This one from dev@list:
The problem is in ReferenceMap.values(). If the GC runs between
checkReferenceQueue() and the last v.get(), then null can be added to the
collection (when using the default WeakValueMap). keySet() and entrySet()
have similar issues.
Previously used class from commons-collections forbid null values so our implementation should do the same, this allows to clear nulls not only explicitly in reference queue check but also inside any operation.
keySet() method will be intact as we can't make any assumption about when returned keys will be used thus it's impractical to additionally clear stale references in that method.