java - Adding header in response in filter?

Question

I need to add the header in each response. I am planning to do below

public class MyFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request,
                                    HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {

        filterChain.doFilter(request, response);
            response.addHeader("Access-Control-Allow-Origin", "*"); 
    }

}

I would like to do it after filterChain.doFilter(request, response) so that once controller process it, i just add header before returning to client. Is it correct ?

But As per How to write response filter?

After chain.doFilter has returned, it's too late to do anything with the response. At this point, entire response was already sent to the client and your code has no access to it.

Above statement does not look right to me. Can't i add header after filterChain.doFilter(request, response) ? If not why ?

i am using spring mvc.

Answer 1

After filterChain.doFilter is called it's too late to do anything with the response. At this point, the entire response was already sent to the client.

You need to build a wrap response into your own classes, pass these wrappers into doFilter method and handle any processing in your wrappers.

There is already a response wrapper: HttpServletResponseWrapper that you can extend. For example:

public class MyResponseRequestWrapper extends HttpServletResponseWrapper{
    public MyResponseRequestWrapper(HttpServletResponse response) {
        super(response);
    }
}

Your filter:

@Override
protected void doFilterInternal(HttpServletRequest request,
                                HttpServletResponse response, FilterChain filterChain)
        throws ServletException, IOException {

    HttpServletResponse myResponse = (HttpServletResponse) response;
    MyResponseRequestWrapper responseWrapper = new MyResponseRequestWrapper(myResponse);
    responseWrapper.addHeader("Access-Control-Allow-Origin", "*");
    filterChain.doFilter(request, myResponse);
}