Log#configure(Properties) uses java.util.Properties to configure log related properties. However, since java.util.Properties is a hashtable, the processing sequence of the configuration items is not guaranteed.
For example, it could process 'java.util.logging.FileHandler.limit=10000000' before 'handlers=java.util.logging.FileHandler'. At this moment, Log#configure() does not recognize any handlers, and hence would not view 'java.util.logging.FileHandler.limit' as related to 'java.util.logging.FileHandler'. Log#configure() would then try to set the log level of the class 'java.util.logging.FileHandler.limit' to '10000000', which is not a valid log level.
One possible solution would be to parse the log configuration properties twice. The first time is to locate the handlers. The second time is to fine tune the handler settings.