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Add two standalone modules: Stream Gatherers (JEP 485) and Scoped Values vs ThreadLocal (JEP 506)

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gatherers/: windowFixed, windowSliding, fold, scan, mapConcurrent, a custom
Gatherer (dedupeConsecutive, takeUntil), and a Collector-vs-Gatherer comparison.
Verified: windowSliding emits one short window on a too-short stream rather than
shrinking to empty like windowFixed; mapConcurrent waits for in-flight siblings
to finish (~901ms) rather than cancelling them when one mapper throws.

scoped-values/: ScopedValue API and rebinding, why plain threads (virtual or
platform) do not inherit a binding while a StructuredTaskScope subtask does,
and the memory cost vs InheritableThreadLocal measured two ways -- a noisy
heap-delta first pass, then a precise jcmd GC.class_histogram object census
(5 shared Carrier objects vs ~700,000 copied ThreadLocalMap/Entry objects for
100,000 threads).

Companion code for the ankurm.com posts "Java Stream Gatherers (JEP 485)" and
"Scoped Values vs ThreadLocal in Java 25: Migration Guide with Virtual Threads".

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FtpJvZfg4nvLvtzgJTDWpB
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Claude
2026-09-24 04:27:12 +00:00
parent f59c1de96d
commit 6b5a918278
25 changed files with 1124 additions and 5 deletions
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docs/15-build-files.md
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@@ -1,6 +1,6 @@
# 15. Build files: what the 21 to 25 upgrade does to Maven, Gradle and Lombok
Prev: [14. The 21 to 25 lane](14-lanes-21-to-25.md) &middot; [Back to the README](../README.md)
Prev: [14. The 21 to 25 lane](14-lanes-21-to-25.md) &middot; Next: [16. Stream Gatherers](16-gatherers.md)
The build half of the [upgrade guide](https://ankurm.com/java-21-to-25-upgrade-guide-ai-prompts/), run by `scripts/upgrade.sh`.
Everything below is a run on JDK 21, 23, 24, 25, 26 or 27; the transcripts are in `output/92` to `output/97`.
@@ -58,4 +58,4 @@ Even on the working versions the JVM prints `WARNING: A Java agent has been load
[96](output/96-jdeps-internals.txt) is `jdeps --jdk-internals` on the class from [`UnsafeWarning.java`](../lanes/src/UnsafeWarning.java): it names `sun.misc.Unsafe` before the JVM prints its run-time warning ([86](output/86-unsafe-and-jni.txt)).
Prev: [14. The 21 to 25 lane](14-lanes-21-to-25.md) &middot; [Back to the README](../README.md)
Prev: [14. The 21 to 25 lane](14-lanes-21-to-25.md) &middot; Next: [16. Stream Gatherers](16-gatherers.md)