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Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

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Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
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2026-09-21 15:08:50 +00:00
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co-authored by Claude Sonnet 5
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docs/10-other-27-changes.md
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# 10. The changes that are not JEPs but will still break a Monday
Prev: [9. JFR and Vector](09-jfr-redaction-and-vector.md) &middot; Next: [11. Recap of Java 26](11-recap-26.md)
Script: [`scripts/other-changes.sh`](../scripts/other-changes.sh). All from real launches; each option runs `java <option> -version` on 26 and on 27 ([60](output/60-removed-options.txt)).
| Option / property | JDK 26 | JDK 27 |
|---|---|---|
| `-noverify`, `-Xverify:none` | deprecation warning | **`Unrecognized option`, the JVM does not start** |
| `-noclassgc` | deprecation warning | **`Unrecognized option`, the JVM does not start** |
| `-XX:+UseCompressedClassPointers` | deprecated in 25 | ignored: "support was removed in 27.0" |
| `-XX:InitiatingHeapOccupancyPercent=40` | accepted | deprecated; use `-XX:G1IHOP` |
| `-XX:+UseGraalJIT` (experimental) | JVMCI error | `Unrecognized VM option` (JVMCI hook removed) |
| `-Djdk.lang.Process.launchMechanism=VFORK` | deprecation banner | "has been removed. Switching to FORK instead" ([61](output/61-behaviour-changes.txt)) |
| `-Djava.locale.useOldISOCodes=true` | works, deprecated | ignored: `Locale("iw").language` is `he` ([61](output/61-behaviour-changes.txt)) |
**The point of the table:** rows 1-2 are the ones that stop a service from starting. They are old flags that live in Dockerfiles and `JAVA_TOOL_OPTIONS` long after anyone remembers why.
Grep your images before you upgrade.
## New public API worth knowing
Found by diffing `javap` dumps ([51](output/51-api-diff-26-to-27.txt)) and exercised once each in [`Api27`](../other-changes/src/Api27.java) ([62](output/62-new-api.txt)):
`Math.acosh/asinh/atanh`, `String.encodedLength(Charset)` (length in bytes without allocating the array), `BigDecimal.rootn`, `KeyStore.getCreationInstant`.
Prev: [9. JFR and Vector](09-jfr-redaction-and-vector.md) &middot; Next: [11. Recap of Java 26](11-recap-26.md)