[aggregat]

We have 2.1 million LOC in Java and we're moving to Java 21 (from 17) in two weeks when we branch for release.

We have a hundreds of third party dependencies across the code base, a lot of the big ones (Hibernate, Spring, a lot of Apache). We write a big web application and maintain a big legacy desktop application in Swing.

We run a dedicated nightly CI job that is on the latest Java release to get early warning for any incompatibilities. After the painful migration from 8 to 9 so many years ago it has been smooth sailing.

In all those version upgrades over all those years and dozens of on premise installations with big customers we have never had a regression or a problem that was caused by the runtime itself.