why a static instance initialized in Application.onCreate loses its value

Question

I initiate a singleton instance in Application.onCreate, this instance has a member mApplicationContext which is initiated by getApplicationContext(), and this is the only place mApplicationContext be assigned value. From the crash log, mApplicationContext becomes null in certain scenarios, my question is in which this would happen?

public class ClassicSingleton {
   private static ClassicSingleton instance = null;
   private Context mApplicationContext = null;
   private ClassicSingleton() {
   }
   public static ClassicSingleton getInstance() {
      if(instance == null) {
         instance = new ClassicSingleton();
      }
      return instance;
   }

   public void initiate(Context context){
        this.mApplicationContext = context;
   }
}

public class MyApplication extends Application{
    @Override
    public void onCreate()
    {
        super.onCreate();
        ClassicSingleton.getInstance().initiate(getApplicationContext());
    }
}

I find similar question here Android static object lifecycle, but it didn't answer my question.

Just guessing since there's no MCVE: Do you have ContentProviders? They are initialised before application onCreate() — laalto, Aug 16 '17 at 13:53
@laalto I don't see any relation with ContentProviders... answer is no — zhanzhan0812, Aug 16 '17 at 14:08
Take a look at this question. https://stackoverflow.com/questions/2002288/static-way-to-get-context-on-android — Dale Wilson, Aug 16 '17 at 14:13
By the way, the code you provided probably won't compile because initiate is private. This does not inspire confidence among those who might try to help you. — Dale Wilson, Aug 16 '17 at 14:17
@DaleWilson thanks, but we can't do this, we're developing lib, ClassicSingleton is in our lib, and the initiate part is in host app. — zhanzhan0812, Aug 16 '17 at 14:21

Dale Wilson · Answer 1 · 2017-08-16T15:25:36.040

Since you are writing a library, don't trust the caller to get it right. Check! i.e:

public void initiate(Context context){
     if (context == null) {
        throw new Error("Attempt to set null context");
     }
     if (mApplicationContext != null) {
        throw new Error("Why are you setting context twice?");
     }
     this.mApplicationContext = context.getApplicationContext();
}

Note the call to getApplicationContext ensures that you don't save an Activity context by mistake. An alternative would be to throw if context != context.getApplicationContext(), but that's probably overkill.

This doesn't fix your bug, but it will help you find it quickly.

Oh-- and you can probably find something better to throw than Error

Even better:

public static ClassicSingleton getInstance() {
   if(instance == null) {
      throw new Error("you forgot to initiate ClassicSingleton!");
   }
   return instance;
}

public static void initiate(Context context){
     if (context == null) {
        throw new Error("Attempt to set null context");
     }
     if (instance == null) {
        instance = new ClassicSingleton();
     }else{
        // optional
        throw new Error("Why are you initializing ClassicSingleton twice?");
     }
     instance.mApplicationContext = context.getApplicationContext();
}

why a static instance initialized in Application.onCreate loses its value

1 Answers1