synchronizers: CountDownLatch vs CyclicBarrier vs Phaser vs Semaphore

One three-round worker pipeline implemented with CountDownLatch (one latch
per round), CyclicBarrier (reusable, with a barrier action), and Phaser
(reusable, plus dynamic mid-run registration), alongside Semaphore solving
the genuinely different problem of bounding concurrent access. Covers
timeout behavior (CyclicBarrier permanently breaks on one timeout, Phaser
doesn't) and virtual-thread compatibility, including the corrected finding
that Object.wait() releases its monitor and never pinned, unlike
Thread.sleep() inside synchronized.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM

synchronizers/output/01-countdownlatch-pipeline.txt

@@ -0,0 +1,17 @@
+$ java CountDownLatchPipeline
+
+main: all 4 workers created, still waiting at the start gate
+main: releasing the start gate
+worker-0: finished round 1
+worker-1: finished round 1
+worker-2: finished round 1
+worker-3: finished round 1
+worker-0: finished round 2
+worker-1: finished round 2
+worker-2: finished round 2
+worker-3: finished round 2
+worker-0: finished round 3
+worker-1: finished round 3
+worker-2: finished round 3
+worker-3: finished round 3
+done

synchronizers/output/02-cyclicbarrier-pipeline.txt

@@ -0,0 +1,18 @@
+$ java CyclicBarrierPipeline
+
+worker-0: finished round 1
+worker-1: finished round 1
+worker-2: finished round 1
+worker-3: finished round 1
+barrier action: round 1 complete, all 4 workers arrived
+worker-0: finished round 2
+worker-1: finished round 2
+worker-2: finished round 2
+worker-3: finished round 2
+barrier action: round 2 complete, all 4 workers arrived
+worker-0: finished round 3
+worker-1: finished round 3
+worker-2: finished round 3
+worker-3: finished round 3
+barrier action: round 3 complete, all 4 workers arrived
+done

synchronizers/output/03-phaser-pipeline.txt

@@ -0,0 +1,21 @@
+$ java PhaserPipeline
+
+worker-0: finished round 1
+worker-1: finished round 1
+worker-2: finished round 1
+worker-3: finished round 1
+phaser onAdvance: phase 1 complete, 4 registered parties -> advancing
+worker-4: registers after round 1 (phaser now has 5 parties)
+worker-0: finished round 2
+worker-1: finished round 2
+worker-2: finished round 2
+worker-3: finished round 2
+worker-4: finished round 2
+phaser onAdvance: phase 2 complete, 5 registered parties -> advancing
+worker-0: finished round 3
+worker-1: finished round 3
+worker-2: finished round 3
+worker-3: finished round 3
+worker-4: finished round 3
+phaser onAdvance: phase 3 complete, 5 registered parties -> advancing
+done

synchronizers/output/04-semaphore-resource-guard.txt

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+$ java SemaphoreResourceGuard
+
+worker-0 round 1: acquired permit, 1 callers concurrently in the resource (availablePermits=0)
+worker-1 round 1: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-2 round 1: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-3 round 1: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-3 round 2: acquired permit, 1 callers concurrently in the resource (availablePermits=1)
+worker-0 round 2: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-1 round 2: acquired permit, 1 callers concurrently in the resource (availablePermits=1)
+worker-2 round 2: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-2 round 3: acquired permit, 1 callers concurrently in the resource (availablePermits=1)
+worker-3 round 3: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+worker-0 round 3: acquired permit, 1 callers concurrently in the resource (availablePermits=1)
+worker-1 round 3: acquired permit, 2 callers concurrently in the resource (availablePermits=0)
+permits=2 maxObservedConcurrency=2
+done

synchronizers/output/05-timeout-behavior.txt

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+$ java TimeoutBehaviorDemo
+
+=== CountDownLatch.await(timeout): expects 2, only 1 ever counts down ===
+await returned: false (false = timed out, latch still usable to re-check later)
+getCount() after timeout: 1
+
+=== CyclicBarrier.await(timeout): expects 2 parties, only 1 arrives ===
+await threw TimeoutException, as expected
+barrier.isBroken() after the timeout: true
+a second, later await() on the SAME barrier: BrokenBarrierException immediately - one timeout poisons the barrier for everyone until reset()
+
+=== Phaser.awaitAdvanceInterruptibly(phase, timeout): expects 2 parties, only 1 arrives ===
+awaitAdvanceInterruptibly threw TimeoutException, as expected
+phaser.getPhase() after the timeout: 0 (unchanged - still phase 0, not advanced or broken)
+a later round on the SAME phaser, once both parties actually arrive: succeeds normally, now at phase 1
+
+=== Semaphore.tryAcquire(timeout): 0 permits available ===
+tryAcquire returned: false (false = timed out, no exception, no broken state)
+a later tryAcquire after a release: true - fully independent of the earlier timeout
+done

synchronizers/output/06-virtual-thread-compatibility.txt

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+$ java -Djdk.tracePinnedThreads=full VirtualThreadCompatibilityDemo   (pre-JDK-24 runtime)
+
+java.version=21.0.10
+jdk.tracePinnedThreads=full
+CountDownLatch.await(): completed, no trace above this line for it
+CyclicBarrier.await(): completed, no trace above this line for it
+Phaser.arriveAndAwaitAdvance(): completed, no trace above this line for it
+Semaphore.acquire(): completed, no trace above this line for it
+done - no pinning trace printed above for any of the four
+
+$ java -Djdk.tracePinnedThreads=full SynchronizedWaitPinsDemo   (pre-JDK-24 runtime, contrast case)
+
+java.version=21.0.10
+jdk.tracePinnedThreads=full
+synchronized + Object.wait(): completed, no pinning trace above this line for it
+VirtualThread[#21]/runnable@ForkJoinPool-1-worker-1 reason:MONITOR
+    java.base/java.lang.VirtualThread$VThreadContinuation.onPinned(VirtualThread.java:199)
+    java.base/jdk.internal.vm.Continuation.onPinned0(Continuation.java:393)
+    java.base/java.lang.VirtualThread.parkNanos(VirtualThread.java:635)
+    java.base/java.lang.VirtualThread.sleepNanos(VirtualThread.java:807)
+    java.base/java.lang.Thread.sleep(Thread.java:507)
+    com.ankurm.synchronizers.SynchronizedWaitPinsDemo.lambda$main$1(SynchronizedWaitPinsDemo.java:48) <== monitors:1
+    java.base/java.lang.VirtualThread.run(VirtualThread.java:329)
+synchronized + Thread.sleep(): completed, pinning trace (if any) appears above this line for it
+done

synchronizers/output/07-round-ordering-correctness.txt

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+-------------------------------------------------------------------------------
+Test set: com.ankurm.synchronizers.RoundOrderingCorrectnessTest
+-------------------------------------------------------------------------------
+Tests run: 41, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.514 s -- in com.ankurm.synchronizers.RoundOrderingCorrectnessTest