Dovrebbe essere definito..
Guarda un po'.. questo è il web.xml....

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN" "http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">
<web-app id="WebApp">
<display-name>Hello</display-name>
<servlet>
<servlet-name>ServletHello</servlet-name>
<display-name>ServletHello</display-name>
<servlet-class>ServletHello</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ServletHello</servlet-name>
<url-pattern>ServletHello</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
</web-app>