[dkersten]

You forgot one: if, for some reason, none of the four mentioned concurrency strategies suit your needs, you can fall back to Javas monitor-based threading model. I don't see why one would want to, but I'm sure there are situations when this actually is the most appropriate mechanism.

[tayssir]

And a possible future feature is something like statically-checked locking, where order-of-acquisition issues are dealt with.