How catch exception from struts action in test class using junit

Hi

i am using junit in my struts application and my test class extends MockStrutsTestCase.
i want to test LoginAction class.For this class,i pass username and password by using addRequestParameter method.
if i pass parameters correctly,the code is working fine.
If i pass incorrect parameter name (for example instead of uname i give name) or missing any parameters, the java class throws NullPointerException.
how can i catch that exception in my test class and display that message.


This is the code for that:




import servletunit.struts.*;

import org.apache.struts.action.*;



public class TestCheckAssociate extends MockStrutsTestCase {


public TestCheckAssociate(String name) {

super(name);

}


protected void setUp() throws Exception {

super.setUp();

}


protected void tearDown() throws Exception {

super.tearDown();

}


public void testSuccessfulAsLogin()

{

setRequestPathInfo("/checkassociate");

addRequestParameter("uname","test@gmail.com");

addRequestParameter("password","123456");

actionPerform();

verifyForward("correct");

assertEquals("test@gmail.com",(String) getSession().getAttribute("asusername"));

}


public void testFailedAsLogin()

{

setRequestPathInfo("/checkassociate");

addRequestParameter("uname","test@gmail.com");

addRequestParameter("password","124456");

actionPerform();

verifyForward("incorrect");

assertNull((String) getSession().getAttribute("asusername"));

}


public void testCheckParameters()

{

ActionRedirect ar = new ActionRedirect("/checkassociate.do");


ar.addParameter("name", "test@gmail.com");

ar.addParameter("password", "123456");


actionPerform();

verifyForward("incorrect");

assertEquals("test@gmail.com",(String) getSession().getAttribute("asusername"));

}


}


please help me in this regard.



Regards

vasu