使用异步任务时，光标变为空？

问题描述

我有一个数据库代码，该数据库代码从表中获取数据并将其存储到游标中，但是当我打开在回收站视图上显示数据的活动时，在主线程上运行数据库代码会使我的应用变慢。因此，我将数据库代码移至asyntask内部类并尝试运行它。

但是，在asyntask在onCreate ()中运行之后，我的回收器视图适配器代码运行了，并且由于游标为空，在游标对象上调用的getItemCount ()引发了null pointer exception。

从调试中，我发现游标变为null的位置，但不明白为什么它变为null。 accessDatabase ()运行时的游标不为null，但是asynctask内部类在onCreate ()中运行后，游标变为null。

onCreate方法中的代码：

protected void onCreate (Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.item_grocery);

    RecyclerView groceryRecycler = (RecyclerView) findViewById(R.id.grocery_recycler_view);

    StartDatabase sd = new StartDatabase();
    sd.execute(databaseHelper);
    Log.d ("blue","sd has ran");

    if (cursor == null) {
        Log.d ("green","cursor is null");//I receive this in the log cat
    }

    captionedImagesAdapter adapter = new captionedImagesAdapter (this,cursor);
    GridLayoutManager layoutManager = new GridLayoutManager(this,2,GridLayoutManager.VERTICAL,false);
    groceryRecycler.setLayoutManager(layoutManager);
    groceryRecycler.setAdapter (adapter);
}

异步任务内部类：

private class StartDatabase extends AsyncTask<MyDatabaseHelper,Void,Boolean> {

    protected void onPreExecute () {}

    @Override
    protected Boolean doInBackground(MyDatabaseHelper... myDatabaseHelpers) {
        try {
            accessDataBase();

            return true;
        } catch (SQLiteException e) {
            return false;

        }
    }

    protected void onPostExecute (Boolean success) {
        if (!success) {
            Toast toast = Toast.makeText(grocery_item.this,"Database unavailable",Toast.LENGTH_SHORT);
            toast.show();
        }

        }
    }
}

访问数据库方法：

public void accessDataBase () {

    try {

        db = databaseHelper.getReadableDatabase();

        cursor = db.query ("GROCERY_DATA",new String[] {"NAME","PATHS"},null,null);

        if (cursor == null) {
            Log.d ("blue","cursor is null");
        }

    } catch (SQLiteException e) {
        e.printStackTrace();
    }

}

我的回收视图适配器：

public class captionedImagesAdapter extends RecyclerView.Adapter <captionedImagesAdapter.ViewHolder> {

private Context context;
private Cursor cursor;

public captionedImagesAdapter (Context context,Cursor cursor) {
    this.context = context;
    this.cursor = cursor;

    if (this.cursor == null) {
        Log.d ("red","cursor is null");//I receive this in the log cat 
    }
}

public captionedImagesAdapter.ViewHolder onCreateViewHolder (ViewGroup parent,int viewType) {
    LayoutInflater inflater = LayoutInflater.from(context);
    CardView cv = (CardView) inflater.inflate(R.layout.card_view,parent,false);

    return new ViewHolder (cv);
}

public void onBindViewHolder(ViewHolder holder,int position) {
    if (!cursor.moveToPosition(position)) {
        return;
    }

    String info_text = cursor.getString (0);
    holder.textView.setText(info_text);


    String image_Path = cursor.getString(1);
    File file = new File (image_Path);

    if (file.exists()) {

        Bitmap bitmap = BitmapFactory.decodeFile (file.getAbsolutePath());

        holder.imageView.setImageBitmap(bitmap);
    }
}

public int getItemCount() {
    return cursor.getCount();//Line throwing the exception
}

public static class ViewHolder extends RecyclerView.ViewHolder {
    private ImageView imageView;
    private TextView textView;

    public ViewHolder(CardView view) {
        super(view);
        imageView = view.findViewById(R.id.info_image);
        textView = view.findViewById(R.id.info_text);
    }
}

}

我收到的例外情况：

java.lang.NullPointerException: Attempt to invoke interface method 'int android.database.Cursor.getCount()' on a null object reference
    at com.myapps.myapplication.captionedImagesAdapter.getItemCount(captionedImagesAdapter.java:58)
    at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:4135)
    at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3939)
    at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4499)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1829)
    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1673)
    at android.widget.LinearLayout.onLayout(LinearLayout.java:1582)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.widget.FrameLayout.layoutChildren(FrameLayout.java:332)
    at android.widget.FrameLayout.onLayout(FrameLayout.java:270)
    at com.android.internal.policy.DecorView.onLayout(DecorView.java:786)
    at android.view.View.layout(View.java:22085)
    at android.view.ViewGroup.layout(ViewGroup.java:6290)
    at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:3333)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2810)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1930)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7988)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1154)
    at android.view.Choreographer.doCallbacks(Choreographer.java:977)
    at android.view.Choreographer.doFrame(Choreographer.java:893)
    at android.view.Choreographer$FrameHandler.handleMessage(Choreographer.java:1082)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7682)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

解决方法

将其设为null的原因之一是由于错误的假设，即Main线程正在等待异步任务完成。那当然是错误的（否则，您将不需要其他线程）。您有不同的解决方案，例如：

• 使用CursorLoader （已弃用）
• AsyncTask （已弃用）完成加载游标后，使用回调通知UI线程。 here
示例
• 研究另一种技术，例如coroutines