Java Static-block Shutdown Hook with System.exit

Question

This code will deadlock:

public class Main {
   static public final Object a = new Object();
   static {
      Runtime.getRuntime().addShutdownHook(new Thread() {
         @Override
         public void run() { if (a == null); }
      });
      System.exit(0);
   }
   static public void main(final String[] args) {}
}

This code will exit normally:

public class Main {
   static public final Object a = new Object();
   static {
      final Object aa = a;
      Runtime.getRuntime().addShutdownHook(new Thread() {
         @Override
         public void run() { if (aa == null); }
      });
      System.exit(0);
   }
   static public void main(final String[] args) {}
}

What is happening?

Answer 1

It is important that classes are not accessed concurrently whilst initialising, so a lock is held.

I guess what is happening in the first case is:

• The main thread holds the initialisation lock for Main.
• Whilst holding the lock, System.exit blocks as it does not return.
• The shutdown hook executes.
• The shutdown tries to access the Main class to read a field, but blocks as the class is initialising.

Hence the deadlock. It's a little clearer if you write if (a == null); as if (Main.a == null);.

In the second case, the value is copied and therefore the shutdown hook does not need to access the Main class.

Moral: Don't mix threads and class initialisation. Gafter and Bloch's Java Puzzlers book has more on this.