This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This means that when navigating through the hierarchy of nested tags using Tag.getParent(), you need to check if the instance returned is actually a TagAdapter, which always encloses a SimpleTag. If so, use the TagAdapter.getAdaptee() method to extract the original SimpleTag.
This is also the reason why the SimpleTagSupport.findAncestorWithClass() method is preferred over the TagSupport.findAncestorWithClass(), because the SimpleTagSupport method automatically unwraps any TagAdapters, exposing the result simply as a JspTag (as with all Tags SimpleTags).