code: @Override
      public int doEndTag() throws JspException {
          String result;                // the eventual result
  
          // add (already encoded) parameters
          String baseUrl = UrlSupport.resolveUrl(url, context, pageContext);
          result = params.aggregateParams(baseUrl);
  
          // if the URL is relative, rewrite it with 'redirect' encoding rules
          HttpServletResponse response =
                  ((HttpServletResponse) pageContext.getResponse());
          if (!UrlUtil.isAbsoluteUrl(result)) {
              result = response.encodeRedirectURL(result);
          }
  
          // redirect!
          try {
              response.sendRedirect(result);
          } catch (java.io.IOException ex) {
              throw new JspTagException(ex.toString(), ex);
          }
  
          return SKIP_PAGE;
      }
docs: gets the right value, encodes it, and prints or stores it