WritableServletResponse

The servlet API states that, for a given response, either the getOutputStream or getWriter method may be called, but not both:

http://docs.oracle.com/javaee/6/api/javax/servlet/ServletResponse.html#getOutputStream() http://docs.oracle.com/javaee/6/api/javax/servlet/ServletResponse.html#getWriter()

Unfiltered response bindings are based on a single outputStream, and support filtering it (unfiltered.response.ResponseFilter) through response function composition. Writing to the underlying response output stream directly would bypass any filters in place, and writing to its writer will produce an InvalidStateException.

If working with software that requries a HttpServletResponse and uses its Writer interface, this wrapper supplies a writer that works with any stream filters in the response function chain.