javademos/README.md

javademos

Runnable companion code for the ankurm.com post "Java 27 Is Out: Every JEP, Plus the Java 26 Changes You Skipped", and for the Java version guides (https://ankurm.com/java-21-to-25-lts-features/). Every number and transcript in the post comes from a file in docs/output/, produced by a script in scripts/.

What was tested against

Thing	Version	Note
JDK 21 (LTS)	Amazon Corretto 21.0.12.1	the start of the 21 to 25 lane
JDK 25 (LTS)	Temurin 25.0.4.1+1	the baseline for "broke on 26"
JDK 26	Amazon Corretto 26.0.2.1	GA 2026-03-17, non-LTS
JDK 27	Amazon Corretto 27+35	GA 2026-09-15, non-LTS
Containers	amazoncorretto:23, :24 (when did it change?), :26, :27	same vendor for both, so only the JDK version changes
JOL	0.17	downloaded and sha1-checked by scripts/object-headers.sh

Next LTS: Java 29, September 2027 (Oracle Java SE Support Roadmap). Layout: g1-container/, object-headers/, jep-tour/ (27 features and broken/ 26-era sources), recap26/ (the Java 26 lane), lanes/ (the 21 to 25 hub demos, the AI prompts in lanes/prompts/ and the build-file demo in lanes/upgrade/), other-changes/, api-diff/.

Quickstart

export JDK21=/path/to/jdk-21 JDK25=/path/to/jdk-25 JDK26=/path/to/jdk-26 JDK27=/path/to/jdk-27   # defaults are in scripts/env.sh
./scripts/jep-tour.sh          # the five 27 feature demos, self-asserting
./scripts/recap26.sh           # the 26 recap lane
./scripts/lanes.sh             # the 21 to 25 hub claims, checked on 21, 23, 24, 25, 26, 27
./scripts/upgrade.sh           # the build-file half: Maven, Gradle, Lombok, --release, jdeps (needs Maven Central)
./scripts/run-all.sh           # regenerate every docs/output/*.txt (about 15-20 minutes; needs Docker)

Timings, RSS, GC counts and the Vector species are machine dependent; treat them as shape, not results. Everything else is byte-stable.

Chapters

#	Chapter
1	What shipped, and what to believe
2	G1 becomes the default, even on one CPU (JEP 523)
3	Compact object headers (JEP 534)
4	Post-quantum TLS (JEP 527)
5	Primitive patterns (JEP 532)
6	Lazy constants (JEP 531)
7	Structured concurrency (JEP 533)
8	PEM encodings (JEP 538)
9	JFR redaction (JEP 536) and the Vector API (JEP 537)
10	Other 27 changes
11	Recap: the Java 26 changes you skipped
12	Version lanes: 25 to 29
13	Upgrade checklist
14	The 21 to 25 lane: every claim in the hub posts, checked
15	Build files: Maven, Gradle and Lombok on the way to 25

Captured output (docs/output/)

File	Produced by	What it is
01-03	g1-default.sh	which collector each JDK picks, by container size; the opt-out
04-06	g1-cost.sh, g1-tuning.sh	what G1 costs on one CPU, and two tested explanations
10-15	object-headers.sh	JOL instance sizes on 26, 27, 27 opted out; the JOL record failure and workaround
20-broken-*	broken-on-27.sh	javac's own messages for 26-era preview code on 27
21-25	jep-tour.sh	the five 27 feature demos
30-32	tls-pq.sh	key-exchange matrix, ClientHello size, the opt-out
40	jfr-redaction.sh	five redaction configurations
50-52	api-diff.sh	public API diffs, 26 to 27, 25 to 26, Vector only
60-62	other-changes.sh	removed options, behaviour changes, new 27 API
70-75	recap26.sh	final-field mutation, AOT cache matrix and startup, removals, 26 API, HTTP/3
92-97	upgrade.sh	Maven release 25 on JDK 21, 25, 27, Gradle 8 vs 9 on JDK 25, --release vs -source/-target, Lombok and Mockito by version, jdeps --jdk-internals
80-91	lanes.sh	the 21 to 25 hub claims on JDK 21, 23, 24, 25, 26, 27: scoped values, structured concurrency, finalization, pinning, ZGC flags, warnings

Not reproduced

jdk.java.net/27 and openjdk.org/jeps returned 403 while this was written; the JEP list was cross-checked against four secondary sources and the JDK binaries themselves. HTTP/3 was not spoken to a real HTTP/3 server (no UDP egress). JEP 522 was not benchmarked. JDK-8377013 could not be reproduced.

MIT licensed, see LICENSE.