Rack::Bug has a nice little framework for displaying firebug-style panels with request information. It should be possible to hook into Rack::Cache so that the number of cache hits, misses, validates, etc. can be recorded and displayed.

Rescuing from Errno::ENOENT does not work for JRuby. JRuby raises an IOError which needs to be handled as well. For (some) more information, please take a look at the following discussion:

http://groups.google.com/group/rack-cache/browse_thread/thread/1bb880ea57037620/5255700919043264?lnk=gst&q=jruby#5255700919043264