Java singleton with weak reference

Question

Here is my singleton class with weak reference.

public class HandheldMapViewProvider  {

    private static WeakReference<HandheldMapViewProvider> mInstance = null;

    private HandheldMapViewProvider(){

    }

    public static synchronized WeakReference<HandheldMapViewProvider> getInstance(){
       if(mInstance == null){
         mInstance = new WeakReference<HandheldMapViewProvider>(new HandheldMapViewProvider());
       }
       return mInstance;
    }

    public void onprint(String data){
        Log.D("TAG",data)
    }


}

Usage of above class is as follow.

 private WeakReference<HandheldMapViewProvider> hereMapViewProvider;

 public void onprint(){
     hereMapViewProvider = HandheldMapViewProvider.getInstance();
     hereMapViewProvider.get().onprint("somevalue");
 }

While calling onprint method first time app get crash sometimes due to get() is null.

Any idea where i am doing it wrong. its not happening all time.

Solution is as below.

public static synchronized HandheldMapViewProvider getInstance(){
  HandheldMapViewProvider mapProvider = mInstance == null ? null :mInstance.get();

  if(mapProvider == null){

       mInstance = new WeakReference<HandheldMapViewProvider>(mapProvider =new HandheldMapViewProvider());
   }

   return mapProvider;

  }

Your main fault is uning the singelton pattern in the first place. https://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons — Timothy Truckle, Jul 17 '17 at 11:23
@EnamulHaque from activity. Mostly same code is working perfectly. some time I am getting null. — Keyur Thumar, Jul 17 '17 at 13:30
Are you calling that activity frequently .. such as 2-3 time repetitively — , Jul 17 '17 at 13:32

score 0 · Answer 1 · answered Jul 17 '17 at 11:45

The problem is that you store HandheldMapViewProvider as a WeakReference. The garbage collector clears the value so get() returns null.

To avoid this behavior just drop the WeakRreference implementation and store the HandheldMapViewProvider as a "regular" static field.

score 0 · Answer 2 · answered Jul 17 '17 at 13:52

Weak reference objects, which do not prevent their referents from being made finalizable, finalized, and then reclaimed.

Garbage collector determines at a certain point in time that which object is weakly reachable. At that time it will atomically clear all weak references to that object and all weak references to any other weakly-reachable objects from which that object is reachable through a chain of strong and soft references. At the same time it will declare all of the formerly weakly-reachable objects to be finalizable.

When we use Weak reference then, At the same time or at some later time it will enqueue those newly-cleared weak references that are registered with reference queues.

It is happening form Garbage Collector...So We don't know when it will happen ..

You can save your code by using try catch block to avoid crashing ..

public void onprint(){
    hereMapViewProvider = HandheldMapViewProvider.getInstance();
    try{
        hereMapViewProvider.get().onprint("somevalue");
    }catch(Exception e){
      Log.e("Err", e.toString()+"");
    }
}

Hi... do get another solution? Please share if any. – Jul 27 '17 at 11:03 — , Jul 27 '17 at 11:03
Yes, will share solution. – Keyur Thumar Jul 27 '17 at 14:08 — Keyur Thumar, Jul 27 '17 at 14:08

score 0 · Answer 3 · answered Jul 29 '17 at 11:19

Solution is as below.

public static synchronized HandheldMapViewProvider getInstance(){
HandheldMapViewProvider mapProvider = mInstance == null ? null :mInstance.get();

if(mapProvider == null){

   mInstance = new WeakReference<HandheldMapViewProvider>(mapProvider =new 
   HandheldMapViewProvider());
}

return mapProvider;

}

Java singleton with weak reference

3 Answers3