Question

我又回来了另一个NullPointerException。它表示标记的行有错误。但是，我无法弄清楚导致问题的原因。我确定id是正确的并确实存在。

这是我的代码：

MainActivity.java：

    package com.bartkoppelmans.wavelength;

    import android.app.Activity;
    import android.content.ComponentName;
    import android.content.Context;
    import android.content.Intent;
    import android.content.ServiceConnection;
    import android.media.MediaPlayer;
    import android.os.Bundle;
    import android.os.IBinder;
    import android.view.GestureDetector;
    import android.view.MotionEvent;
    import android.view.View;
    import android.widget.FrameLayout;
    import android.widget.ImageView;
    import android.widget.ProgressBar;
    import android.widget.TextView;
    import android.widget.Toast;

    import java.text.SimpleDateFormat;
    import java.util.Date;
    import java.util.TimeZone;
    import java.util.Timer;
    import java.util.TimerTask;

    public class MainActivity extends Activity implements GestureDetector.OnGestureListener {

        GestureDetector gestureDetector;

        public static final boolean isCreated = false;
        public MediaPlayer mediaPlayer;
        Timer timer;
        boolean musicBound=false;
        private MusicService musicService;
        private Intent playIntent;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            gestureDetector = new GestureDetector( this );

            ImageView playButton = (ImageView) findViewById(R.id.albumImageView);
            playButton.setOnClickListener(new View.OnClickListener() {
                public void onClick(View v) {
                    playButtonPressed();
                }
            });

            FrameLayout frameLayout = (FrameLayout) findViewById(R.id.musicSelectionOpenerFrameLayout);
            frameLayout.setOnClickListener(new View.OnClickListener() {
                public void onClick(View v) {
                    Intent intent = new Intent(MainActivity.this, MusicSelection.class);
                    startActivity(intent);
                    overridePendingTransition(R.anim.slide_up_in, R.anim.slide_up_out);
                }
            });
        }

        //connect to the service
        public final ServiceConnection musicConnection = new ServiceConnection(){

            @Override
            public void onServiceConnected(ComponentName name, IBinder service) {
                MusicService.MusicBinder binder = (MusicService.MusicBinder)service;
                //get service
                musicService = binder.getService();
                musicBound = true;
            }

            @Override
            public void onServiceDisconnected(ComponentName name) {
                musicBound = false;
            }
        };

        //start and bind the service when the activity starts
        @Override
        public void onStart() {
            super.onStart();
            if(playIntent==null){
                playIntent = new Intent(this, MusicService.class);
                bindService(playIntent, musicConnection, Context.BIND_AUTO_CREATE);
                startService(playIntent);
            }
        }

        @Override
        protected void onNewIntent(Intent intent) {
            super.onNewIntent(intent);
            overridePendingTransition(R.anim.slide_down_in, R.anim.slide_down_out);
        }

        @Override
        protected void onDestroy() {
            mediaPlayer.release();
            mediaPlayer = null;
            super.onStop();
        }

        public void playButtonPressed() {
            if  (isCreated)  {
                if ( mediaPlayer.isPlaying() ) {
                    mediaPlayer.pause();
                } else {
                    mediaPlayer.start();
                }
            }else {
                updateTimer();
                Toast.makeText(getApplicationContext(), "Please select song", Toast.LENGTH_LONG).show();
            }
        }

        @Override
        public boolean onDown(MotionEvent e) {
            return false;
        }

        @Override
        public void onShowPress(MotionEvent e) {

        }

        @Override
        public boolean onSingleTapUp(MotionEvent e) {
            return false;
        }

        @Override
        public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
            return false;
        }

        @Override
        public void onLongPress(MotionEvent e) {

        }

        @Override
        public boolean onFling(MotionEvent start, MotionEvent finish, float velocityX, float velocityY) {
            return false;
        }

        @Override
        public boolean onTouchEvent(MotionEvent me) {
            return gestureDetector.onTouchEvent(me);
        }

        public void updateTimer() {
            timer = new Timer();
            timer.scheduleAtFixedRate(new TimerTask() {
                @Override
                public void run() {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
----------------------------final TextView timerTextView = (TextView)findViewById(R.id.timerTextView);
                            final ProgressBar timerProgressBar = (ProgressBar)findViewById(R.id.mainProgressbar);
                            if (mediaPlayer != null && mediaPlayer.isPlaying()) {
                                timerTextView.post(new Runnable() {
                                    @Override
                                    public void run() {
                                        int position = 5;
                                        int duration = 10;
                                        timerProgressBar.setMax(duration);
                                        SimpleDateFormat df = new SimpleDateFormat("HH:mm:ss");
                                        df.setTimeZone(TimeZone.getTimeZone("UTC"));
                                        final String time = df.format(new Date(position)) + " / " + df.format(new Date(duration));
                                        timerTextView.setText(time);
                                        timerProgressBar.setProgress(position);
                                    }
                                });
                            } else {
                                timer.cancel();
                                timer.purge();
                            }
                        }
                    });
                }
            }, 0, 100);
        }

    }

MusicService.java：

package com.bartkoppelmans.wavelength;

import android.app.Service;
import android.content.Intent;
import android.media.AudioManager;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Binder;
import android.os.Handler;
import android.os.IBinder;
import android.os.PowerManager;
import android.util.Log;

import java.util.Timer;

public class MusicService extends Service implements MediaPlayer.OnPreparedListener, MediaPlayer.OnErrorListener, MediaPlayer.OnCompletionListener {

    //media player
    private MediaPlayer mediaPlayer;
    //binder
    private final IBinder musicBind = new MusicBinder();
    Timer timer;
    Handler handler;

    public void onCreate(){
        handler = new Handler();
        //create the service
        super.onCreate();
        //create player
        mediaPlayer = new MediaPlayer();
        //initialize
        initMusicPlayer();
    }

    public void initMusicPlayer(){
        //set player properties
        mediaPlayer.setWakeMode(getApplicationContext(),
                PowerManager.PARTIAL_WAKE_LOCK);
        mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
        //set listeners
        mediaPlayer.setOnPreparedListener(this);
        mediaPlayer.setOnCompletionListener(this);
        mediaPlayer.setOnErrorListener(this);
    }

    //binder
    public class MusicBinder extends Binder {
        MusicService getService() {
            return MusicService.this;
        }
    }

    //activity will bind to service
    @Override
    public IBinder onBind(Intent intent) {
        return musicBind;
    }

    //release resources when unbind
    @Override
    public boolean onUnbind(Intent intent){
        mediaPlayer.stop();
        mediaPlayer.release();
        return false;
    }

    //play a song
    public void playSong(Uri songUri){
        //play
        mediaPlayer.reset();

        //set the data source
        try{
            mediaPlayer.setDataSource(getApplicationContext(), songUri);
        }
        catch(Exception e){
            Log.e("MUSIC SERVICE", "Error setting data source", e);
        }
        mediaPlayer.prepareAsync();

        MainActivity mainActivity = new MainActivity();
        mainActivity.updateTimer();
    }

    public void setSong(Uri newSongUri)  {
        Intent intent = new Intent(getApplicationContext(), MainActivity.class);
        startActivity(intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK));
        //getApplicationContext().startActivity(intent);

        playSong(newSongUri);
    }

    @Override
    public void onCompletion(MediaPlayer mediaPlayer) {
    }

    @Override
    public boolean onError(MediaPlayer mediaPlayer, int what, int extra) {
        return false;
    }

    @Override
    public void onPrepared(MediaPlayer mediaPlayer) {
        //start playback
        mediaPlayer.start();
    }

    private void runOnUiThread(Runnable runnable) {
        handler.post(runnable);
    }

    public int getDuration(){
        int duration = mediaPlayer.getDuration();
        return duration;
    }
    public int getPosition(){
        int position = mediaPlayer.getCurrentPosition();
        return position;
    }
}

Java堆栈跟踪：

12-31 15:52:40.088  28371-28371/com.bartkoppelmans.wavelength E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.bartkoppelmans.wavelength, PID: 28371
    java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:1965)
            at com.bartkoppelmans.wavelength.MainActivity$4$1.run(MainActivity.java:161)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:157)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

您无法在“活动与服务”之间进行通信

在MusicService内：

    MainActivity mainActivity = new MainActivity();
    mainActivity.updateTimer();

您的空指针是因为当您说new MainActivity()时，您创建的活动未附加到生命周期，因此onCreate和其他方法未被调用，因此没有onCreate表示否{ {1}}表示使用setContentView找到的所有观看均为空

要在服务和活动之间进行通信，请使用以下方法之一：

http://www.vogella.com/tutorials/AndroidServices/article.html#servicecommunication

http://developer.android.com/guide/components/bound-services.html#Messenger

Answer 2

使用您提供的信息进行调试真的非常困难。正如@duffymo所说，VM 总是正确。解决NPE的方法是查看堆栈跟踪，转到代码中的相应行，然后在该行上修改为null的变量。

在您的情况下，导致错误的原因是您在MainActivity中创建的playSong实例中没有根视图。 findViewById期望安装的视图，并且当它为null时失败。

您可能无法创建活动的新实例并期望它们正常运行。只有框架才能做到这一点。您可能需要找到一种从MainAcitivity.onStart更新计时器的方法。

NullPointerException（已更正）

2 个答案: