Lazy Initialization

Question

Josh Bloch and Neal Gafter posted the following code puzzler. The thread in the static block blocks if you do "t.join()" but "t.join(10)" lets the program finish. Is this due to the private static variable which is being lazily initialized? Thanks for any mind-opening explanations!
public class Lazy {
private static boolean initialized = false;
static {
Thread t = new Thread(new Runnable() {
public void run() {
initialized = true;
t. start();
try {
t.join();
} catch (InterruptedException e) {
throw new AssertionError(e);
public static void main(String[] args) {
System.out.println(initialized);
}

Answer 1

o, when the thread tries to update the value of
the 'initialized' variable, it cannot because the
lock for the Class object is held by a different
Thread.
But.... the Thread is NOT synchronized, so why should
it care about locks? It's a sort of class
initialization magic, right?The section in the JVM spec just before the link I provided states:
=============
A class or interface type T will be initialized immediately before one of the following occurs:
* T is a class and an instance of T is created.
* T is a class and a static method of T is invoked.
* A nonconstant static field of T is used or assigned.
=============
Note the last item.
Since the static initializer has not exited, the Class has not been initialized, therefore , the attempt to assign a value to 'initialized' in the Thread attepts to initialize the class (again) but finds it locked and the lock will never be released due to the join().
When join(10) is used, the static block exits, and releases the lock. On release of the lock, any waiting threads are notified, and from the spec:
=============
If the class or interface has already been initialized, then no further action is required. Release the lock on the Class object and complete normally
=============
So, the second attempt to initialize the class (in the thread) finds the class already initialized, and therefore the initialization simply releases the lock and ends.
Jim S.