code: protected ContextHandler initContextHandler(ServletContext servletContext) {
  		ContextHandler.Context scontext = ContextHandler.getCurrentContext();
  		if (scontext == null) {
  			if (servletContext instanceof ContextHandler.Context) {
  				return ((ContextHandler.Context) servletContext)
  						.getContextHandler();
  			} else {
  				throw new IllegalArgumentException("The servletContext "
  						+ servletContext + " "
  						+ servletContext.getClass().getName() + " is not "
  						+ ContextHandler.Context.class.getName());
  			}
  		} else {
  			return ContextHandler.getCurrentContext().getContextHandler();
  		}
  	}
docs: Compute the field _contextHandler.<br/>
  In the case where the DefaultServlet is deployed on the HttpService it is
  likely that this method needs to be overwritten to unwrap the
  ServletContext facade until we reach the original jetty's ContextHandler.
  
  @param servletContext The servletContext of this servlet.
  @return the jetty's ContextHandler for this servletContext.