I found the cause of the problem.
Tomcat's implementation of javax.servlet.http.HttpServlet implements
the doGet method like this:
protected void doHead(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
NoBodyResponse response = new NoBodyResponse(resp);
doGet(req, response);
response.setContentLength();
}
The problem occurs when a HTTP HEAD request arrives for a page that
isn't in the page cache.
When a HTTP HEAD request is received for a page that is not in the
cache, the caching filter prepares a response wrapper that will
capture the request (content generated by the application's servlets),
puts that wrapper in the place of the real response, for the filters
and servlets next in the chain, and gives the execution to the next
filter/servlet in chain..
But, for HTTP HEAD requests, the caching wrapper never gets to the
servlets that generate content, because the above method wraps it in
NoBodyResponse, a response wrapper that throws away everything that is
writen to it, like /dev/null does... servlets write content to
NoBodyResponse, and the content is lost.
On the way back, the caching filter inspects the response wrapper it
created, sees 200 response code, and decides that the page was
generated well, and caches it... (the content doesn't get cached
because NoBodyResponse didnt forward it to the response it wrapped -
caching filter's response wrapper)
I will fix my caching filter to watch for this special case, and that
will solve the problem in our applications... but I wonder if it is
okay that Tomcat (silently) wraps the response inside a wrapper like
NoBodyResponse that eats content.. or was it my fault that I didn't
explicitly make difference between GET and HEAD requests when thinking
about caching, probably because of lack of expirience..
--
Why?
Because YES!
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