尝试进入同步块时发生IllegalstateException

当进入同步块时引发IllegalstateException是什么意思?我有时会在线程的run方法中看到:

    public void run() {
        while (true) {
            int n = 0;
            synchronized (service) { // IllegalStateException
                n = processPendingRequests();
            }

            /*
             * If n > 0, we processed at least one element, in which case we
             * immediately check the queue again until it was empty.
             */
            if (n == 0) {
                sleep();
                continue;
            }
        }
    }

可以 service 对象导致 IllegalStateException ?怎么用?