When starting a test Sol instance, it locks up sometimes. We took a thread dump and all threads are trying to load classes via Class.forName() and are stuck in that method. One of these threads got one step further into the <clinit> of TrieField where it creates an internal static instance of TrieDateField (circular dependency). I don't know why this locks up exactly, but this code smells anyway. So I removed that instance and made the used methods static in TrieDateField.
This does not completely remove the circular dependency, but at least it is no more in <clinit>. For the future someone may extract a util class to remove the circular dependency.