Running the following program shows the Y class/thread accessing statics in the X class/thread while X's <clinit> is still underway.

Fails in HotSpot 1.3.1 & 1.4. Fails in CVM.
Works in exact VM (classic as well?) and IBM's 1.3.0 JIT.
Cliff Click


class X extends Thread {
  public static final boolean _P; // TRUE after clinit

  // Class initializer
  static {
    // _P is FALSE here

    Y.fiddle_with_X(true);

    // Stall for a long time to allow adversary thread to get in
    try { sleep(2500); } catch (InterruptedException ix) {}

    _P = true; // set to TRUE here
  }

}

class Y extends Thread {
  public static void boolean b ) {

    if( b ) {
      System.out.println("Called from INSIDE X::<clinit>. Access to X's statics is OK. Expect to see false: ");
      System.out.println(X._P);

    } else {
      System.out.println("Called from OUTSIDE X::<clinit>. Access to X's statics should hang");
      System.out.println("If this prints TRUE , then we hung until X finished initializing");
      System.out.println("If this prints FALSE, then we peeked at X's statics before <clinit> was done");
      System.out.println(X._P);
    }
  }

  // What happens when we kick off a new Y?
  public void run() {

    // Sleep until X's <clinit> is underway
    try { sleep(150); } catch (InterruptedException ix) {}

    fiddle_with_X(false);
  }
}


public class clinit {

  public static void main( String args[] ) {

    // Make an adversarial Y thread.
    // It sleeps a little to let X::<clinit> get underway
    Y y = new Y();
    y.start();

    // Invoke X.<clinit> It will sleep for 2500.
    // Meanwhile Y should wake-up and access X's statics.
    boolean b = X._P;

    System.out.println("Called when all done. Access to X's statics is OK. Expect to see final value of true: ");
    System.out.println(b);

  }

}


----

A slightly harder test is to only access X._P once in fiddle_with_X,
catching any VM that resolves the constant pool entry and quickens
the getstatic the first time through:

  public static void fiddle_with_X( boolean b ) {

    boolean B = X._P;
    if( b ) {
System.out.println("Called from INSIDE X::<clinit>. Access to X's statics is OK. Expect to see false: ");
System.out.println(B);

    } else {
System.out.println("Called from OUTSIDE X::<clinit>. Access to X's statics should hang");
System.out.println("If this prints TRUE , then we hung until X finished initializing");
System.out.println("If this prints FALSE, then we peeked at X's statics before <clinit> was done");
System.out.println(B);
    }
  }

This trips up every VM I've tried it with (1.1.8, 1.2, and the latest CVM.)

###@###.### 2001-08-20