Multi-user database access is possible if Derby is running inside a server framework.
If more than one client application tries to modify the same data, the connection that gets the table first gets the lock on the data (either specific rows or the entire table). The second connection has to wait until the first connection commits or rolls back the transaction in order to access the data. If two connections are only querying and not modifying data, they can both access the same data at the same time because they can each get a shared lock.