Null check and listeners in java

Question

Very often I see code like this:

Listener mListener;

public void setListener(Listener listener){
   mListener=listener;
}

public void fooFunction(){
   ...
   ...
   if (mListener!=null){
      mListener.notifyFoo();
   }
}

My question is: what if between the null check and the notifyFoo() another thread calls setListener(null)? Is that possible? or does the compiler make it atomical

Answer 1

You can synchronize methods

public synchronized void setListener(Listener listener){
   mListener=listener;
}

public synchronized void fooFunction(){
   ...
   ...
   if (mListener!=null){
       mListener.notifyFoo();
   }
}

Or if you want better granularity of locks

private Object monitor = new Object();

public void setListener(Listener listener){
   synchronized (monitor) {
       mListener=listener;
   }
}

public void fooFunction(){
   ...
   ...
   synchronized (monitor) {
        if (mListener!=null){
           mListener.notifyFoo();
       }
   }
}

Answer 2

You can avoid that by making your block Synchronized

public synchronized void setListener(Listener listener) {
    ....
}

public synchronized void fooFunction() {
    ....
}

Answer 3

The compiler doesn't make anything atomic by default. True, there is a chance of the listener being set to null between the two lines. You may use synchronized to avoid this.

Answer 4

Synchronisation is overkill here. Just take a copy of the member variable before checking it for null :

Listener mListener;

public void setListener(Listener listener){
   mListener=listener;
}

public void fooFunction(){
   ...
   ...
   Listener listener = mListener;
   if (listener!=null){
      listener.notifyFoo();
   }
}

Answer 5

Yes, it possible. Mark your fooFunction and setListener as synchronized and it should make this code multithread-safe