android - 应用程序退出后崩溃？

Question

我正在制作一个应用程序，其中有多个 Horizo​​ntalScrollview ，总体来说应用程序运行良好，但是当用户退出 Boom 应用程序崩溃时，logcast 告诉我它在 onPostExecute 中有多个标记:

rootView = inflater.inflate(R.layout.three_fragment, container, false);
    L1= (LinearLayout) rootView.findViewById(R.id.LinearLayout1);
    L3= (TextView) rootView.findViewById(R.id.CheckTest);
    prog =(ProgressBar)rootView.findViewById(R.id.progressBar);



        booksList = new ArrayList<Books>();



        new JSONAsyncTask().execute("http://192.168.30.3/actors/book_tab2.php");    

        centerLockHorizontalScrollview = (CenterLockHorizontalScrollview) rootView.findViewById(R.id.HMscrollView); 

          adapter = new CardHomeAdapter(getActivity(), R.layout.row_home_card, booksList);


        centerLockHorizontalScrollview.setAdapter(TabThreeFragment.this, adapter); 

            rootView.setLayoutParams(new FrameLayout.LayoutParams(FrameLayout.LayoutParams.MATCH_PARENT,FrameLayout.LayoutParams.MATCH_PARENT ));   


            }   
        }

执行后:

protected void onPostExecute(Boolean result) {

    adapter.notifyDataSetChanged();

    if(result == false)

        Toast.makeText(getActivity(), "Unable to fetch data from server", Toast.LENGTH_LONG).show(); 
    if(booksList != null) {


        adapter = new CardHomeAdapter(getActivity().getApplicationContext(), R.layout.row_home_card, booksList);  
       centerLockHorizontalScrollview.setAdapter(TabThreeFragment.this, adapter); 
    }

错误:

03-07 11:49:21.478: E/AndroidRuntime(9703): FATAL EXCEPTION: main
    03-07 11:49:21.478: E/AndroidRuntime(9703): java.lang.NullPointerException
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at br.liveo.fragments.TabThreeFragment$JSONAsyncTask.onPostExecute(TabThreeFragment.java:344)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at br.liveo.fragments.TabThreeFragment$JSONAsyncTask.onPostExecute(TabThreeFragment.java:1)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask.finish(AsyncTask.java:631)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.Handler.dispatchMessage(Handler.java:99)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.Looper.loop(Looper.java:137)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.app.ActivityThread.main(ActivityThread.java:4961)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at java.lang.reflect.Method.invokeNative(Native Method)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at java.lang.reflect.Method.invoke(Method.java:511)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
    03-07 11:49:21.478: E/AndroidRuntime(9703):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

当您退出应用程序时，仍然需要执行后执行，它会尝试调用 Activity 来显示 toast 或创建新的适配器。但是由于您的应用程序已被终止，因此该 Activity 不再存在，因此您会收到空指针异常。在显示 toast 或创建新适配器之前检查 if(getActivity() != null)，或者在 Activity 的 ondestroy 函数中终止任务。

protected void onPostExecute(Boolean result) {
if(getActivity() != null)
{
    adapter.notifyDataSetChanged();

    if(result == false)

        Toast.makeText(getActivity(), "Unable to fetch data from server", Toast.LENGTH_LONG).show(); 
    if(booksList != null) {


        adapter = new CardHomeAdapter(getActivity().getApplicationContext(), R.layout.row_home_card, booksList);  
       centerLockHorizontalScrollview.setAdapter(TabThreeFragment.this, adapter); 
}
    }