It's not clear for me from the documentation if a db4o server in java without persistent class can handle unicity constraint, I see nothing saying no, but the way to configure it on the server we need the class on the classpath. (and I don't want that in that case).
Same question for indexes ?
Thanks in advance.
My recommendation: Don't do a db4o server without the persistent classes available. That leads to countless issues.
Anyway, you should be able to just specify the full qualified class-name as a string and it should work.
config.objectClass("your.package.YourClass").objectField("id").indexed(true);
config.add(new UniqueFieldValueConstraint("your.package.YourClass", "id"));