聊聊artemis的ClientProducerCredits

public interface ClientProducerCredits {
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   void acquireCredits(int credits) throws ActiveMQException;
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   void receiveCredits(int credits);
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   void receiveFailCredits(int credits);
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   boolean isBlocked();
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   void init(SessionContext sessionContext);
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   void reset();
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   void close();
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   void incrementRefCount();
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   int decrementRefCount();
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   void releaseOutstanding();
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   SimpleString getAddress();
}

public abstract class AbstractProducerCreditsImpl implements ClientProducerCredits {
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   protected int pendingCredits;
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   private final int windowSize;
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   protected volatile boolean closed;
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   protected boolean blocked;
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   protected final SimpleString address;
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   private final ClientSessionInternal session;
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   protected int arriving;
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   private int refCount;
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   protected boolean serverRespondedWithFail;
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   protected SessionContext sessionContext;
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   public AbstractProducerCreditsImpl(final ClientSessionInternal session,
                                      final SimpleString address,
                                      final int windowSize) {
      this.session = session;
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      this.address = address;
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      this.windowSize = windowSize / 2;
   }
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   @Override
   public SimpleString getAddress() {
      return address;
   }
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   @Override
   public void init(SessionContext sessionContext) {
      // We initial request twice as many credits as we request in subsequent requests
      // This allows the producer to keep sending as more arrive, minimising pauses
      checkCredits(windowSize);
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      this.sessionContext = sessionContext;
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      this.sessionContext.linkFlowControl(address, this);
   }
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   @Override
   public void acquireCredits(final int credits) throws ActiveMQException {
      checkCredits(credits);
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      actualAcquire(credits);
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      afterAcquired(credits);
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   }
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   protected void afterAcquired(int credits) throws ActiveMQAddressFullException {
      // check to see if the blocking mode is FAIL on the server
      synchronized (this) {
         pendingCredits -= credits;
      }
   }
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   protected abstract void actualAcquire(int credits);
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   @Override
   public boolean isBlocked() {
      return blocked;
   }
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   @Override
   public void receiveFailCredits(final int credits) {
      serverRespondedWithFail = true;
      // receive credits like normal to keep the sender from blocking
      receiveCredits(credits);
   }
​
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   @Override
   public void receiveCredits(final int credits) {
      synchronized (this) {
         arriving -= credits;
      }
   }
​
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   @Override
   public synchronized void reset() {
      // Any pendingCredits credits from before failover won't arrive, so we re-initialise
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      int beforeFailure = pendingCredits;
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      pendingCredits = 0;
      arriving = 0;
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      // If we are waiting for more credits than what's configured, then we need to use what we tried before
      // otherwise the client may starve as the credit will never arrive
      checkCredits(Math.max(windowSize * 2, beforeFailure));
   }
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   @Override
   public void close() {
      // Closing a producer that is blocking should make it return
      closed = true;
   }
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   @Override
   public synchronized void incrementRefCount() {
      refCount++;
   }
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   @Override
   public synchronized int decrementRefCount() {
      return --refCount;
   }
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   public abstract int getBalance();
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   protected void checkCredits(final int credits) {
      int needed = Math.max(credits, windowSize);
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      int toRequest = -1;
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      synchronized (this) {
         if (getBalance() + arriving < needed) {
            toRequest = needed - arriving;
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            pendingCredits += toRequest;
            arriving += toRequest;
         }
      }
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      if (toRequest != -1) {
         requestCredits(toRequest);
      }
   }
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   private void requestCredits(final int credits) {
      session.sendProducerCreditsMessage(credits, address);
   }
}

public class ClientProducerCreditsImpl extends AbstractProducerCreditsImpl {
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   private final Semaphore semaphore;
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   public ClientProducerCreditsImpl(ClientSessionInternal session, SimpleString address, int windowSize) {
      super(session, address, windowSize);
​
​
      // Doesn't need to be fair since session is single threaded
      semaphore = new Semaphore(0, false);
​
   }
​
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   @Override
   protected void afterAcquired(int credits) throws ActiveMQAddressFullException {
      // check to see if the blocking mode is FAIL on the server
      synchronized (this) {
         super.afterAcquired(credits);
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         if (serverRespondedWithFail) {
            serverRespondedWithFail = false;
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            // remove existing credits to force the client to ask the server for more on the next send
            semaphore.drainPermits();
            pendingCredits = 0;
            arriving = 0;
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            throw ActiveMQClientMessageBundle.BUNDLE.addressIsFull(address.toString(), credits);
         }
      }
   }
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   @Override
   protected void actualAcquire(int credits) {
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      boolean tryAcquire;
      synchronized (this) {
         tryAcquire = semaphore.tryAcquire(credits);
      }
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      if (!tryAcquire && !closed) {
         this.blocked = true;
         try {
            while (!semaphore.tryAcquire(credits, 10, TimeUnit.SECONDS)) {
               // I'm using string concatenation here in case address is null
               // better getting a "null" string than a NPE
               ActiveMQClientLogger.LOGGER.outOfCreditOnFlowControl("" + address);
            }
         } catch (InterruptedException interrupted) {
            Thread.currentThread().interrupt();
            throw new ActiveMQInterruptedException(interrupted);
         } finally {
            this.blocked = false;
         }
      }
   }
​
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   @Override
   public synchronized void reset() {
      // Any pendingCredits credits from before failover won't arrive, so we re-initialise
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      semaphore.drainPermits();
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      super.reset();
   }
​
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   @Override
   public void close() {
      super.close();
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      // Closing a producer that is blocking should make it return
      semaphore.release(Integer.MAX_VALUE / 2);
   }
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   @Override
   public void receiveCredits(final int credits) {
      synchronized (this) {
         super.receiveCredits(credits);
      }
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      semaphore.release(credits);
   }
​
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   @Override
   public synchronized void releaseOutstanding() {
      semaphore.drainPermits();
   }
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   @Override
   public int getBalance() {
      return semaphore.availablePermits();
   }
​
​
}