Head First Servlets and JSP, 2nd Edition

Getting an arbitrary ancestor

There is another mechanism you can use if you want to, say, skip some nesting levels and go straight to a grandparent or something even further up the tag nesting hierarchy. The method is in both TagSupport and SimpleTagSupport (although they have slightly different behavior), and it’s called findAncestorWithClass().

Getting an immediate parent using getParent()

OuterTag parent = (OuterTag) getParent();

Getting an arbitrary ancestor using findAncestorWithClass()

WayOuterTag ancestor = (WayOuterTag) findAncestorWithClass(this, WayOuterTag.class);

The Container walks the tag nesting hierarchy until it finds a tag that’s an instance of this class. It returns the first one, so there’s no way to say “skip the first tag you see that’s an instance of WayOuterTag.class and give me the second instance instead...” So if you really know for a fact that you wanted the second instance of a tag ancestor of that type, you’ll just have to get the return value of findAncestorWithClass(), and then call getParent() or findAncestorWithClass() on it.

You will not be tested on any details of using findAncestorWithClass(). All you need to know for the exam is that it exists!

Exercise

Key differences between Simple and Classic tags

	Simple tags	Classic tags
Tag interfaces
Support implementation classes
Key lifecycle methods that YOU might implement
How you write to the response output ...