protected void redirectToLogin(ServletRequest request, ServletResponse response) throws IOException {
        String loginUrl = getLoginUrl();
        WebUtils.issueRedirect(request, response, loginUrl);
    }
Convenience method for subclasses that merely acquires the {@link #getLoginUrl() getLoginUrl} and redirects
the request to that url.
<p/>
<b>N.B.</b>  If you want to issue a redirect with the intention of allowing the user to then return to their
originally requested URL, don't use this method directly.  Instead you should call
{@link #saveRequestAndRedirectToLogin(javax.servlet.ServletRequest, javax.servlet.ServletResponse)
saveRequestAndRedirectToLogin(request,response)}, which will save the current request state so that it can
be reconstructed and re-used after a successful login.

@param request  the incoming <code>ServletRequest</code>
@param response the outgoing <code>ServletResponse</code>
@throws IOException if an error occurs.
web/src/main/java/org/apache/shiro/web/filter/AccessControlFilter.java