Unravel › Java concepts › Concurrency

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At a glance

1. 1CountDownLatch latch = new CountDownLatch(3); each worker calls latch.countDown() when done; the coordinator calls latch.await() and continues when the count reaches 0It is single-use.
2. 2CyclicBarrier(4, action) makes four threads call await() and all continue together, then it resets and can be reused for the next phase
3. 3Semaphore sem = new Semaphore(5); sem.acquire(); try { ..} finally { sem.release(); } limits concurrency, for example to protect a connection pool or a rate-limited API.
4. 4Phaser generalises the barrier to a changing number of parties and several phases

• Always call release() and countDown() in finally; a failed worker that never counts down leaves the waiter stuck forever
• A semaphore with one permit is a lock that any thread can release, which makes it a poor mutex but a useful signal
• They are built on AbstractQueuedSynchronizer (AQS), a lock-free state counter plus a queue of waiting threads

import java.util.concurrent.*;public class StartUp {  static void runAll(ExecutorService pool, Runnable[] jobs) throws InterruptedException {    CountDownLatch done = new CountDownLatch(jobs.length);    Semaphore limit = new Semaphore(2);    for (Runnable job : jobs) {      pool.execute(() -> {        try { limit.acquire(); try { job.run(); } finally { limit.release(); } }        catch (InterruptedException e) { Thread.currentThread().interrupt(); }        finally { done.countDown(); }      });    }    if (!done.await(10, TimeUnit.SECONDS)) throw new IllegalStateException("timeout");  }}

1. line 4 uses CountDownLatch, the core of Latches, barriers and semaphores
2. line 5 uses Semaphore, the core of Latches, barriers and semaphores
3. line 8 uses acquire, the core of Latches, barriers and semaphores
4. line 10 uses countDown, the core of Latches, barriers and semaphores
5. line 13 uses await, the core of Latches, barriers and semaphores

How Unravel recognises it

When you paste code, Unravel looks for these signs of latches, barriers and semaphores. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: CountDownLatch CyclicBarrier Semaphore Phaser Exchanger
• Method calls: countDown() await() acquire() release() arriveAndAwaitAdvance() tryAcquire() getCount()

Part of a learning path

• Concurrency beyond the basics · previous: Interruption and InterruptedException · next: Producer-consumer with blocking queues

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