Modernize Java concurrency patterns: Java 8–17 updates + Java 21 doc stubs

[Copilot]

Summary

Implements the modernization plan agreed in the previous session. The build target is raised to Java 17 (the highest version available in the current environment), with full working code for all Java 8–17 APIs and Javadoc-only stubs for Java 21 Project Loom features to serve as a migration guide.

All 53 source files compile cleanly (mvn compile — BUILD SUCCESS).

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Changes by Phase

Phase 1 — Build configuration

• pom.xml — compiler plugin bumped from 3.7.0 → 3.12.1; source/target raised from 11 → 17

Phase 2 — Java 8–17 modernizations

File	Change
ThreadSafeDateFormat.java	ThreadLocal.withInitial() + immutable DateTimeFormatter (no ThreadLocal needed)
ThreadLocalConfinement.java	ThreadLocal.withInitial() lambda form
ImmutableObject.java	MyImmutableObject inner class → Java 16 record
EventKeeper.java	Event → Java 16 record (with defensive byte[] copy in compact constructor); switch → Java 14 switch expression
UsingParallelStreams.java	Collectors.toList() → Stream.toList() (Java 16)
UsingConcurrentCollections.java	New demo of Java 8 bulk ops: merge, compute, forEach(threshold,…), reduceValues, search

Phase 3 — Java 9 APIs

File	Change
UsingReactiveStreams.java	Full java.util.concurrent.Flow implementation: SubmissionPublisher → PrintSubscriber (back-pressure with request(1)), DoublingProcessor pipeline, three demos
UsingCompletableFuture.java	Added orTimeout() and completeOnTimeout() (Java 9) with runnable examples
UsingAtomics.java	Added VarHandleCounter using MethodHandles.lookup().findVarHandle(), getAndAdd, getVolatile, compareAndExchange (Java 9)

Phase 4 — Java 21 documentation stubs

Files compile on Java 17 (no executable code). Each contains a detailed Javadoc description with runnable code examples in {@code} blocks, ready to be activated when the project upgrades to JDK 21.

• UsingVirtualThreads.java — Thread.ofVirtual(), newVirtualThreadPerTaskExecutor(), carrier threads, pinning, isVirtual()
• UsingStructuredConcurrency.java — StructuredTaskScope.ShutdownOnFailure and ShutdownOnSuccess
• UsingScopedValues.java — ScopedValue vs ThreadLocal comparison table, nested rebinding, structured concurrency integration
• UsingExecutors.java — added Javadoc for the planned newVirtualThreadPerTaskExecutor() method

Phase 5 — New pattern stubs (Java 21)

• VirtualThreadProducerConsumer.java — how virtual threads simplify the producer-consumer pattern
• StructuredTaskConvergence.java — StructuredTaskScope replacing CyclicBarrier with automatic cancellation
• StructuredBackgroundTaskExecutor.java — structured concurrency comparison table vs BackgroundTaskExecutor

Phase 6 — README

• Added Java version coverage table (Java 8 → Java 17 → Java 21 stubs)
• All new features and patterns listed with links
• Preamble updated to reflect Java 8–17 coverage