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:
This issue is related to the following feature request:
An easy solution that will enable using ObjectDB as a separate bundle and accessing it from other bundles / Eclipse plug ins is required.
ObjectDB - Fast Object Database for Java (JPA/JDO)