code: public void doTag()
              throws JspException
      {
          // First look for the exception in the pageflow/struts request attribute.  If it's not there,
          // look for it in the request attribute the container provides for web.xml-configured error
          // pages.
          InternalStringBuilder results = new InternalStringBuilder(128);
          PageContext pageContext = getPageContext();
          Throwable e = (Throwable) pageContext.getAttribute(Globals.EXCEPTION_KEY, PageContext.REQUEST_SCOPE);
  
          if (e == null) {
              ServletRequest req = pageContext.getRequest();
              e = (Throwable) req.getAttribute("javax.servlet.error.exception");
              if (e == null) {
                  e = (Throwable) req.getAttribute("javax.servlet.jsp.jspException");
              }
          }
  
          if (!_showStackTrace && _showDevModeStackTrace) {
              boolean devMode = !AdapterManager.getServletContainerAdapter(pageContext.getServletContext()).isInProductionMode();
              if (devMode)
                  _showStackTrace = true;
          }
  
          if (e != null) {
              if (_showMessage) {
                  String msg = e.getMessage();
                  // if we have message lets output the exception name and the name of the
                  if ((msg != null) && (msg.length() > 0)) {
                      if (!_showStackTrace)
                          msg = e.getClass().getName() + ": " + msg;
                      results.append(HtmlExceptionFormatter.format(msg, e, _showStackTrace));
                  }
                  else {
                      results.append(HtmlExceptionFormatter.format(e.getClass().getName(), e, _showStackTrace));
                  }
              }
              else {
                  results.append(HtmlExceptionFormatter.format(null, e, _showStackTrace));
              }
              ResponseUtils.write(pageContext, results.toString());
          }
      }
docs: Render the exception text based on the display attributes.
  @throws JspException if a JSP exception has occurred