jdk-21-26-notes.md

JDK 21 to 26 Notes

Treat JDK behavior as version-sensitive.

Defaults

• Repository baseline: JDK 21
• Current local runtime may be newer (e.g. JDK 26)
• Advice about inlining, intrinsics, vectorization, and loop optimizations must be checked on the active runtime

What stays stable enough

• Fewer allocations usually helps
• Fewer copies usually helps
• Monomorphic hot calls are easier to inline than megamorphic ones
• Primitive, contiguous loop shapes are friendlier to optimization than object-heavy callback stacks

What must be verified

• Whether a specific JDK method lowers to an intrinsic on this runtime
• Whether SuperWord or related loop optimizations fire for this loop shape
• Whether a call chain fully inlines on this runtime
• Whether scalar replacement / escape analysis removes the expected allocation
• Whether benchmark results carry across JDK 21 and JDK 26

Reporting rule

When giving low-level JVM explanations, say which JDK you are talking about.

Good:

• On JDK 26 this loop appears to inline fully and the benchmark improves by 12%.
• On the JDK 21 baseline, verify the same claim before treating it as settled.

Bad:

• HotSpot will optimize this.
• The JVM should vectorize it.

Upgrade rule

If a change is intended for the repo baseline, prefer evidence on JDK 21.

If only a newer runtime is available locally, say that clearly and lower confidence until the baseline JVM is checked.