The RequestHandler implementation that's being used with native carries an implementation error:
It does cast the InvocationContext to a concrete implementation instead of relying on the interface. This forces clients like the ESB to work with that particular implementation, ServletRequestHandler in this case.

Here's the code:

public class RequestHandlerImpl implements RequestHandler
{

[...]

public void handleWSDLRequest(Endpoint endpoint, OutputStream outputStream, InvocationContext context)
{
[...]
ServletRequestContext reqContext = (ServletRequestContext)context;
HttpServletRequest req = reqContext.getHttpServletRequest();
[...]