Examples of java.util.concurrent.Semaphore

java.util.concurrent.Semaphore
a particularly efficient data structure; just for demo protected Object[] items = ... whatever kinds of items being managed protected boolean[] used = new boolean[MAX_AVAILABLE]; protected synchronized Object getNextAvailableItem() { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (!used[i]) { used[i] = true; return items[i]; } } return null; // not reached } protected synchronized boolean markAsUnused(Object item) { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (item == items[i]) { if (used[i]) { used[i] = false; return true; } else return false; } } return false; } }
Before obtaining an item each thread must acquire a permit from the semaphore, guaranteeing that an item is available for use. When the thread has finished with the item it is returned back to the pool and a permit is returned to the semaphore, allowing another thread to acquire that item. Note that no synchronization lock is held when {@link #acquire} is called as that would prevent an itemfrom being returned to the pool. The semaphore encapsulates the synchronization needed to restrict access to the pool, separately from any synchronization needed to maintain the consistency of the pool itself.
A semaphore initialized to one, and which is used such that it only has at most one permit available, can serve as a mutual exclusion lock. This is more commonly known as a binary semaphore, because it only has two states: one permit available, or zero permits available. When used in this way, the binary semaphore has the property (unlike many {@link Lock}implementations), that the "lock" can be released by a thread other than the owner (as semaphores have no notion of ownership). This can be useful in some specialized contexts, such as deadlock recovery.
The constructor for this class optionally accepts a fairness parameter. When set false, this class makes no guarantees about the order in which threads acquire permits. In particular, barging is permitted, that is, a thread invoking {@link #acquire} can be allocated a permit ahead of athread that has been waiting - logically the new thread places itself at the head of the queue of waiting threads. When fairness is set true, the semaphore guarantees that threads invoking any of the {@link #acquire() acquire} methods are selected to obtain permits in the order inwhich their invocation of those methods was processed (first-in-first-out; FIFO). Note that FIFO ordering necessarily applies to specific internal points of execution within these methods. So, it is possible for one thread to invoke {@code acquire} before another, but reach the ordering point afterthe other, and similarly upon return from the method. Also note that the untimed {@link #tryAcquire() tryAcquire} methods do nothonor the fairness setting, but will take any permits that are available.
Generally, semaphores used to control resource access should be initialized as fair, to ensure that no thread is starved out from accessing a resource. When using semaphores for other kinds of synchronization control, the throughput advantages of non-fair ordering often outweigh fairness considerations.
This class also provides convenience methods to {@link #acquire(int) acquire} and {@link #release(int) release} multiplepermits at a time. Beware of the increased risk of indefinite postponement when these methods are used without fairness set true.
Memory consistency effects: Actions in a thread prior to calling a "release" method such as {@code release()}happen-before actions following a successful "acquire" method such as {@code acquire()}in another thread. @since 1.5 @author Doug Lea

        final int count=names.length;


        final ConcurrentStartupChannel[] channels=new ConcurrentStartupChannel[count];
        try {
            // Create a semaphore and take all its permits
            Semaphore semaphore=new Semaphore(count);
            semaphore.acquire(count);


            // Create activation threads that will block on the semaphore
            for(int i=0;i < count;i++) {
                if(i == 0)
                    channels[i]=new ConcurrentStartupChannel(names[i], semaphore);
                else
                    channels[i]=new ConcurrentStartupChannel((JChannel)channels[0].getChannel(), names[i], semaphore);
                channels[i].start();
                semaphore.release(1);
                if(i == 0)
                    Util.sleep(1500); // sleep after the first node to educe the chances of a merge
            }


            // Make sure everyone is in sync
            Channel[] tmp=new Channel[channels.length];
            for(int i=0; i < channels.length; i++)
                tmp[i]=channels[i].getChannel();


            Util.blockUntilViewsReceived(30000, 500, tmp);
            System.out.println(">>>> all nodes have the same view <<<<");


            // Re-acquire the semaphore tickets; when we have them all we know the threads are done
            boolean acquired=semaphore.tryAcquire(count, 20, TimeUnit.SECONDS);
            if(!acquired) {
                log.warn("Most likely a bug, analyse the stack below:");
                log.warn(Util.dumpThreads());
            }

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@Test(groups = Global.FLUSH, sequential = false)
public class FlushTest extends ChannelTestBase {


    @Test
    public void testSingleChannel() throws Exception {
        Semaphore s = new Semaphore(1);
        FlushTestReceiver receivers[] = new FlushTestReceiver[] { new FlushTestReceiver("c1", s, 0,
                        FlushTestReceiver.CONNECT_ONLY) };
        receivers[0].start();
        s.release(1);


        // Make sure everyone is in sync
        Channel[] tmp = new Channel[receivers.length];
        for (int i = 0; i < receivers.length; i++)
            tmp[i] = receivers[i].getChannel();
        Util.blockUntilViewsReceived(60000, 1000, tmp);


        // Reacquire the semaphore tickets; when we have them all
        // we know the threads are done
        s.tryAcquire(1, 60, TimeUnit.SECONDS);
        receivers[0].cleanup();
        Util.sleep(1000);


        checkEventStateTransferSequence(receivers[0]);
    }

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        int count = names.length;


        List<FlushTestReceiver> channels = new ArrayList<FlushTestReceiver>(count);
        try {
            // Create a semaphore and take all its permits
            Semaphore semaphore = new Semaphore(count);
            semaphore.acquire(count);


            // Create channels and their threads that will block on the
            // semaphore
            boolean first = true;
            for (String channelName : names) {
                FlushTestReceiver channel = null;
                if (first)
                    channel = new FlushTestReceiver(channelName, semaphore, 0, connectType);
                else {
                    channel = new FlushTestReceiver((JChannel) channels.get(0).getChannel(),
                                    channelName, semaphore, 0, connectType);
                }
                channels.add(channel);


                // Release one ticket at a time to allow the thread to start working
                channel.start();
                semaphore.release(1);
                if (first)
                    Util.sleep(3000); // minimize changes of a merge happening
                first = false;
            }


            Channel[] tmp = new Channel[channels.size()];
            int cnt = 0;
            for (FlushTestReceiver receiver : channels)
                tmp[cnt++] = receiver.getChannel();
            Util.blockUntilViewsReceived(30000, 1000, tmp);


            // Reacquire the semaphore tickets; when we have them all
            // we know the threads are done
            semaphore.tryAcquire(count, 40, TimeUnit.SECONDS);


            Util.sleep(1000); //let all events propagate...
            for (FlushTestReceiver app : channels)
                app.getChannel().setReceiver(null);
            for (FlushTestReceiver app : channels)

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    private final ExecutorService executor;
    private final Semaphore semaphore;


    public BoundedWorkQueueExecutor(ExecutorService exec, int bound) {
        this.executor = exec;
        this.semaphore = new Semaphore(bound);
    }

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    }


    public BoundedWorkQueueExecutor(int threadPoolSize, int workQueueSize, String threadName) {
        this.executor = ExecutorFactory.newFixedThreadPool(threadPoolSize, threadName);
        int bound = workQueueSize + threadPoolSize;
        this.semaphore = new Semaphore(bound);
    }

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    * </ol>
    */
   public void testReadUncommitted() throws Throwable
   {
      final LockStrategy s = new LockStrategyReadUncommitted();
      final Semaphore sem = new MyFIFOSemaphore(1);
      final CyclicBarrier barrier = new CyclicBarrier(2);


      Thread t1 = new Thread("t1")
      {
         Lock lock = null;


         public void run()
         {
            try
            {
               sem.acquire(); // we're first to the semaphore


               // log("waiting on barrier");
               barrier.await(); // wait until t2 joins us
               // log("passed barrier");
               lock = s.readLock();
               lock.tryLock(TIMEOUT, TimeUnit.MILLISECONDS);
               log("1st read: value is " + value);
               assertEquals(10, value);
               sem.release(); // give t2 time to make the modification
               TestingUtil.sleepThread(100);


               sem.acquire(); // to read the uncommitted modification by t2
               log("2nd read: value is " + value + "; we should see t2's uncommitted change (20)");
               assertEquals(20, value); // we're seeing the modification by t2 before t2 committed (a.k.a. released the lock)
               sem.release();
               TestingUtil.sleepThread(100);


               sem.acquire(); // to read the committed change by t2
               log("3rd read: value is still " + value + "; we should see t2's committed change");
               assertEquals(20, value);
            }
            catch (Throwable ex)
            {
               t1_ex = ex;
            }
            finally
            {
               if (lock != null)
                  lock.unlock();
               sem.release();
            }
         }
      };




      Thread t2 = new Thread("t2")
      {
         Lock lock = null;


         public void run()
         {
            try
            {
               TestingUtil.sleepThread(100);
               barrier.await();
               sem.acquire();
               lock = s.writeLock();
               lock.tryLock(TIMEOUT, TimeUnit.MILLISECONDS);
               log("changing value from " + value + " to 20");
               value = 20;
               sem.release(); // now t1 can read the uncommitted modification
               TestingUtil.sleepThread(100);


               sem.acquire(); // to unlock the lock
               log("committing the TX");
               lock.unlock();
            }
            catch (Throwable ex)
            {
               t2_ex = ex;
            }
            finally
            {
               if (lock != null)
                  lock.unlock();
               sem.release();
            }
         }
      };


      t1.start();

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    public void testStateTransferWhileSending() throws Exception {


        StateTransferApplication[] apps=new StateTransferApplication[APP_COUNT];
        try {
            Semaphore semaphore=new Semaphore(APP_COUNT);
            semaphore.acquire(APP_COUNT);


            int from=0, to=MSG_SEND_COUNT;
            for(int i=0;i < apps.length;i++) {
                if(i == 0)
                    apps[i]=new StateTransferApplication(semaphore, names[i], from, to);
                else
                    apps[i]=new StateTransferApplication((JChannel)apps[0].getChannel(), semaphore, names[i], from, to);
                from+=MSG_SEND_COUNT;
                to+=MSG_SEND_COUNT;
            }


            for(int i=0;i < apps.length;i++) {
                StateTransferApplication app=apps[i];
                app.start();
                semaphore.release();
                //avoid merge
                Util.sleep(3000);
            }


            // Make sure everyone is in sync
            Channel[] tmp=new Channel[apps.length];
            for(int i=0; i < apps.length; i++)
                tmp[i]=apps[i].getChannel();


            Util.blockUntilViewsReceived(60000, 1000, tmp);


            // Reacquire the semaphore tickets; when we have them all
            // we know the threads are done
            boolean acquired=semaphore.tryAcquire(apps.length, 30, TimeUnit.SECONDS);
            if(!acquired) {
                log.warn("Most likely a bug, analyse the stack below:");
                log.warn(Util.dumpThreads());
            }

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    protected final Map<RequestContext, Thread> _runningThreads = new ConcurrentIdentityHashMap<RequestContext, Thread>(12);
    protected final ExecutorService _executors = ExecutorFactory.newCachedThreadPool(60L, "ParallelQP");


    public QueryProcessor(ResponseListener handler) {
        super(handler);
        this._throttle = new Semaphore(ThrottedRemoteSequenceProxy.NUM_THROTTLE);
    }

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        this._throttle = throttle;
    }


    public ThrottedRemoteSequenceProxy(Sequence<Item> delegate) {
        super(delegate);
        this._throttle = new Semaphore(NUM_THROTTLE);
    }

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        this._throttle = new Semaphore(NUM_THROTTLE);
    }


    @Override
    public void run() {
        final Semaphore throttle = _throttle;
        throttle.acquireUninterruptibly();
        try {
            compute();
        } finally {
            throttle.release();
        }
    }

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