# 13. Upgrade checklist: 25 to 27 (or to 26)

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Ordered by "will stop a service from starting" first.

1. **Grep for flags that no longer exist.** `-noverify`, `-Xverify:none` and `-noclassgc` make a 27 JVM refuse to start ([60](output/60-removed-options.txt)). Look in Dockerfiles, `JAVA_TOOL_OPTIONS`, systemd units and start scripts.
2. **Pin your collector in small containers.** If nobody sets one, 27 gives you G1 on one CPU, where 26 gave you Serial ([02](02-g1-default.md)). Decide on purpose, and set `-Xmx` or `-XX:MaxRAMPercentage`.
3. **Run your tests once with `--illegal-final-field-mutation=deny` on 26 or later.** Mocking, DI and serialisation libraries are the usual offenders ([11](11-recap-26.md)).
4. **Compile everything on 26 and 27.** `java.applet`, `JApplet` and `Thread.stop()` are gone ([73](output/73-removed-in-26.txt)).
5. **Test TLS through your real network path.** JDK 27 clients send a 1573-byte ClientHello ([04](04-post-quantum-tls.md)). If a middlebox objects, `-Djdk.tls.namedGroups=x25519,secp256r1` restores the old behaviour.
6. **Search for hard-coded object sizes.** Estimators, capacity plans and tests that assume a 12-byte header ([03](03-compact-headers.md)).
7. **Do not ship preview APIs from a library.** Structured concurrency, lazy constants and PEM changed API between 26 and 27, and primitive patterns is now on its fifth preview ([12](12-lanes-25-to-29.md)).
8. **Check custom JFR redaction settings.** A list without `+` replaces the defaults ([09](09-jfr-redaction-and-vector.md)).
9. **Rebuild AOT caches per JDK and per collector** ([11](11-recap-26.md)).
10. **Decide whether to move at all.** 26 and 27 are non-LTS: 26's premier support ends September 2026 and 27's in March 2027. Staying on 25 and planning for 29 is a legitimate answer ([12](12-lanes-25-to-29.md)).

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