Servlet filter for caching

Question

I'm creating a servlet filter for caching. The idea is to cache the response body to memcached. The response body is generated by (result is a string):

 response.getWriter().print(result);

My question is that, since the response body will be put into the memcached without modification, do I still need to create a customized HttpServletResponseWrapper? Can anyone provide any skeleton code for this filter?

Answer 1

You need to be able to capture the servlet's output in your filter.

For this, you need to inject a customized HttpServletResponseWrapper that collects all output sent to getWriter().print() somewhere so that you can hand it off to memcached.

Maybe something along the lines of:

ByteArrayOutputStream baos = new ByteArrayOutputStream(3000); 
final PrintWriter w = new PrintWriter(new OutputStreamWriter(baos, "UTF-8"));

HttpServletResponse wrapper = new HttpServletResponseWrapper(response) {

    @Override
    public PrintWriter getWriter() throws IOException {
        return w;
    }

};

If this is a bigger project and you have more control over the network infrastructure, it might also be a good idea to not do this in Java, but just use a separate proxy server in front of the Servlet container. You can control what should be cached by the usual Cache-Control HTTP headers (which you can set using a filter if the servlet does not already do it).