Build 2.2.9_05 adds this new feature, which should be very useful in tests.
You can override an existing database file by adding ;drop to the url:
EntityManagerFactory emf =
A new configuration line was added to specify database file name extensions for which drop is enabled:
<extensions drop="temp,tmp" />
This is a defensive restriction to avoid deleting databases by mistake, but the conventional odb extension could also be added to that configuration setting.
In client-server mode this configuration should be set on the server side.
ObjectDB - Fast Object Database for Java (JPA/JDO)