Rails static page caching won't generally work with Java ServletFilter unless some serious hack with the cache directory is found

NB: you don't even need a Rails application to understand why things won't work properly. A simple custom ServletFilter logging something when called would prove you we need to find an other solution with JRuby-Rack.

So suppose you adhere to Rails Restful conventions (like resources) and you have a 'topics' resources for instance.
Then, in a normal Rails application (like served by Mongrel) if you do

POST /topics -> that creates a new topic using the create method of TopicsController
GET /topics -> that read the topics index and list all the topics, using the index method of TopicsController
GET /topics/:id -> renders the topic with the provided :id using the show method in TopicController

Now suppose you want to cache the /topics/:id show page with the cache_page :show statement inside your TopicsController
Normally, the first time you request a topic, a /topics/:id.html file will be created inside your RAILS_ROOT/public directory. This is true as well when deploying as a WAR.
( good explanations about page caching to be found here: http://www.railsenvy.com/2007/2/28/rails-caching-tutorial#pagecaching )

BUT, as soon as the 'topics' directory gets created inside your Java webapp RAILS_ROOT/public directory, any attempt to create a new topic will fail, both on Tomcat6 and Glassfish v2 at least!

Why? Because at least on those Java servers, if you do;
POST /topics while a "/topics" directory exists inside your RAILS_ROOT/public directory
then the server will answer an empty content with the following HTTP headers:
HTTP/1.1 302 Moved Temporarily
Server: Apache-Coyote/1.1
Location: http://localhost:8080/topics/
Transfer-Encoding: chunked
Date: Sun, 20 Jul 2008 00:16:39 GMT

But the thing is that the server answers this without even calling any ServletFilter! Also notice that the ServletFilters are usually called even when requesting a static content. What screws it up here is the presence of a '/topics' directory which isn't a file but matches the request path. Same thing happens when performing a GET on paths having a corresponding public directory.

So the RackFilter isn't even called when doing so. So you have no chance to answer properly to the request at all!
You can easily reproduce that behavior with that command line:
>curl -d "" http://localhost:8080/topics
(try both: with and without a '/topics' directory inside your RAILS_ROOT/public directory)

Moreover, to screw things a bit more, using Firefox3 at least, the browser send a GET /topics/ after the POST /topics returned a 302 header with the Location: http://localhost:8080/topics/ (notice the '/' at the end of the path) . Then the GET /topics/ will might be served but that would generally not do what you would expect with the POST /topics action. In the Restful case, you'll be redirected to the topics list instead of creating a resource!

Since the RackFilter isn't even called, static page caching on Java servers won't generally work out of the box.

I see two solutions now:
1) append '/' at the end of your post (or get) request where there could exist a public directory: doing POST /topics/ works while POST /topics will fail. But this is not the nice default Rails behavior so you'll need to be hack your POST urls in the forms of your Rails application. Not very nice.

2) configure your Rails application to put the cached files elsewhere, like in your environment.rb:
config.action_controller.page_cache_directory = RAILS_ROOT + "/public/my_cache_path/"
Then, inside the RackFilter, we should prefix request path with the my_cache_path inside the maybeAppendHtmlToPath method. I'll try that to see were I get.

Hope this helps. Don't hesitate to ask me more details if required. Best regards,

Raphaël Valyi.