code: @Override
      public int doStartTag() throws JspException {
          // Sanity check
          if (context != null
                  && (!context.startsWith("/") || !url.startsWith("/"))) {
              throw new JspTagException(
                      Resources.getMessage("IMPORT_BAD_RELATIVE"));
          }
  
          // reset parameter-related state
          urlWithParams = null;
          params = new ParamSupport.ParamManager();
  
          // check the URL
          if (url == null || url.equals("")) {
              throw new NullAttributeException("import", "url");
          }
  
          // Record whether our URL is absolute or relative
          isAbsoluteUrl = UrlUtil.isAbsoluteUrl(url);
  
          try {
              // If we need to expose a Reader, we've got to do it right away
              if (varReader != null) {
                  r = acquireReader();
                  pageContext.setAttribute(varReader, r);
              }
          } catch (IOException ex) {
              throw new JspTagException(ex.toString(), ex);
          }
  
          return EVAL_BODY_INCLUDE;
      }
docs: determines what kind of import and variable exposure to perform