ClickJacking Filter to add X-FRAME-OPTIONS in response

Question

In order to tackle clickJacking and blocking my site to be opened by iframe I have created a servlet filter in which I am adding below line to add "X-FRAME-OPTIONS" response header. But when I run page and see response headers of that page I never get this header in there. Any Idea why?

public void doFilter(
        ServletRequest request, ServletResponse response, FilterChain chain
        ) throws IOException, ServletException
    {

        HttpServletResponse res = (HttpServletResponse)response;
        chain.doFilter(request, response);

        //Specify the mode
        res.addHeader("X-FRAME-OPTIONS", "DENY");
    }

Answer 1

You need to add the header before calling doFilter. By the time control returns from doFilter the headers and body have already been sent, so your addHeader is ignored.