One of our DBAs answered this question and I thought to post it in the upgrade forum ...

Java enabled database means "the DB is ready to handle requests coming from Java based applications".
A database does this by creating a JVM internally to handle the Java related requests.
In Oracle the JVM gets installed by including the component “JServer JAVA Virtual Machine” during the installation (Other DBs would have different approaches).

To verify if the Oracle DB is 'Java enabled' query the table “all_registry_banners“; see the sample below ...
QUERY Select * from all registry banners where banner like '%JAVA%'
RESULT JServer JAVA Virtual Machine Release 11.X.X.X.X - QA