There has been a long email thread on core-libs-dev about deadlocks happening when Logger was recursively invoked by a custom formatter.
However there are more subtle cases where the same kind of deadlocks could occur: for instance, when using a simple formatter, a recursive call to Logger may occur if one of the message parameters passed to the formatter is an object whose toString() invokes a methods that happens to do some logging.

I believe we should at least recommend - as a general programming rule - to avoid calling Logger from within toString(), hashCode(), equals(), or j.u.l.Formatter. We should also investigate whether we could make handlers safer wrt to recursive logging.