Java Servlet Filter - Modify response headers based on status

Question

I'm trying to write a servlet filter that will add headers to the response depending on the status of the request. I know I have to wrap the response with a HttpServletResponseWrapper before passing to the chain.doFilter but the headers never get sent, so I'm obviously missing something very obvious.

Code looks something like:

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException
{
   HttpServletResponse httpServletResponse = (HttpServletResponse) response;
   HttpServletResponseWrapper responseWrapper = new HttpServletResponseWrapper(httpServletResponse);

   chain.doFilter(request, responseWrapper);

   if(responseWrapper.getStatus() < 400)
   {
      responseWrapper.addHeader("X-Custom-Foobar", "abc");
   }
}

Is there something I have to capture in the wrapper to prevent the response from going out to the client until the check is complete?

Answer 1

So the frustrating part about this spec is that you have to completely intercept the ServletOutputStream and buffer it. I ended up following the example here :: https://stackoverflow.com/a/11027170/76343

The base class HttpServletResponseWrapper is a complete passthrough and as soon as the output stream is closed, all further modifications to the response are mute.

Unfortunately there doesn't seem to be a more elegant way to accomplish this.