Still a work in progress.

I'm considering removing the async listener and use the packet listener
for both sync and async processing.

ProtocolLib/src/com/comphenix/protocol/PacketStream.java

@@ -0,0 +1,55 @@
+package com.comphenix.protocol;
+
+import java.lang.reflect.InvocationTargetException;
+
+import org.bukkit.entity.Player;
+
+import com.comphenix.protocol.events.PacketContainer;
+
+/**
+ * Represents a object capable of sending or receiving packets.
+ * 
+ * @author Kristian
+ */
+public interface PacketStream {
+	
+	/**
+	 * Send a packet to the given player.
+	 * @param reciever - the reciever.
+	 * @param packet - packet to send.
+	 * @throws InvocationTargetException - if an error occured when sending the packet.
+	 */
+	public void sendServerPacket(Player reciever, PacketContainer packet) 
+			throws InvocationTargetException;
+
+	/**
+	 * Send a packet to the given player.
+	 * @param reciever - the reciever.
+	 * @param packet - packet to send.
+	 * @param filters - whether or not to invoke any packet filters.
+	 * @throws InvocationTargetException - if an error occured when sending the packet.
+	 */
+	public void sendServerPacket(Player reciever, PacketContainer packet, boolean filters)
+			throws InvocationTargetException;
+
+	/**
+	 * Simulate recieving a certain packet from a given player.
+	 * @param sender - the sender.
+	 * @param packet - the packet that was sent.
+	 * @throws InvocationTargetException If the reflection machinery failed.
+	 * @throws IllegalAccessException If the underlying method caused an error.
+	 */
+	public void recieveClientPacket(Player sender, PacketContainer packet) 
+			throws IllegalAccessException, InvocationTargetException;
+
+	/**
+	 * Simulate recieving a certain packet from a given player.
+	 * @param sender - the sender.
+	 * @param packet - the packet that was sent.
+	 * @param filters - whether or not to invoke any packet filters.
+	 * @throws InvocationTargetException If the reflection machinery failed.
+	 * @throws IllegalAccessException If the underlying method caused an error.
+	 */
+	public void recieveClientPacket(Player sender, PacketContainer packet, boolean filters)
+			throws IllegalAccessException, InvocationTargetException;
+}

ProtocolLib/src/com/comphenix/protocol/ProtocolManager.java

@@ -17,7 +17,6 @@
 
 package com.comphenix.protocol;
 
-import java.lang.reflect.InvocationTargetException;
 import java.util.List;
 import java.util.Set;
 
@@ -35,7 +34,7 @@ import com.google.common.collect.ImmutableSet;
  * Represents an API for accessing the Minecraft protocol.
  * @author Kristian
  */
-public interface ProtocolManager {
+public interface ProtocolManager extends PacketStream {
 
 	/**
 	 * Retrieves a list of every registered packet listener.
@@ -67,46 +66,6 @@ public interface ProtocolManager {
 	 */
 	public void removePacketListeners(Plugin plugin);
 
-	/**
-	 * Send a packet to the given player.
-	 * @param reciever - the reciever.
-	 * @param packet - packet to send.
-	 * @throws InvocationTargetException - if an error occured when sending the packet.
-	 */
-	public void sendServerPacket(Player reciever, PacketContainer packet) 
-			throws InvocationTargetException;
-
-	/**
-	 * Send a packet to the given player.
-	 * @param reciever - the reciever.
-	 * @param packet - packet to send.
-	 * @param filters - whether or not to invoke any packet filters.
-	 * @throws InvocationTargetException - if an error occured when sending the packet.
-	 */
-	public void sendServerPacket(Player reciever, PacketContainer packet, boolean filters)
-			throws InvocationTargetException;
-
-	/**
-	 * Simulate recieving a certain packet from a given player.
-	 * @param sender - the sender.
-	 * @param packet - the packet that was sent.
-	 * @throws InvocationTargetException If the reflection machinery failed.
-	 * @throws IllegalAccessException If the underlying method caused an error.
-	 */
-	public void recieveClientPacket(Player sender, PacketContainer packet) 
-			throws IllegalAccessException, InvocationTargetException;
-
-	/**
-	 * Simulate recieving a certain packet from a given player.
-	 * @param sender - the sender.
-	 * @param packet - the packet that was sent.
-	 * @param filters - whether or not to invoke any packet filters.
-	 * @throws InvocationTargetException If the reflection machinery failed.
-	 * @throws IllegalAccessException If the underlying method caused an error.
-	 */
-	public void recieveClientPacket(Player sender, PacketContainer packet, boolean filters)
-			throws IllegalAccessException, InvocationTargetException;
-
 	/**
 	 * Constructs a new encapsulated Minecraft packet with the given ID.
 	 * @param id - packet ID.

ProtocolLib/src/com/comphenix/protocol/async/AsyncFilterManager.java

@@ -1,6 +1,12 @@
 package com.comphenix.protocol.async;
 
-import java.util.concurrent.Future;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.logging.Logger;
+
+import org.bukkit.plugin.Plugin;
+
+import com.comphenix.protocol.PacketStream;
+import com.comphenix.protocol.events.PacketEvent;
 
 /**
  * Represents a filter manager for asynchronous packets.
@@ -9,9 +15,77 @@ import java.util.concurrent.Future;
  */
 public class AsyncFilterManager {
 
+	private PacketProcessingQueue processingQueue;
+	private PacketSendingQueue sendingQueue;
 	
-	public Future<Void> registerAsyncHandler() {
+	private PacketStream packetStream;
+	private Logger logger;
 	
+	// The likely main thread
+	private Thread mainThread;
+	
+	// Current packet index
+	private AtomicInteger currentSendingIndex = new AtomicInteger();
+	
+	public AsyncFilterManager(Logger logger, PacketStream packetStream) {
+		this.sendingQueue = new PacketSendingQueue();
+		this.processingQueue = new PacketProcessingQueue(sendingQueue);
+		this.packetStream = packetStream;
+		
+		this.logger = logger;
+		this.mainThread = Thread.currentThread();
 	}
 	
+	public ListenerToken registerAsyncHandler(Plugin plugin, AsyncListener listener) {
+		ListenerToken token = new ListenerToken(plugin, mainThread, this, listener);
+		
+		processingQueue.addListener(token, listener.getSendingWhitelist());
+		return token;
+	}
+	
+	public void unregisterAsyncHandler(ListenerToken listenerToken) {
+		if (listenerToken == null)
+			throw new IllegalArgumentException("listenerToken cannot be NULL");
+		
+		listenerToken.cancel();	
+	}
+	
+	// Called by ListenerToken
+	void unregisterAsyncHandlerInternal(ListenerToken listenerToken) {
+		// Just remove it from the queue
+		processingQueue.removeListener(listenerToken, listenerToken.getAsyncListener().getSendingWhitelist());
+	}
+	
+	public void enqueueSyncPacket(PacketEvent syncPacket, int sendingDelta, long timeoutDelta) {
+		AsyncPacket asyncPacket = new AsyncPacket(packetStream, syncPacket, 
+				currentSendingIndex.getAndIncrement() + sendingDelta,
+				System.currentTimeMillis(),
+				timeoutDelta);
+		
+		// Start the process
+		sendingQueue.enqueue(asyncPacket);
+		processingQueue.enqueuePacket(asyncPacket);
+	}
+	
+	public PacketStream getPacketStream() {
+		return packetStream;
+	}
+
+	public Logger getLogger() {
+		return logger;
+	}
+
+	PacketProcessingQueue getProcessingQueue() {
+		return processingQueue;
+	}
+
+	PacketSendingQueue getSendingQueue() {
+		return sendingQueue;
+	}
+
+	public void cleanupAll() {
+		// Remove all listeners
+		
+		// We don't necessarily remove packets, as this might be a part of a server reload
+	}
 }

ProtocolLib/src/com/comphenix/protocol/async/AsyncListener.java

@@ -1,5 +1,17 @@
 package com.comphenix.protocol.async;
 
+import org.bukkit.plugin.Plugin;
+
+import com.comphenix.protocol.events.ListeningWhitelist;
+
 public interface AsyncListener {
 	public void onAsyncPacket(AsyncPacket packet);
+	
+	public ListeningWhitelist getSendingWhitelist();
+	
+	/**
+	 * Retrieve the plugin that created this async packet listener.
+	 * @return The plugin, or NULL if not available.
+	 */
+	public Plugin getPlugin();
 }

ProtocolLib/src/com/comphenix/protocol/async/AsyncPacket.java

@@ -1,8 +1,12 @@
 package com.comphenix.protocol.async;
 
 import java.io.Serializable;
+import java.lang.reflect.InvocationTargetException;
+import java.util.Iterator;
 
+import com.comphenix.protocol.PacketStream;
 import com.comphenix.protocol.events.PacketEvent;
+import com.comphenix.protocol.injector.PrioritizedListener;
 import com.google.common.primitives.Longs;
 
 /**
@@ -12,6 +16,11 @@ import com.google.common.primitives.Longs;
  */
 public class AsyncPacket implements Serializable, Comparable<AsyncPacket> {
 	
+	/**
+	 * Signal an end to the packet processing.
+	 */
+	static final AsyncPacket INTERUPT_PACKET = new AsyncPacket();
+	
 	/**
 	 * Generated by Eclipse.
 	 */
@@ -20,13 +29,23 @@ public class AsyncPacket implements Serializable, Comparable<AsyncPacket> {
 	/**
 	 * Default number of milliseconds until a packet will rejected.
 	 */
-	public static final int DEFAULT_TIMEOUT_DETLA = 60000;
+	public static final int DEFAULT_TIMEOUT_DELTA = 60000;
 	
 	/**
 	 * The original synchronized packet.
 	 */
 	private PacketEvent packetEvent;
 	
+	/**
+	 * The packet stream responsible for transmitting the packet when it's done processing.
+	 */
+	private transient PacketStream packetStream;
+	
+	/**
+	 * Current list of async packet listeners.
+	 */
+	private transient Iterator<PrioritizedListener<ListenerToken>> listenerTraversal;
+	
 	// Timeout handling
 	private long initialTime;
 	private long timeout;
@@ -35,18 +54,39 @@ public class AsyncPacket implements Serializable, Comparable<AsyncPacket> {
 	private long originalSendingIndex;
 	private long newSendingIndex;
 	
+	// Whether or not the packet has been processed by the listeners
+	private volatile boolean processed;
+	
+	private AsyncPacket() {
+		// Used by the poision pill pattern
+	}
+	
+	/**
+	 * Determine whether or not this is a signal for the async listener to interrupt processing.
+	 * @return Interrupt packet processing.
+	 */
+	boolean isInteruptPacket() {
+		// This is only possble if we're dealing with the poision pill packet
+		return packetEvent == null || packetStream == null;
+	}
 	
 	/**
 	 * Create a container for asyncronous packets.
 	 * @param packetEvent - the synchronous packet event.
 	 * @param initialTime - the current time in milliseconds since 01.01.1970 00:00.
 	 */
-	public AsyncPacket(PacketEvent packetEvent, long sendingIndex, long initialTime) {
+	public AsyncPacket(PacketStream packetStream, PacketEvent packetEvent, long sendingIndex, long initialTime, long timeoutDelta) {
+		if (packetEvent == null)
+			throw new IllegalArgumentException("packetEvent cannot be NULL");
+		if (packetStream == null)
+			throw new IllegalArgumentException("packetStream cannot be NULL");
+		
+		this.packetStream = packetStream;
 		this.packetEvent = packetEvent;
 		
 		// Timeout
 		this.initialTime = initialTime;
-		this.timeout = initialTime + DEFAULT_TIMEOUT_DETLA;
+		this.timeout = initialTime + timeoutDelta;
 		
 		// Sending index
 		this.originalSendingIndex = sendingIndex;
@@ -69,14 +109,6 @@ public class AsyncPacket implements Serializable, Comparable<AsyncPacket> {
 		return timeout;
 	}
 	
-	/**
-	 * Sets the time the packet will be forcefully rejected.
-	 * @param timeout - the time to reject the packet, in milliseconds since 01.01.1970 00:00.
-	 */
-	public void setTimeout(long timeout) {
-		this.timeout = timeout;
-	}
-
 	/**
 	 * Retrieve the order the packet was originally transmitted.
 	 * @return The original packet index.
@@ -113,6 +145,74 @@ public class AsyncPacket implements Serializable, Comparable<AsyncPacket> {
 		return packetEvent;
 	}
 	
+	/**
+	 * Retrieve the packet ID of the underlying packet.
+	 * @return Packet ID.
+	 */
+	public int getPacketID() {
+		return packetEvent.getPacketID();
+	}
+	
+	/**
+	 * Retrieve the packet stream responsible for transmitting this packet.
+	 * @return The packet stream.
+	 */
+	public PacketStream getPacketStream() {
+		return packetStream;
+	}
+
+	/**
+	 * Sets the output packet stream responsible for transmitting this packet.
+	 * @param packetStream - new output packet stream.
+	 */
+	public void setPacketStream(PacketStream packetStream) {
+		this.packetStream = packetStream;
+	}
+
+	/**
+	 * Retrieve whether or not this packet has been processed by the async listeners.
+	 * @return TRUE if it has been processed, FALSE otherwise.
+	 */
+	boolean isProcessed() {
+		return processed;
+	}
+
+	/**
+	 * Sets whether or not this packet has been processed by the async listeners.
+	 * @param processed - TRUE if it has, FALSE otherwise.
+	 */
+	void setProcessed(boolean processed) {
+		this.processed = processed;
+	}
+
+	/**
+	 * Retrieve iterator for the next listener in line.
+	 * @return Next async packet listener iterator.
+	 */
+	Iterator<PrioritizedListener<ListenerToken>> getListenerTraversal() {
+		return listenerTraversal;
+	}
+
+	/**
+	 * We're done processing. Send the packet.
+	 */
+	void sendPacket() {
+		try {
+			// We only support server packets at this stage
+			packetStream.sendServerPacket(packetEvent.getPlayer(), packetEvent.getPacket(), false);
+		} catch (InvocationTargetException e) {
+			e.printStackTrace();
+		}
+	}
+	
+	/**
+	 * Set the iterator for the next listener.
+	 * @param listenerTraversal - the new async packet listener iterator.
+	 */
+	void setListenerTraversal(Iterator<PrioritizedListener<ListenerToken>> listenerTraversal) {
+		this.listenerTraversal = listenerTraversal;
+	}
+
 	@Override
 	public int compareTo(AsyncPacket o) {
 		if (o == null)

ProtocolLib/src/com/comphenix/protocol/async/ListenerToken.java

@@ -1,38 +1,139 @@
 package com.comphenix.protocol.async;
 
 import java.util.concurrent.ArrayBlockingQueue;
+import java.util.logging.Level;
+
+import org.bukkit.plugin.Plugin;
 
 public class ListenerToken {
 
+	// Default queue capacity
+	private static int DEFAULT_CAPACITY = 1024;
+	
 	// Cancel the async handler
 	private volatile boolean cancelled;
 	
+	// The packet listener
+	private AsyncListener listener;
+	
+	// The original plugin
+	private Plugin plugin;
+	
+	// The filter manager
+	private AsyncFilterManager filterManager;
+	
+	// List of queued packets
+	private ArrayBlockingQueue<AsyncPacket> queuedPackets = new ArrayBlockingQueue<AsyncPacket>(DEFAULT_CAPACITY);
+
+	// Minecraft main thread
+	private Thread mainThread;
+	
+	public ListenerToken(Plugin plugin, Thread mainThread, AsyncFilterManager filterManager, AsyncListener listener) {
+		if (filterManager == null)
+			throw new IllegalArgumentException("filterManager cannot be NULL");
+		if (listener == null)
+			throw new IllegalArgumentException("listener cannot be NULL");
+		
+		this.plugin = plugin;
+		this.mainThread = mainThread;
+		this.filterManager = filterManager;
+		this.listener = listener;
+	}
+	
 	public boolean isCancelled() {
 		return cancelled;
 	}
 
+	public AsyncListener getAsyncListener() {
+		return listener;
+	}
+
 	/**
 	 * Cancel the handler.
 	 */
 	public void cancel() {
-		cancelled = true;
+		// Remove the listener as quickly as possible
+		close();
+		
+		// Poison Pill Shutdown
+		queuedPackets.clear();
+		queuedPackets.add(AsyncPacket.INTERUPT_PACKET);
 	}
 	
+	/**
+	 * Queue a packet for processing.
+	 * @param packet - a packet for processing.
+	 * @throws IllegalStateException If the underlying packet queue is full.
+	 */
+	public void enqueuePacket(AsyncPacket packet) {
+		if (packet == null)
+			throw new IllegalArgumentException("packet is NULL");
 		
-	public void beginListener(AsyncListener asyncListener) {
+		queuedPackets.add(packet);
+	}
+	
+	/**
+	 * Entry point for the background thread that will be processing the packet asynchronously.
+	 * <p>
+	 * <b>WARNING:</b>
+	 * Never call this method from the main thread. Doing so will block Minecraft.
+	 */
+	public void listenerLoop() {
+		// Danger, danger!
+		if (Thread.currentThread().getId() == mainThread.getId()) 
+			throw new IllegalStateException("Do not call this method from the main thread.");
 		
 		try {
-			AsyncPacket packet = processingQueue.take();
+			mainLoop:
+			while (!cancelled) {
+				AsyncPacket packet = queuedPackets.take();
 				
-			// Now, 
-			asyncListener.onAsyncPacket(packet);
+				// Handle cancel requests
+				if (packet == null || packet.isInteruptPacket()) {
+					break;
+				}
 				
+				// Here's the core of the asynchronous processing
+				try {
+					listener.onAsyncPacket(packet);
+				} catch (Throwable e) {
+					// Minecraft doesn't want your Exception.
+					filterManager.getLogger().log(Level.SEVERE, 
+							"Unhandled exception occured in onAsyncPacket() for " + getPluginName(), e);
+				}
+				
+				// Now, get the next non-cancelled listener
+				for (; packet.getListenerTraversal().hasNext(); ) {
+					ListenerToken token = packet.getListenerTraversal().next().getListener();
+					
+					if (!token.isCancelled()) {
+						token.enqueuePacket(packet);
+						continue mainLoop;
+					}
+				}
+				
+				// There are no more listeners - queue the packet for transmission
+				filterManager.getSendingQueue().signalPacketUpdate(packet);
+				filterManager.getProcessingQueue().signalProcessingDone();
+			}
 			
 		} catch (InterruptedException e) {
-			
+			// We're done
 		}
 		
+		// Clean up
+		close();
+	}
 	
+	private void close() {
+		// Remove the listener itself
+		if (!cancelled) {
+			filterManager.unregisterAsyncHandlerInternal(this);
+			cancelled = true;
+		}
+	}
 	
+	private String getPluginName() {
+		return plugin != null ? plugin.getName() : "UNKNOWN";
 	}
 }

ProtocolLib/src/com/comphenix/protocol/async/PacketProcessingQueue.java

@@ -1,17 +1,19 @@
 package com.comphenix.protocol.async;
 
+import java.util.Collection;
+import java.util.Iterator;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.Semaphore;
 
-import com.comphenix.protocol.concurrency.SortedCopyOnWriteArray;
+import com.comphenix.protocol.concurrency.AbstractConcurrentListenerMultimap;
+import com.comphenix.protocol.injector.PrioritizedListener;
 
 /**
- * Handles the processing of a certain packet type.
+ * Handles the processing of every packet type.
  * 
  * @author Kristian
  */
-class PacketProcessingQueue {
-
+class PacketProcessingQueue extends AbstractConcurrentListenerMultimap<ListenerToken> {
 
 	/**
 	 * Default maximum number of packets to process concurrently.
@@ -32,43 +34,86 @@ class PacketProcessingQueue {
 	// Queued packets for being processed
 	private ArrayBlockingQueue<AsyncPacket> processingQueue;
 	
-	// Packet listeners
-	private SortedCopyOnWriteArray<>
+	// Packets for sending
+	private PacketSendingQueue sendingQueue;
 
-	public PacketProcessingQueue() {
-		this(DEFAULT_QUEUE_LIMIT, DEFAULT_MAXIMUM_CONCURRENCY);
+	public PacketProcessingQueue(PacketSendingQueue sendingQueue) {
+		this(sendingQueue, DEFAULT_QUEUE_LIMIT, DEFAULT_MAXIMUM_CONCURRENCY);
 	}
 	
-	public PacketProcessingQueue(int queueLimit, int maximumConcurrency) {
+	public PacketProcessingQueue(PacketSendingQueue sendingQueue, int queueLimit, int maximumConcurrency) {
+		super();
 		this.processingQueue = new ArrayBlockingQueue<AsyncPacket>(queueLimit);
 		this.maximumConcurrency = maximumConcurrency;
 		this.concurrentProcessing = new Semaphore(maximumConcurrency);
+		this.sendingQueue = sendingQueue;
 	}
 	
-	public boolean queuePacket(AsyncPacket packet) {
+	/**
+	 * Enqueue a packet for processing by the asynchronous listeners.
+	 * @param packet - packet to process.
+	 * @return TRUE if we sucessfully queued the packet, FALSE if the queue ran out if space.
+	 */
+	public boolean enqueuePacket(AsyncPacket packet) {
 		try {
 			processingQueue.add(packet);
 			
 			// Begin processing packets
-			processPacket();
+			signalBeginProcessing();
 			return true;
 		} catch (IllegalStateException e) {
 			return false;
 		}
 	}
 	
-	public void processPacket() {
-		if (concurrentProcessing.tryAcquire()) {
+	/**
+	 * Called by the current method and each thread to signal that a packet might be ready for processing.
+	 */
+	public void signalBeginProcessing() {
+		while (concurrentProcessing.tryAcquire()) {
 			AsyncPacket packet = processingQueue.poll();
 			
 			// Any packet queued?
 			if (packet != null) {
+				Collection<PrioritizedListener<ListenerToken>> list = getListener(packet.getPacketID());
 				
+				if (list != null) {
+					Iterator<PrioritizedListener<ListenerToken>> iterator = list.iterator();
+					
+					if (iterator.hasNext()) {
+						packet.setListenerTraversal(iterator);
+						iterator.next().getListener().enqueuePacket(packet);
+						continue;
+					}
+				}
+				
+				// The packet has no listeners. Just send it.
+				sendingQueue.signalPacketUpdate(packet);
+				signalProcessingDone();
+				
+			} else {
+				// No more queued packets. 
+				return;
 			}
  		}
 	}
 	
+	/**
+	 * Called when a packet has been processed.
+	 */
+	public void signalProcessingDone() {
+		concurrentProcessing.release();
+	}
+
+	/**
+	 * Retrieve the maximum number of packets to process at any given time.
+	 * @return Number of simultaneous packet to process.
+	 */
 	public int getMaximumConcurrency() {
 		return maximumConcurrency;
 	}
+	
+	public void removeListeners() {
+		for (PrioritizedListener<ListenerToken> token : )
+	}
 }

ProtocolLib/src/com/comphenix/protocol/async/PacketSendingQueue.java

@@ -0,0 +1,41 @@
+package com.comphenix.protocol.async;
+
+import java.util.concurrent.PriorityBlockingQueue;
+
+/**
+ * Represents packets ready to be transmitted to a client.
+ * @author Kristian
+ */
+class PacketSendingQueue {
+
+	private PriorityBlockingQueue<AsyncPacket> sendingQueue;
+	
+	/**
+	 * Enqueue a packet for sending. 
+	 * @param packet
+	 */
+	public void enqueue(AsyncPacket packet) {
+		sendingQueue.add(packet);
+	}
+	
+	/**
+	 * Invoked when one of the packets have finished processing.
+	 */
+	public synchronized void signalPacketUpdate(AsyncPacket packetUpdated) {
+		
+		// Mark this packet as finished
+		packetUpdated.setProcessed(true);
+		
+		// Transmit as many packets as we can
+		while (true) {
+			AsyncPacket current = sendingQueue.peek();
+			
+			if (current != null && current.isProcessed()) {
+				current.sendPacket();
+				sendingQueue.poll();
+			} else {
+				break;
+			}
+		}
+	}
+}

ProtocolLib/src/com/comphenix/protocol/concurrency/AbstractConcurrentListenerMultimap.java

@@ -1,6 +1,7 @@
 package com.comphenix.protocol.concurrency;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
@@ -16,8 +17,8 @@ import com.comphenix.protocol.injector.PrioritizedListener;
 public abstract class AbstractConcurrentListenerMultimap<TListener> {
 
 	// The core of our map
-	protected ConcurrentMap<Integer, SortedCopyOnWriteArray<PrioritizedListener<TListener>>> listeners = 
-		  new ConcurrentHashMap<Integer, SortedCopyOnWriteArray<PrioritizedListener<TListener>>>();
+	private ConcurrentMap<Integer, SortedCopyOnWriteArray<PrioritizedListener<TListener>>> listeners = 
+		new ConcurrentHashMap<Integer, SortedCopyOnWriteArray<PrioritizedListener<TListener>>>();
 	
 	/**
 	 * Adds a listener to its requested list of packet recievers.
@@ -91,4 +92,15 @@ public abstract class AbstractConcurrentListenerMultimap<TListener> {
 		
 		return removedPackets;
 	}
+	
+	/**
+	 * Retrieve the registered listeners, in order from the lowest to the highest priority.
+	 * <p>
+	 * The returned list is thread-safe and doesn't require synchronization.
+	 * @param packetID - packet ID.
+	 * @return Registered listeners.
+	 */
+	public Collection<PrioritizedListener<TListener>> getListener(int packetID) {
+		return listeners.get(packetID);
+	}
 }

ProtocolLib/src/com/comphenix/protocol/injector/PacketFilterManager.java

@@ -46,6 +46,7 @@ import org.bukkit.plugin.Plugin;
 import org.bukkit.plugin.PluginManager;
 
 import com.comphenix.protocol.ProtocolManager;
+import com.comphenix.protocol.async.AsyncFilterManager;
 import com.comphenix.protocol.events.*;
 import com.comphenix.protocol.reflect.FieldAccessException;
 import com.comphenix.protocol.reflect.FuzzyReflection;
@@ -112,6 +113,9 @@ public final class PacketFilterManager implements ProtocolManager {
 	// Error logger
 	private Logger logger;
 	
+	// The async packet handler
+	private AsyncFilterManager asyncFilterManager;
+	
 	/**
 	 * Only create instances of this class if protocol lib is disabled.
 	 */
@@ -126,11 +130,20 @@ public final class PacketFilterManager implements ProtocolManager {
 			this.classLoader = classLoader;
 			this.logger = logger;
 			this.packetInjector = new PacketInjector(classLoader, this, connectionLookup);
+			this.asyncFilterManager = new AsyncFilterManager(logger, this);
 		} catch (IllegalAccessException e) {
 			logger.log(Level.SEVERE, "Unable to initialize packet injector.", e);
 		}
 	}
 	
+	/**
+	 * Retrieve the current async packet filter manager.
+	 * @return Async filter manager.
+	 */
+	public AsyncFilterManager getAsyncFilterManager() {
+		return asyncFilterManager;
+	}
+	
 	/**
 	 * Retrieves how the server packets are read.
 	 * @return Injection method for reading server packets.
@@ -656,6 +669,9 @@ public final class PacketFilterManager implements ProtocolManager {
 			injection.cleanupAll();
 		}
 		
+		// Clean up async handlers
+		asyncFilterManager.cleanupAll();
+		
 		// Remove packet handlers
 		if (packetInjector != null)
 			packetInjector.cleanupAll();

ProtocolLib/src/com/comphenix/protocol/injector/SortedPacketListenerList.java

@@ -1,16 +1,16 @@
 package com.comphenix.protocol.injector;
 
+import java.util.Collection;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
 import com.comphenix.protocol.concurrency.AbstractConcurrentListenerMultimap;
-import com.comphenix.protocol.concurrency.SortedCopyOnWriteArray;
 import com.comphenix.protocol.events.PacketAdapter;
 import com.comphenix.protocol.events.PacketEvent;
 import com.comphenix.protocol.events.PacketListener;
 
 /**
- * A thread-safe implementation of a listener multimap.
+ * Registry of synchronous packet listeners.
  * 
  * @author Kristian
  */
@@ -22,22 +22,20 @@ class SortedPacketListenerList extends AbstractConcurrentListenerMultimap<Packet
 	 * @param event - the packet event to invoke.
 	 */
 	public void invokePacketRecieving(Logger logger, PacketEvent event) {
-		SortedCopyOnWriteArray<PrioritizedListener<PacketListener>> list = listeners.get(event.getPacketID());
+		Collection<PrioritizedListener<PacketListener>> list = getListener(event.getPacketID());
 		
 		if (list == null)
 			return;
 
-		// We have to be careful. Cannot modify the underlying list when sending notifications.
-		synchronized (list) {
-			for (PrioritizedListener<PacketListener> element : list) {
-				try {
-					element.getListener().onPacketReceiving(event);
-				} catch (Throwable e) {
-					// Minecraft doesn't want your Exception.
-					logger.log(Level.SEVERE, 
-							"Exception occured in onPacketReceiving() for " + 
-								PacketAdapter.getPluginName(element.getListener()), e);
-				}
+		// The returned list is thread-safe
+		for (PrioritizedListener<PacketListener> element : list) {
+			try {
+				element.getListener().onPacketReceiving(event);
+			} catch (Throwable e) {
+				// Minecraft doesn't want your Exception.
+				logger.log(Level.SEVERE, 
+						"Exception occured in onPacketReceiving() for " + 
+							PacketAdapter.getPluginName(element.getListener()), e);
 			}
 		}
 	}
@@ -48,21 +46,19 @@ class SortedPacketListenerList extends AbstractConcurrentListenerMultimap<Packet
 	 * @param event - the packet event to invoke.
 	 */
 	public void invokePacketSending(Logger logger, PacketEvent event) {
-		SortedCopyOnWriteArray<PrioritizedListener<PacketListener>> list = listeners.get(event.getPacketID());
+		Collection<PrioritizedListener<PacketListener>> list = getListener(event.getPacketID());
 		
 		if (list == null)
 			return;
 		
-		synchronized (list) {
-			for (PrioritizedListener<PacketListener> element : list) {
-				try {
-					element.getListener().onPacketSending(event);
-				} catch (Throwable e) {
-					// Minecraft doesn't want your Exception.
-					logger.log(Level.SEVERE, 
-							"Exception occured in onPacketReceiving() for " + 
-								PacketAdapter.getPluginName(element.getListener()), e);
-				}
+		for (PrioritizedListener<PacketListener> element : list) {
+			try {
+				element.getListener().onPacketSending(event);
+			} catch (Throwable e) {
+				// Minecraft doesn't want your Exception.
+				logger.log(Level.SEVERE, 
+						"Exception occured in onPacketReceiving() for " + 
+							PacketAdapter.getPluginName(element.getListener()), e);
 			}
 		}
 	}