicplayer.Service.MediaPlayerService.skipToNext(MediaPlayerService.java:293)
at com.example.developer.musicplayer.Activity.PlayerActivity$2.onClick(PlayerActivity.java:61)
at android.view.View.performClick(View.java:5198)
at android.view.View$PerformClick.run(View.java:21147)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
答案 0 :(得分：0)
这不是您使用Service的方式。要么：
摆脱extends Service并在MediaPlayerService中修复其余代码，或者
正确地使用Service，而不是自己创建一个实例（不要创建new MediaPlayerService()），而是使用startService()或也许使用bindService()
您当前的代码为您提供了MediaPlayerService且未正确初始化。结果，您从Service继承的方法（例如getApplicationContext()）将不起作用。
The documentation介绍了如何使用Service。 Android应用程序开发方面的书籍也是如此-free older editions中的one of my books包含了大量有关使用服务的资料。
答案 1 :(得分：0)
尝试这样：
new StorageUtil(this).storeAudioIndex(audioIndex);