如何从ServletResponse获取内容类型？

Question

I'm implementing a filter, which overrides the doFilter method, to insert in the response header some "cache-control" values. The thing is that I only want that values to be set in the responses with a JSON content type. I tried to call response.getContentType before set the new header values but it always evaluates to null.

 @Override
public void doFilter(final ServletRequest servletRequest, final ServletResponse servletResponse,
                     final FilterChain filterChain) throws IOException, ServletException {

    try {

        final HttpServletResponse response = (HttpServletResponse) servletResponse;

        if(response != null) {

            final HttpServletResponseWrapper wrapper = new HttpServletResponseWrapper(response);

            if(wrapper.containsHeader(HttpHeaders.CONTENT_TYPE)
                    && wrapper.getHeader(HttpHeaders.CONTENT_TYPE).equals("application/json")) {

                response.setHeader("Expires", "Mon, 23 Aug 1982 12:00:00 GMT");
                response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate");
            }
        }

    filterChain.doFilter(servletRequest, servletResponse);
    }

    catch(final Exception e) {
        LOG.error(e.getLocalizedMessage());
    }
}

Answer 1

The difficulty with servlet filters is that they are too coarse-grained for that kind of functionality. The doFilter method is "all-or-nothing", that is there is nothing set in the response before it is called and the entire response is already committed after it returns so it's too late then to add any header.

What you can do instead is to wrap the response and intercept the setHeader method in the wrapper, then pass that to doFilter . There is a helper class HttpServletResponseWrapper designed for that purpose.