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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
This commit is contained in:
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/output/99-memory-heap-delta.txt
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$ java --enable-preview -Xmx6g ScopedValueVsThreadLocalMemory 500000 (JDK 25)
threads per scenario: 500000, 5 bound values each, forked via StructuredTaskScope
CHECK ok : every one of 1000 subtasks read all 5 scoped values correctly
CHECK ok : every one of 1000 subtasks read all 5 inherited ThreadLocal values correctly
CHECK ok : every one of 500000 subtasks read all 5 scoped values correctly
ScopedValue: 500000 live subtasks, 5 bound values each, heap delta = 531.8 MB (1115 bytes/thread)
CHECK ok : every one of 500000 subtasks read all 5 inherited ThreadLocal values correctly
ThreadLocal: 500000 live subtasks, 5 inherited values each, heap delta = 546.1 MB (1145 bytes/thread)
ThreadLocal used 1.0x the heap ScopedValue used, for holding the identical 5 values live on 500000 threads
exit=0