ObjectDB can be used in Eclipse plugin, but currently it has to be embedded in the plugin and cannot be used from other plugins in which it is not embedded.
More details can be found in the following forum threads:
Eclipse plugin problem (using ObjectDB as a separate bundle)
ClassCastException of same object type
javax.persistence.PersistenceException
This issue is related to the following feature request:
Issue #257 - Better OSGI Compatibility
An easy solution that will enable using ObjectDB as a separate bundle and accessing it from other bundles / Eclipse plug ins is required.