JCacheFilter code logic error

final PageKey key = new PageKey(key(servletRequest), gzip);
Page page = cache.get(key);
if (page == null)

{ .... }

if (page.status == SC_OK) {
checkResponse(httpServletResponse);

the last line always throw exception for the first time the specified url is requested,because the response is commited in the previous if.

I think the logic of doFilter should be like this:

String key=getKeyFromRequest();
Page page=cache.get(key);
if(page==null){
chain.doFilter();
cache.put(key,response);
}else{
response.write(cache);
}