FreeMarker

To be clear, actually I haven't used this patch, as I did other
refactorings too. But it uses ConcurrentHashMap when available, so it's
similar in that respect.

This is now fixed in the 2.3.x head. However, 2.3.20 is only expected on
the end of May. But until that, I think it's already safe to use.

I believe it's not safe to call those methods outside synchronization,
because the new items are added to the map concurrently. Or is there
something in the API that says otherwise?

But anyway, decreasing locking on the class cache is hight priority TODO,
so whenever 2..3.20 is released it will be almost surely part of it. (The
problem that will addressed there is that the map remains locked during
introspection. After all, doing a Map.get inside sync. block is not a big
deal, until the instrospection starts blocking gets. Using
ConcurrentHashMap on Java 1.5+ was also considered, but I'm nor sure what
was the result... maybe that it's overhead killed its advantage if there's
was no long locking anyway.)

I have just attached your patch (sent to me in e-mail) that should be
backward compatible...

It imports imports java.util.concurrent.ConcurrentHashMap... that's there
only since J2SE 1.5. 2.3.x only requires 1.2.