使用异步任务时光标变为空？

Posted 2023-05-07

【中文标题】使用异步任务时光标变为空？

【英文标题】：My cursor becomes null when I use async task?

【发布时间】：2020-12-25 15:28:04

【问题描述】：

我有一个数据库代码，它从我的表中获取数据并将其存储到我的光标中，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减慢我的应用程序的速度。所以我将数据库代码移动到一个 asyntask 内部类并尝试运行它。

但是，在 onCreate () 中运行 asyntask 后，我的回收器视图适配器代码运行并且在游标对象上调用的 getItemCount () 会抛出 null pointer exception，因为游标为空。

通过调试，我发现光标在哪里变为空，但不明白为什么它变为空。 accessDatabase ()运行时的游标不为空，但在onCreate ()运行asynctask内部类后游标变为空。

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, null, 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);
    

我收到的异常：

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)

【问题讨论】：

在你的异步任务的 postexecute() 中设置你的适配器，你应该很高兴

主线程不会等到完成 AsyncTask。我认为这是获取空值的问题。您可以将整个适配器类放在 onPostExecute() 中。

【参考方案1】：

您将其设为 null 的原因之一是错误地假设主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）。您有不同的选择来修复它，例如：

使用CursorLoader （已弃用） 当 AsyncTask（已弃用）完成加载光标时，使用回调通知 UI 线程。一个例子here 研究不同的技术，例如coroutines

【讨论】：

- 考虑任何其他异步解决方案（Rx、线程池、协程......）。 AsyncTask 和 Loaders 已经过时了。

关于异步任务的公平点。不适用于装载机 imo

developer.android.com/guide/components/loaders "从 Android P (API 28) 开始，加载程序已被弃用。"

我错过了。谢谢@Kikiwa