我有一个数据库代码，它从我的表中获取数据&将其存储到我的光标中，但当我打开在回收器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。所以我把数据库代码移到了一个asyntask内部类&试着运行它。
然而，在asyntask在onCreate ()中运行后，我的回收器视图适配器代码将运行&对游标对象调用的CCD_ 2由于游标为空而抛出CCD_。
通过调试，我发现了光标变为null的地方，但不明白为什么它变为null。当CCD_ 4运行时光标不为空，但在异步任务内部类在CCD_。
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();
}
}
}
}
访问数据库的方法：
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)
将其设为null的原因之一是错误地认为主线程正在等待异步任务完成。当然，这是错误的(否则你就不需要不同的线程(。你有不同的选项来修复它，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。这里有一个例子 - 研究不同的技术，比如协同程序