the doFilter method of Filter has this prototype
public void doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
throws java.io.IOException,
ServletException

You can get the output stream using the following code
ServletOutputStream out = response.
getOutputStream ();

It is the responsibility of the container (implementing servlet api 2.3 or above) to call the doFilter method with the correct args -- even in the case of a web service.

Hope it helps.

Venkatesh