This makes the parts of the API that get extended more clear in regards to what behavior is required (abstract), and what behavior can not be overridden (final). In any application that does not override handle*, this is a trivial change.

I'd prefer to keep these methods overridable, since it allows for more flexibility in applications.

Can you provide a little more detail on which methods, specifically, you intend to preserve, and what the reasoning is? The handler behavior isn't something I see a reason to override - is this the concern? The rest of the commit should be innocuous, and if needed I'd like to pick out the offensive parts so the rest can be merged.