Claude
ff6130bede
Payment model, permits and the three modifiers, default-branch trap, generic and recursive hierarchies, visitor comparison, run-time enforcement, modules. Transcripts from JDK 21, 25 and 27. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a
javademos
Runnable companion code for the ankurm.com post "Java 27 Is Out: Every JEP, Plus the Java 26 Changes You Skipped", for the Java version guides
(https://ankurm.com/java-21-to-25-lts-features/), and for two standalone deep dives: Stream Gatherers (JEP 485) and Scoped Values vs
ThreadLocal (JEP 506). 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/,
gatherers/ (JEP 485, standalone post), scoped-values/ (JEP 506 vs ThreadLocal, standalone post).
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/gatherers.sh # Stream Gatherers (JEP 485): every built-in gatherer, a custom one, mapConcurrent
./scripts/scoped-values.sh # Scoped Values vs ThreadLocal (JEP 506): API, inheritance, memory (needs jcmd on PATH)
./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
Standalone article modules (no docs/ folder)
These modules keep only sources, a run.sh and their transcripts (output/). The explanation lives in the article itself, in collapsible sections.
| Module | Article | What it demonstrates |
|---|---|---|
jep512/ |
Compact Source Files and Instance Main Methods in Java 25 (JEP 512) | java Hello.java on JDK 21 vs 25 vs 27, the class javac builds around void main(), implicit java.base imports, java.lang.IO, the measured launch order, a private-constructor difference between 25 and 27 |
jep513/ |
Flexible Constructor Bodies in Java 25 (JEP 513): Validate Before super() | statements before super(...)/this(...), the parent-calls-child bug and its fix, twelve compile errors that remain with the 25 and 27 wordings side by side, --release rejection |
jep511/ |
Module Import Declarations in Java 25 (JEP 511) | import module java.base; against seven single imports, ambiguity errors and how single-type, on-demand and same-package names beat a module import, what one module import does and does not bring in (java.sql does not give java.base; java.se needs --add-modules), named modules, JEP 512 compact files, jshell's default imports, identical bytecode |
patterns/ |
Pattern Matching in Java: switch, Record Patterns and Primitive Patterns (JDK 21 to 27) | type and record patterns, guards and dominance errors, sealed exhaustiveness (with JDK 27's new missing patterns lines), MatchException from a throwing accessor and from separate compilation, unnamed patterns, primitive patterns with --enable-preview and the preview class-file trap |
sealed/ |
Sealed Classes and Interfaces: Modelling Domains with Exhaustive switch | a Payment model, permits and the three subclass modifiers, the default trap, generic and recursive hierarchies, the visitor for comparison, run-time enforcement and modules |
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 |
98 |
gatherers.sh |
the four built-in gatherers, mapConcurrent + virtual threads, a custom Gatherer, Collector vs Gatherer |
99 |
scoped-values.sh |
ScopedValue API and rebinding, why plain threads don't inherit a binding, memory vs ThreadLocal two ways (heap delta, then jcmd object census) |
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.