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

Here's the bytecode for the deadlocked example:

public class Main extends java.lang.Object{
public static final java.lang.Object a;

public Main();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   return

static {};
  Code:
   0:   new     #2; //class java/lang/Object
   3:   dup
   4:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   7:   putstatic       #3; //Field a:Ljava/lang/Object;
   10:  invokestatic    #4; //Method java/lang/Runtime.getRuntime:()Ljava/lang/Runtime;
   13:  new     #5; //class Main$1
   16:  dup
   17:  invokespecial   #6; //Method Main$1."<init>":()V
   20:  invokevirtual   #7; //Method java/lang/Runtime.addShutdownHook:(Ljava/lang/Thread;)V
   23:  iconst_0
   24:  invokestatic    #8; //Method java/lang/System.exit:(I)V
   27:  return

}

And here's the bytecode for the case that finishes normally:

public class Main extends java.lang.Object{
public static final java.lang.Object a;

public Main();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   return

static {};
  Code:
   0:   new     #2; //class java/lang/Object
   3:   dup
   4:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   7:   putstatic       #3; //Field a:Ljava/lang/Object;
   10:  getstatic       #3; //Field a:Ljava/lang/Object;
   13:  astore_0
   14:  invokestatic    #4; //Method java/lang/Runtime.getRuntime:()Ljava/lang/Runtime;
   17:  new     #5; //class Main$1
   20:  dup
   21:  aload_0
   22:  invokespecial   #6; //Method Main$1."<init>":(Ljava/lang/Object;)V
   25:  invokevirtual   #7; //Method java/lang/Runtime.addShutdownHook:(Ljava/lang/Thread;)V
   28:  iconst_0
   29:  invokestatic    #8; //Method java/lang/System.exit:(I)V
   32:  return

}

The bytecode is obviously different. I'll either come up with the answer or someone else who understands the internals of the JVM will help.

Answer 2

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.