One Derby instance for each Java Virtual Machine (JVM) You could potentially have two instances of a system running on the same machine at the same time. Each instance must run in a different Java Virtual Machine (JVM). Multiple JVMs accessingprohibition of single systemprohibition of

If you use the embedded driver, two separate instances of cannot access the same database. If a instance attempts to access a running database, an error message appears, and a stack trace appears in the derby.log file. If you want more than one instance to be able to access the same database, you can use the Network Server.

If a instance uses the in-memory database capability for its database connection, the database exists only within the JVM of that instance. Another instance could refer to the same database name, but it would not be referring to the same actual database, and no error would result.