android.app.Application cannot be instantiated due to NullPointerException

问题:

I'm kind of a noob in the android world, and doing a pet project for exercising. It's a very simple reminder-like app with just two activities. One is a customized ListView display the existing alarms. There are some buttons in it to start the other one, which is for add/edit alarms. There is a button in it lead to the previous ListView activity.

There is a weird situation I ran into recently. My app works fine. But the problem is, whenever I trigger the add/edit activity then go back to the ListView, and re-run(or I should say re-install?) the app. An error message will popup. But it will only show up briefly and the the app will start.

The error message I caught in the log says:

FATAL EXCEPTION: main java.lang.RuntimeException: Unable to instantiate application android.app.Application: java.lang.NullPointerException at android.app.LoadedApk.makeApplication(LoadedApk.java:482) at android.app.ActivityThread.handleBindApplication(ActivityThread.java:3909) at android.app.ActivityThread.access$1300(ActivityThread.java:122) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1184) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4340) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.NullPointerException at android.app.LoadedApk.initializeJavaContextClassLoader(LoadedApk.java:362) at android.app.LoadedApk.getClassLoader(LoadedApk.java:305) at android.app.LoadedApk.makeApplication(LoadedApk.java:474) ... 11 more 

It doesn't explicitly point out where is wrong in my codes. So I don't have a clue about how to correct it. Does anyone encountered similar problem? Any suggestion will be appreciated!

Here is codes of add/edit activity:

public class EditEntry extends Activity {     private AutoCompleteTextView foodNameTextView;     private DatePicker datePicker;     // store values in AutoCompleteTextView & DatePicker     private String foodName;     private Calendar foodDate;      @Override     protected void onCreate(Bundle savedInstanceState)     {         super.onCreate(savedInstanceState);         setContentView(R.layout.edit);         // dummy selections for AutoCompleteTextView         String[] foodList = new String[]{"meat", "fruit", "vega"};         // instantiate AutoCompleteTextView & DatePicker         ArrayAdapter arrayAdapter = new ArrayAdapter(this, R.layout.food_list_dropdown, foodList);         foodNameTextView = (AutoCompleteTextView)findViewById(R.id.foodName);         foodNameTextView.setAdapter(arrayAdapter);         datePicker = (DatePicker)findViewById(R.id.date_picker);         // get intent from ReminderList.         Intent intent = getIntent();         // get extras from intent. Return null if intent is sent from "add" action.         foodName = intent.getStringExtra("foodName");         foodDate = (Calendar) intent.getSerializableExtra("foodDate");         // set default values for widgets if it is an "edit" action.         if (null != foodName)         {             foodNameTextView.setText(foodName);             datePicker.init(foodDate.get(Calendar.YEAR), foodDate.get(Calendar.MONTH), foodDate.get(Calendar.DAY_OF_MONTH),                      new OnDateChangedListener()                     {   // will implement date input check later.                         @Override                         public void onDateChanged(DatePicker view, int year, int monthOfYear, int dayOfMonth)   {}                     });         }          // Submit will add/modify the data in xml file. Back will start ReminderList activity         Button submit = (Button)findViewById(R.id.entry_submit);         Button back = (Button)findViewById(R.id.entry_back);          submit.setOnClickListener(new SubmitButtonListener());         back.setOnClickListener(new OnClickListener()         {             @Override             public void onClick(View v)             {                 Intent intent = new Intent();                 intent.setClass(EditEntry.this, FoodReminderList.class);                 startActivity(intent);             }         });     }      // add or modify data in xml file     @SuppressWarnings("unused")     class SubmitButtonListener implements OnClickListener     {         Calendar foodDate = Calendar.getInstance();          @Override         public void onClick(View v)         {             XmlUtil xmlUtil = new XmlUtil();             // determine if it is an "edit" action.             if (null != foodName)             {                 FoodInfo foodInfo = new FoodInfo(foodName, foodDate);                 // delete the old data entry                 xmlUtil.deleteEntry(foodInfo);                 // cancel old alarm                 FoodReceiver alarm = new FoodReceiver(EditEntry.this, foodDate, false);             }             // get new input values              foodName = foodNameTextView.getText().toString();             foodDate.set(datePicker.getYear(), datePicker.getMonth(), datePicker.getDayOfMonth(), 0, 0, 0);             // update xml file             FoodInfo foodInfo = new FoodInfo(foodName, foodDate);             xmlUtil.updateEntry(foodInfo);             // set new alarm             FoodReceiver alarm = new FoodReceiver(EditEntry.this, foodDate, true);             // popup toast confirming the submit             Toast.makeText(EditEntry.this, "Reminder Added", Toast.LENGTH_SHORT).show();             // clear widgets             foodNameTextView.setText("");             Calendar currentDate = Calendar.getInstance();             datePicker.updateDate(currentDate.get(Calendar.YEAR), currentDate.get(Calendar.MONTH), currentDate.get(Calendar.DAY_OF_MONTH));         }     } } 

The Manifest goes like this:

回答1:

I had exactly the same error, although with other and more simpler code than yours. I found out that the problem occurred only if I didn't close the app in the emulator after re-run it from eclipse. So if I close it before run it from eclipse, everything works fine. But since I am really new to android, I have no clue why this is so. I previous test apps this was never a problem.

回答2:

I did some more tests and lockon the problem in the anonymous inner class for Back button, since the error will appear whenever I clicked this before restarting the app. If I switch activities using the "go back" button on the phone, everything is fine.

I added one line in the inner class

EditEntry.this.finish(); 

It does solve the problem. I tried to test it more systematical this time. Let's say the code without the above line is A, and the code with this line is B. Scenario goes like follow

1.run A then A --> Error

2.run A then B --> Error

3.run B then A --> OK

4.run B then B --> OK

Therefore, I assume it is some kind of problem about the activity stack?

回答3:

Actually, the real solution to this problem is a folder permissions issue. Make sure you are operating in a folder that you can modify. I just fixed that same problem for my self by making sure I was in a folder with the right permissions.

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