Granted this is an extreme case but large, complex critical systems need a certain amount of stability and upgrades across Java releases are not quite as easy as just touching the servers.

Would you agree that the breakage between 1.4 and 1.5, 1.5 and 1.6, 1.6 and 1.7, 1.7 and 1.8 is not comparable to the breakage that Jigsaw alone introduced between 1.8 and 1.9?

Yes, I do agree.

However stuff like JDBC interface changes comes up as breaking changes in previous releases.

In what concerns Java 9, Mark Reinhold keeps doing presentations about how all Java relevant frameworks and tools are already Java 9 compliant. Including many that they validated against in Maven repositories.

Last presentation I saw was at FOSDEM.

So it appears to be a major issue to anyone wanting to migrate Java 9 without updating the frameworks as well, having some kind of special case library, or using APIs that were always documented as internal.

The QA outreach page [0] suggests maybe about half the projects are ready. The ones that aren't include Netty and SLF4J. Those aren't small projects. Is that page outdated or does "JDK9 ready" not really mean "JDK9 ready"?

EDIT: Maybe it's outdated; SLF4J and Netty do claim to be JDK9-ready.

[0]: https://wiki.openjdk.java.net/display/quality/Quality+Outrea...