Update comment in Javadoc of ServletRequestPathFilter DispatcherServlet relating to DispatcherServlet

[sergiuprdn]

The following behavior documented by org.springframework.web.filter.ServletRequestPathFilter does not hold true with current version:

DispatcherServlet will parse and cache the RequestPath if it detects that parsed PathPatterns are enabled for any HandlerMapping but it will skip doing that if ServletRequestPathUtils#PATH_ATTRIBUTE already exists.

This PR adds the logic to skip parse and cache RequestPath if ServletRequestPathUtils.PATH_ATTRIBUTE already exists.

The exception where multiple parsing must happen is for Forward requests.

[rstoyanchev]

Thanks for bringing this up. You're right that the Javadoc reflects original intent rather than current behavior.

For a FORWARD, the filter would also get involved, so technically the DispatcherServlet can always decide to stick with the existing one. The problem is that when there is no Filter, on FORWARD we would find a previous value, and the path would need to be parsed again. This is why in the end DispatcherServlet ended up with always doing its own parsing, while the Filter is there mostly if anything in the Filter chain relies on the parsed path.

There are also other scenarios with ERROR and ASYNC dispatches, and I'm not sure it's worth doing anything more here. We might just have to correct the Javadoc of the filter accordingly.

[sergiuprdn]

Updated javadoc as indicated by PR title. Please check