FacesImpl-viewhandlerImpl

    public String getActionURL(FacesContext context, String viewId) {

        if (context == null) {
            String message = MessageUtils.getExceptionMessageString
                (MessageUtils.NULL_PARAMETERS_ERROR_MESSAGE_ID, "context");
            throw new NullPointerException(message);
        }
        if (viewId == null) {
            String message = MessageUtils.getExceptionMessageString
                (MessageUtils.NULL_PARAMETERS_ERROR_MESSAGE_ID, "viewId");
            throw new NullPointerException(message);
        }

        if (viewId.charAt(0) != '/') {
            String message =
                  MessageUtils.getExceptionMessageString(
                        MessageUtils.ILLEGAL_VIEW_ID_ID,
                        viewId);
            if (logger.isLoggable(Level.SEVERE)) {
                logger.log(Level.SEVERE, "jsf.illegal_view_id_error", viewId);
            }
        throw new IllegalArgumentException(message);
        }

        // Acquire the context path, which we will prefix on all results
        ExternalContext extContext = context.getExternalContext();
        String contextPath = extContext.getRequestContextPath();

        // Acquire the mapping used to execute this request (if any)
        String mapping = Util.getFacesMapping(context);

        // If no mapping can be identified, just return a server-relative path
        if (mapping == null) {
            return (contextPath + viewId);
        }

        // Deal with prefix mapping
        if (Util.isPrefixMapped(mapping)) {
            if (mapping.equals("/*")) {
                return (contextPath + viewId);
            } else {
                return (contextPath + mapping + viewId);
            }
        }

        // Deal with extension mapping
        int period = viewId.lastIndexOf('.');
        if (period < 0) {
            return (contextPath + viewId + mapping);
        } else if (!viewId.endsWith(mapping)) {
            return (contextPath + viewId.substring(0, period) + mapping);
        } else {
            return (contextPath + viewId);
        }

    }