Days before JDK 27's scheduled release, OpenJDK developers emergency-reverted a P1-level C2 compiler intrinsic memory alias bug (JDK-8390590) that caused incorrect optimization of EncodeISOArray, choosing a clean backout over risky patching to ensure release stability.
JDK 27 changes the default garbage collector from a resource‑aware Serial GC to G1, which can unexpectedly increase CPU usage, alter latency and throughput for small‑container Java services even when no code or JVM flags are changed, so teams must verify the GC in use and benchmark both collectors under realistic limits.
JDK 27’s third preview (JEP 531) removes the isInitialized() and orElse() methods from the LazyConstant API, adds a Set.ofLazy() factory method, and refines the lazy‑constant design to improve composability, startup performance, thread safety, and JVM optimizations.
JDK 27 introduces JEP 534, making compact object headers the default layout, shrinking the 12‑byte header to 8 bytes (a 33% reduction), which lowers heap memory consumption by over 20%, cuts CPU time by 8‑30% in benchmarks, reduces GC pressure, and requires no code changes.
