Solr 9 isn’t supporting Java 8, and even Solr 7 was commonly packaged for Java 11. This is the most cumbersome upgrade because of Java EE being transferred to Eclipse under the name Jakarta EE, the transition took some years to complete for all dependencies.

It affects both the Java binding of the MusicBrainz XML Metadata (MMD) Schema, and MusicBrainz Solr itself.