When ReferenceHandler thread is notified?

Question

We know, thread ReferenceHandler is responsible for enqueue pending Reference instance to ReferenceQueue, see this code in Reference$ReferenceHandler.run():

public void run() {
    for (;;) {

    Reference r;
    synchronized (lock) {
        if (pending != null) {
        r = pending;
        Reference rn = r.next;
        pending = (rn == r) ? null : rn;
        r.next = r;
        } else {
        try {
            lock.wait();
        } catch (InterruptedException x) { }
        continue;
        }
    }

    // Fast path for cleaners
    if (r instanceof Cleaner) {
        ((Cleaner)r).clean();
        continue;
    }

    ReferenceQueue q = r.queue;
    if (q != ReferenceQueue.NULL) q.enqueue(r);
    }
}
}

If pending queue is null, then this thread is waiting on lock;

My question is when this thread is notified? When pending instance is modified?

Answer 1

From the code

/* Object used to synchronize with the garbage collector.  The collector
 * must acquire this lock at the beginning of each collection cycle.  It is
 * therefore critical that any code holding this lock complete as quickly
 * as possible, allocate no new objects, and avoid calling user code.
 */
static private class Lock { };
private static Lock lock = new Lock();

This implies that the collector will notify() when it has finished and needs the ReferenceHandler to wake up.