The error log

Once you create or connect to a database within a system, Derby begins outputting information and error messages, if any. Typically, Derby writes this information to a log called derby.log in the system directory, although you can also have Derby send messages to a stream, using a property. By default, Derby overwrites derby.log when you start the system. You can configure Derby to append to the log with the derby.infolog.append property. For information on setting this and other properties, see Tuning Derby.

Related concepts
One Derby instance for each Java Virtual Machine
Booting databases
Defining the system directory
Double-booting system behavior
Recommended practices
Related tasks
Shutting down the system