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I have written a publisher-subscriber system for a multi-threaded project I am working on. It is my understanding that when using SynchronizedList structural operations are supposed to be thread-safe, but iteration requires that the list be wrapped in a synchronized block.

For these reasons, I can't figure out why I'm getting a comod exception here. It appears that I have taken all the necessary precautions to prevent this: all iterations are in synchronized blocks, and the list itself is private so it can't be accessed from outside the class.

Relevant code is below:
 * Intermediary which receives messages and distributes them to subscribers
 * that have expressed interest in those messages.
 * @author Kevin J. Burns
public class Publisher {
	private List<subscription> subscriptions = 
			Collections.synchronizedList(new ArrayList<>());
	private static final Publisher instance = new Publisher();
	private Publisher() {
		// Does nothing special except prevent instantiation.
	 * Registers a subscription to a subset of messages.
	 * @param sub An object that can receive a message when sent
	 * @param type RTTI info for the message type of interest.
	 * @param filter A filter to use to determine whether a future message
	 * matches this subscription 
	 * @return a reference to the subscription, in case subscribers wish to
	 * keep track of their subscriptions
	public Subscription subscribe(Subscriber sub, 
			Class type, Predicate<message> filter) {
		 * Implementation note:
		 * While it would be good to make the generic clause more specific
		 * (thereby reducing type-checking code in filters), I'm not sure how 
		 * to instantiate generics from RTTI info.
		Subscription ret = new Subscription();
		ret.subscriber = sub;
		ret.messageType = type;
		ret.filter = filter;
		System.out.println("Subscribing " + sub + " to " + type.toString());
		synchronized (this.subscriptions) {
		return ret;
	 * Removes a subscription.
	 * @param s The subscription to remove.
	public void unsubscribe(Subscription s) {
		System.out.println("Unsubscribing " + s.subscriber + " from " + s.messageType.toString());
		synchronized (subscriptions) {
	 * Causes a message to be published to appropriate subscribers
	 * @param m the message to publish
	public void publish(Message msg) {
		System.out.println("Publishing" + msg.toString());
		synchronized (subscriptions) {
			Iterator<subscription> it = subscriptions.iterator();
			while (it.hasNext()) {
				Subscription sub =;
				if (sub.test(msg)) {
	 * Cancels all subscriptions belonging to a particular subscriber. If this
	 * is used, the subscriber does not have to keep up with subscriptions.
	 * @param sub Subscriber to remove
	public void cancelAllSubscriptions(Subscriber sub) {
		System.out.println("Unsubscribing " + sub.toString());
		ArrayList<subscription> toRemove = new ArrayList<>();
		synchronized(this.subscriptions) {
			Iterator<subscription> it = subscriptions.iterator();
			while (it.hasNext()) {
				Subscription s =;
				if (s.subscriber == sub) toRemove.add(s);

What I have tried:

I've tried removing the synchronized blocks on the list modification methods; I have consulted here, Oracle's documentation on synchronized collections, and even putting a volatile 'busy' variable in that hangs the thread until busy == false, then sets busy to true before processing and to false when finished (that just ended up making one of my threads hang, and it's not a good solution anyway). I've toyed with the idea of some kind of task queue, but before I go to that trouble I thought I would ask the community whether there is something obvious that I'm missing.

Thank you in advance for your help.
Updated 20-Oct-16 5:08am
Richard MacCutchan 20-Oct-16 12:53pm
All your code looks OK, but I cannot actually test it. I can only suggest you use the debugger to try and capture more information when the exception occurs.
Henrik Jonsson 20-Oct-16 15:18pm
Check the stack trace to see whether you end up in a call to remove or add while you are iterating from the same thread.
kjburns1980 20-Oct-16 16:56pm
Thank you for your help. Henrik, the problem was occurring on a lambda on a task queue (the code path is pretty straightforward and I wasn't able to find anything suspicious), so it's entirely possible that there may be a problem with the task queue.

I found CopyOnWriteArrayList, and it's only good they say for situations where iteration constitutes 90% of activity on the list. For this purpose, it's at about 65%, which I can live with for now. Plus, if the problem is on the task queue, this will at least take care of things for now so I can move on.

Thank you both for your answers.

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