One Derby instance for each Java Virtual Machine

You could potentially have two instances of a Derby system (JVM) running on the same machine at the same time. Each instance must run in a different JVM. Two separate instances of Derby must not access the same database. For example, in an embedded environment, an application that accesses Derby databases starts up the local JDBC driver, which starts up an instance of Derby. If you start another application, such as ij, and connect to the same database, severe database corruption can result. See Double-booting system behavior.

Related concepts
Booting databases
Defining the system directory
The error log
Double-booting system behavior
Recommended practices
Related tasks
Shutting down the system