使用Proxool连接池， 当在重启Tomcat容器时，出现 Exception in Thread "HouseKeeper" java.lang.NullPointerException 空指针异常错误。 原因是：
This is because Proxool is not being shutdown properly. If the JVM stops then Proxool recognises that and shuts down gracefully, but if you redeploy Proxool into some environments (for example, a servlet container) then Proxool needs to be explicitly told so by calling ProxoolFacade.shutdown(). If you have a servlet container then you could put it in the servlet’s destroy() method. Alternatively, use the ServletConfigurator to both configure and shutdown Proxool
