Due to change to Java 11 as minimum version (
LUCENE-8738), the usage of the old java.utilObservable pattern is deprecated and cannot be used anymore in Lucene/Solr. LUCENE-8738 added a rewritten, more type-safe oimplementation of the observer pattern, but it looks like the is overengenieered. As there is only one listener registered, it would be enough to just call the method on SolrCores class (pkg-protected). On LUCENE-8738, Erick Erickson suggested to move the callback method to queue closes of core in CoreContainer instead, so all the abstractions can be removed.