java - getApplicationContext() 上的空指针异常

Question

我尝试了以下代码来扩展 Activity ，它运行得很好，但现在我更改了代码以使用扩展 fragment 。我在互联网上搜索并找到了 getActivity() 函数来调用 query() 和其他函数。我尝试了这个，但现在它给出了一个错误:

错误

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.admin.funtube_mp4/com.example.admin.funtube_mp4.Main}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2404)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5653)
            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:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.admin.funtube_mp4.HomeFragmant.init_phone_video_grid(HomeFragmant.java:65)
            at com.example.admin.funtube_mp4.HomeFragmant.onCreateView(HomeFragmant.java:49)
            at android.app.Fragment.performCreateView(Fragment.java:1700)
            at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:890)
            at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1062)
            at android.app.BackStackRecord.run(BackStackRecord.java:684)
            at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1453)
            at android.app.Activity.performStart(Activity.java:5550)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5653)
            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:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)

代码

public class HomeFragmant extends Fragment{
    private Cursor videocursor;
    private int video_column_index;
    ListView videolist;

    List<String> listpath = new ArrayList<String>();
    String[] paths=new String[100];
    int count;
    String[] thumbColumns = { MediaStore.Video.Thumbnails.DATA, MediaStore.Video.Thumbnails.VIDEO_ID };
    String path;
    File file;

    public HomeFragmant() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View rootView = inflater.inflate(R.layout.fragment_home, container, false);
        videolist = (ListView) getActivity(). findViewById(R.id.PhoneVideoList);
        init_phone_video_grid();

        return rootView;

    }
    private void init_phone_video_grid() {
        System.gc();
        String[] proj = { MediaStore.Video.Media._ID, MediaStore.Video.Media.DATA, MediaStore.Video.Media.DISPLAY_NAME, MediaStore.Video.Media.SIZE };

      //  file = new File(Environment.getExternalStorageDirectory() + File.separator + "Funtube/UserData/Videos/");
       // videocursor =getActivity().getContentResolver().query(MediaStore.Video.Media.EXTERNAL_CONTENT_URI, proj, MediaStore.Images.Media.DATA + " LIKE ? ", new String[]{"%" + file.getAbsolutePath().toString() + "%"}, null);
        videocursor = getActivity().getContentResolver().query(MediaStore.Video.Media.EXTERNAL_CONTENT_URI,proj, null, null, null);
//        videocursor =getActivity().managedQuery(MediaStore.Video.Media.EXTERNAL_CONTENT_URI, proj, MediaStore.Images.Media.DATA + " LIKE ? ", new String[]{"%" + file.getAbsolutePath().toString() + "%"}, null);

        count = videocursor.getCount();

        videolist.setAdapter(new VideoAdapter(getActivity().getApplicationContext()));
        videolist.setOnItemClickListener(videogridlistener);
    }

    private AdapterView.OnItemClickListener videogridlistener = new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView parent, View v, int position,long id) {
            System.gc();
            video_column_index = videocursor.getColumnIndexOrThrow(MediaStore.Video.Media.DATA);
            videocursor.moveToPosition(position);
            String filename = videocursor.getString(video_column_index);
            Intent intent = new Intent(getActivity(),ViewVideo.class);
            intent.putExtra("videofilename", filename);
            startActivity(intent);
        }
    };

    public class VideoAdapter extends BaseAdapter {
        private Context vContext;

        public VideoAdapter(Context c) {
            vContext = c;
        }

        public int getCount() {
            return count;
        }

        public Object getItem(int position) {
            return position;
        }

        public long getItemId(int position) {
            return position;
        }

        public View getView(int position, View convertView, ViewGroup parent) {
            System.gc();
            ViewHolder holder;
            String id = null;
            convertView = null;
            if (convertView == null) {
                convertView = LayoutInflater.from(vContext).inflate(R.layout.listitem, parent, false);
                holder = new ViewHolder();
                holder.txtTitle = (TextView) convertView.findViewById(R.id.txtTitle);
                holder.txtSize = (TextView) convertView.findViewById(R.id.txtSize);
                holder.thumbImage = (ImageView) convertView.findViewById(R.id.imgIcon);

                video_column_index = videocursor.getColumnIndexOrThrow(MediaStore.Video.Media.DISPLAY_NAME);
                videocursor.moveToPosition(position);
                id = videocursor.getString(video_column_index);
                video_column_index = videocursor.getColumnIndexOrThrow(MediaStore.Video.Media.SIZE);
                videocursor.moveToPosition(position);
                // id += " Size(KB):" +
                // videocursor.getString(video_column_index);
                holder.txtTitle.setText(id);
                holder.txtSize.setText(" Size(KB):"+ videocursor.getString(video_column_index));

                String[] proj = { MediaStore.Video.Media._ID, MediaStore.Video.Media.DISPLAY_NAME, MediaStore.Video.Media.DATA };
                @SuppressWarnings("deprecation")
                Cursor cursor = getActivity().getContentResolver().query(MediaStore.Video.Media.EXTERNAL_CONTENT_URI, proj, MediaStore.Video.Media.DISPLAY_NAME + "=?", new String[]{id}, null);
                cursor.moveToFirst();
                long ids = cursor.getLong(cursor
                        .getColumnIndex(MediaStore.Video.Media._ID));

                ContentResolver crThumb = getActivity().getContentResolver();
                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inSampleSize = 1;
                Bitmap curThumb = MediaStore.Video.Thumbnails.getThumbnail(
                        crThumb, ids, MediaStore.Video.Thumbnails.MICRO_KIND,
                        options);
                holder.thumbImage.setImageBitmap(curThumb);
                curThumb = null;

            } /*
             * else holder = (ViewHolder) convertView.getTag();
             */
            return convertView;
        }
    }

    static class ViewHolder {

        TextView txtTitle;
        TextView txtSize;
        ImageView thumbImage;
    }
}

Answer 1

不要在onCreateView中调用getActivity()，您无法保证Activity已在此处完全创建。使用 onActivityCreated 方法获取对 Activity 的引用。

除非您担心泄漏 Activity，否则您可以只使用 getActivity()，无需检索 ApplicationContext 来创建您的适配器恕我直言。

来自docs about Fragments :

Caution: If you need a Context object within your Fragment, you can call getActivity(). However, be careful to call getActivity() only when the fragment is attached to an activity. When the fragment is not yet attached, or was detached during the end of its lifecycle, getActivity() will return null.