Spider does not exit when success

[code4craft]

Sometimes the thread does not exit when success in multi-thread mode.
I guess there is some problem in the wait-notify mechanism:

if (request == null) {
                if (threadAlive.get() == 0 && exitWhenComplete) {
                    break;
                }
                // wait until new url added
                waitNewUrl();
            }

[code4craft]

After I tested in SpiderTest.testWaitAndNotify(), I confirmed the wait method has some problem.
I just add double-check in waitNewUrl() method.

    private void waitNewUrl() {
        try {
            newUrlLock.lock();
            //double check
            if (threadAlive.get() == 0 && exitWhenComplete) {
                return;
            }
            try {
                newUrlCondition.await();
            } catch (InterruptedException e) {
            }
        } finally {
            newUrlLock.unlock();
        }
    }