code: public void doTag()
              throws JspException
      {
          JspTag parentTag = SimpleTagSupport.findAncestorWithClass(this, IFormattable.class);
  
          // if there are errors we need to either add these to the parent AbstractBastTag or report an error.
          if (hasErrors()) {
              if (parentTag instanceof IFormattable) {
                  IFormattable parent = (IFormattable) parentTag;
                  parent.formatterHasError();
              }
              reportErrors();
              return;
          }
  
          if (parentTag instanceof IFormattable) {
              StringFormatter formatter = new StringFormatter();
              formatter.setPattern(_pattern);
              formatter.setTruncate(truncate);
              IFormattable parent = (IFormattable) parentTag;
              parent.addFormatter(formatter);
          }
          else {
              String s = Bundle.getString("Tags_FormattableParentRequired");
              registerTagError(s, null);
              reportErrors();
          }
      }
docs: Create the internal Formatter instance and perform the formatting.
  @throws JspException if a JSP exception has occurred