Adding header in response in filter?

From The Java EE Tutorial

A filter that modifies a response must usually capture the response before it is returned to the client. To do this, you pass a stand-in stream to the servlet that generates the response. The stand-in stream prevents the servlet from closing the original response stream when it completes and allows the filter to modify the servlet’s response.

To pass this stand-in stream to the servlet, the filter creates a response wrapper that overrides the getWriter or getOutputStream method to return this stand-in stream. The wrapper is passed to the doFilter method of the filter chain. Wrapper methods default to calling through to the wrapped request or response object. This approach follows the well-known Wrapper or Decorator pattern described in Design Patterns,

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);
}

I use this in my project with Spring 3.0.x:

public class MyFilter extends OncePerRequestFilter {
    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException
    {
        response.addHeader("headerName", "headerValue");
        filterChain.doFilter(request, response);
    }
}

Works fine.