How can I map a "root" Servlet so that other scripts are still runnable?

Use an empty pattern, e.g.

<servlet-mapping>
    <servlet-name>MainServlet</servlet-name>
    <url-pattern></url-pattern>
</servlet-mapping>

The servlet 3.0 spec has clarified this:

The empty string ("") is a special URL pattern that exactly maps to the application's context root

So it should at least work on a 3.0 container, and I've verified that it works on Jetty 8

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Using the welcome-file element of web.xml worked for me, on app engine. Here's mine:

<web-app>
    <servlet>
        <servlet-name>RootServlet</servlet-name>
        <servlet-class>com.foo.RootServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>RootServlet</servlet-name>
        <url-pattern>/foo</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>foo</welcome-file>
    </welcome-file-list>
</web-app>

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The original question doesn't mention that they're trying to map a root servlet on App Engine - it's easy on Tomcat (and other servlet containers as far as I know) but App Engine isn't a normal servlet container.

My normal way of building a web application with servlets is to extend HttpServlet, add a "page" object with title, content, errors, messages etc. and for output forward to a JSP template. This has been an absolute nightmare getting working in App Engine.

JSP files can't be "named" without a "/" at the beginning.

JSP files can't be in a subdirectory

Servlets can't be mapped to the root of your application with a "/" url-pattern

Here's my web.xml (edited for brevity) which finally worked.

<web-app>
  <servlet>
    <!-- this servlet needs to redirect to a NamedDispatcher
         called "template" -->
    <servlet-name>Home</servlet-name>
    <servlet-class>my.domain.HomeSv</servlet-class>
  </servlet>
  <servlet>
    <!-- jsp file must apparently be in root directory and have "/" at
         start of path -->
    <servlet-name>template</servlet-name>
    <jsp-file>/template.jsp</jsp-file>
  </servlet>
  <servlet-mapping>
    <!-- map your home servlet to somewhere other than "/" -->
    <servlet-name>Home</servlet-name>
    <url-pattern>/home</url-pattern>
  </servlet-mapping>
  <welcome-file-list>
    <!-- make your Home servlet the welcome file -->
    <welcome-file>home</welcome-file>
  </welcome-file-list>
</web-app>

I haven't been particularly scientific about validating all this - but it seems to work for me now and I'm pretty happy about that.

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You can create a welcome file named index.jsp in the root with the following code using JSTL or otherwise.

<c:redirect url="/main"/>

So in the web.xml file you will have this:

<welcome-file-list>
    <welcome-file>index.jsp</welcome-file>        
</welcome-file-list>

So anyone requesting the root will be redirected to /main. Now your servlet can be mapped to main.

<servlet-mapping>
    <servlet-name>MainServlet</servlet-name>
    <url-pattern>/main</url-pattern>
</servlet-mapping>

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Try just to remove '*' from the pattern, i.e.

<url-pattern>/</url-pattern>

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A solution is mentioned in another thread URL Pattern for servlet mapping in web.xml using URLrewrite -> http://tuckey.org/urlrewrite/

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Have you tried the below? (Note the missing *, which is a wild card and is the reason that your configuration catches everything.)

<servlet-mapping>
        <servlet-name>MainServlet</servlet-name>
        <url-pattern>/index.jsp</url-pattern>
</servlet-mapping>

(Edited as per comments from just /.)

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You can't redirect to WEB-INF. A servlet container will never serve requests for documents in that folder.

If you want your application (not just a servlet, but the entire application) to be available under the root context ("/" of "http://www.domainname.com/"), then you need to set up a context entry for it - not a servlet mapping.

With Tomcat, you add a new <Context> mapping (in one of about 3 different possible places).

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