Now I know the problem. It is the logging of Class Unloading on Tomcat V6.029. Making log level "Error" instead of default "New" or Adding "-XX:-TraceClassUnloading" to the java VM options is workaround. Also upgrading to Tomcat V6.0.32 is OK. The logging of Tomcat V6.0.32 is modified on Tomcat V6.0.29.