Default Servlet
A mapping that contains the pattern <url-pattern>/</url-pattern> matches a request if no other pattern matches. This is the default mapping. The servlet mapped to this pattern is called the default servlet.
By explicitly providing a default mapping, you can ensure that requests with malformed URLs find their way to the application rather than simply returning an error. A common practice is to map the default servlet to the application’s home page, or you might map to a page with more helpful error handling.
The servlet-mapping element below maps the Welcome servlet instance to the default mapping.
For the context that contains this element, any request that is not handled by another mapping is forwarded to the Welcome servlet.