how to specify the contextPath

Answer 1

That's the path in the URL after the domain where the webapplication should listen on.

If you set the context path to /foo, then the webapp will listen on http://example.com/foo and all pages/servlets will be available there in the /foo.

Here you're setting the context to /, which means that the webapp should listen on http://example.com. You're also creating a new servlet which intercepts on all requests (/*). So every request which goes through http://example.com would pass this servlet.

If you get a 404, then either the request URL is wrong, or the servlet failed to start.