You can specify a welcome-file-list in web.xml and specify index.jsp, for example:
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
Then in index.jsp, you can put this at the top:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<META HTTP-EQUIV="Refresh" CONTENT="0;URL=initLogin.action">
Assuming you have something like this in struts.xml:
<action name="*Login" method="{1}" class="com.abc.LoginAction">
<result name="login">/login.jsp</result>
<result name="register">/register.jsp</result>
<result name="success">/success.jsp</result>
</action>
This will redirect to your LoginAction class with an init action.
So then if I went to http://localhost:8080/MyApp, this is the default action it would go to.