WriteLock does not work.

Hi everyone,

I would like to implement a 'preemptible' mutex with the WriteLock example
in the recipe. My method is like this.

1. Acquire the WriteLock and get the resources
2. Set a watch on the lock's root node with getChildren() repeatedly, using
a separate ZooKeeper client
3. If only one child (self) is found, sleep
4. The callback function only wakes up the main thread
5. If the watch is triggered and > 1 children are found, release the
resources and unlock the WriteLock

In my design, I will execute the jar multiple times. Each time a new one is
started, the old one should be forced to release the mutex. It works. The
first execution does stop. However, the second process seems to lose its
watch on the first one, because the callback function I passed to the
WriteLock object is never evoked. The callback function is used to
synchronize the locking request and is thus very important.

Can anyone see any flaws in my design, or provide a different way that
works? Thanks~

Best regards,
mdzfirst