What is the use of setContextDirectory()/setConfigFile()/setServletConfigFile() in MockStrutsTestCase class?

Trying to write a junit struts test case using MockStrutsTestCase and my directory structure is like this

MyBillingApp |--src |--webRoot |--config |--WEB-INF | |--struts-config.xml | |--struts-invoice.xml | |--struts-bills.xml | |--struts-common.xml |--web.xml

Then what should I set in setContextDirectory()/setConfigFile()/setServletConfigFile() in order to run my test successfully.