zookeeper-3.4.2: test case NioNettySuiteHammerTest sometime failed during "ant test"

[lulynn_2008]

 Hello,
I am running "ant test" to zookeeper-3.4.2. And found NioNettySuiteHammerTest 
failed sometime. The failure seems is caused by connection refused. My question 
is:
1. why this UT is failed?
2. why this UT just failed sometime?
Thank you.