A DESCRIPTION OF THE REQUEST :
private Hashtable<String,LoggerWeakRef> namedLoggers = new Hashtable<String,LoggerWeakRef>();

should be converted to:

private Map<String,LoggerWeakRef> namedLoggers = new ConcurrentHashMap<String,LoggerWeakRef>();

JUSTIFICATION :
It improves logging performance due to not having lock contention on retrieval.

Retrieving a logger happens way more often then adding a new logger.

already done in other java.lang.util.* classes:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6991380

also see:
http://www.ibm.com/developerworks/java/library/j-jtp07233/index.html#N101CD
http://www.codercorp.com/blog/java/why-concurrenthashmap-is-better-than-hashtable-and-just-as-good-hashmap.html