concurrency8LocksAndCoordination

1package pkg7concurrency;2 3/*4 * concurrency8LocksAndCoordination.java5 * -------------------------6 * java.util.concurrent coordination tools: ReentrantLock, ReadWriteLock,7 * CountDownLatch, Semaphore, and a deadlock-avoidance note.8 */9import java.util.concurrent.*;10import java.util.concurrent.locks.*;11import java.util.concurrent.atomic.AtomicInteger;12 13public class concurrency8LocksAndCoordination {14 15    public static void main(String[] args) throws InterruptedException {16        countDownLatchDemo();17        readWriteLockDemo();18        semaphoreDemo();19    }20 21    // CountDownLatch: wait for N tasks to complete before proceeding.22    static void countDownLatchDemo() throws InterruptedException {23        int workers = 4;24        CountDownLatch latch = new CountDownLatch(workers);25        for (int i = 0; i < workers; i++) {26            int id = i;27            new Thread(() -> {28                try { Thread.sleep(20 * id); } catch (InterruptedException ignored) {}29                latch.countDown();              // signal completion30            }).start();31        }32        latch.await();                          // block until count reaches 033        System.out.println("[CountDownLatch] all " + workers + " workers finished");34    }35 36    // ReadWriteLock: many readers OR one writer (improves read-heavy throughput).37    static void readWriteLockDemo() throws InterruptedException {38        ReadWriteLock rw = new ReentrantReadWriteLock();39        AtomicInteger shared = new AtomicInteger(0);40 41        Runnable writer = () -> {42            rw.writeLock().lock();43            try { shared.incrementAndGet(); } finally { rw.writeLock().unlock(); }44        };45        Runnable reader = () -> {46            rw.readLock().lock();47            try { shared.get(); } finally { rw.readLock().unlock(); }48        };49 50        Thread[] ts = new Thread[6];51        for (int i = 0; i < ts.length; i++) ts[i] = new Thread(i % 2 == 0 ? reader : writer);52        for (Thread t : ts) t.start();53        for (Thread t : ts) t.join();54        System.out.println("[ReadWriteLock] writes applied, shared = " + shared.get());55    }56 57    // Semaphore: limit concurrent access to a resource (e.g. a connection pool).58    static void semaphoreDemo() throws InterruptedException {59        Semaphore permits = new Semaphore(2);   // only 2 at a time60        AtomicInteger maxConcurrent = new AtomicInteger();61        AtomicInteger current = new AtomicInteger();62 63        Thread[] ts = new Thread[6];64        for (int i = 0; i < ts.length; i++) {65            ts[i] = new Thread(() -> {66                try {67                    permits.acquire();68                    int now = current.incrementAndGet();69                    maxConcurrent.accumulateAndGet(now, Math::max);70                    Thread.sleep(20);71                    current.decrementAndGet();72                    permits.release();73                } catch (InterruptedException ignored) {}74            });75        }76        for (Thread t : ts) t.start();77        for (Thread t : ts) t.join();78        System.out.println("[Semaphore] max concurrent observed = " + maxConcurrent.get() + " (cap = 2)");79    }80}