JDK 27's nine JEPs are frozen, and two of them change your defaults

Two phrases in the HotSpot titles do the work. "In all environments" ^[6] only means something if some environments currently get a collector other than G1. "By default" ^[7] only means something if compact object headers are currently opt-in. Neither of those changes requires anything from you to take effect, and that is the whole difference between them and the five preview and incubator items on the list ^[16]. Code that never touches Lazy Constants or the Vector API is untouched by their arrival. A heap gets the collector the JVM picks, read notes or not.

The argument about that is over. The feature set was fixed when the main line forked to the stabilization repository in early June 2026 ^[2], and only critical bugs approved through the Fix-Request process can move now ^[3]. That leaves roughly three months of stabilization ^[18] in which the available lever is not the JDK's default but your own command line.

It is worth counting what the nine actually are. Structured concurrency arrives as a seventh preview, after six previews in JDK 21 through 26 and two incubator rounds in 19 and 20 ^[12], which makes JDK 27 the ninth consecutive release to carry it ^[15]. The Vector API enters a twelfth incubation with no substantial implementation change since JDK 25, and will not be promoted to preview until the Project Valhalla features it depends on are themselves available as previews ^[13]. Primitive types in patterns reaches a fifth preview, with tighter dominance checks in switch and a revised definition of unconditional exactness ^[11]. These are the items with the longest paper trail and the least effect on a service you deploy tomorrow.

The third HotSpot item is the one that lands in an existing operations pipeline rather than a language feature backlog. JEP 536 has JDK Flight Recorder redact sensitive data before a recording completes, and the source names command-line arguments among the data in scope ^[8]. If your recordings are currently scrubbed after the fact, part of that job moves inside the JVM, and the part that stays yours is the part you still have to demonstrate to whoever asks.

The release itself is the second non-LTS since JDK 25 ^[4][14], so the population that adopts it in September is self-selecting: teams who follow releases. That is the audience least likely to be surprised by a default collector change, and the one whose bug reports determine what the next set of adopters inherits. According to InfoQ's summary of the release candidate, declared by Oracle's Mark Reinhold ^[1], the nine JEPs span four categories ^[5]; the categories are how the release is described, not how it is felt. Five things you must request, two things that happen anyway, one change to your telemetry, and one post-quantum key exchange for TLS 1.3 ^[10] is a more useful partition for anyone planning an upgrade window.