Forget it: Concurrent Modification Exception in ChannelLinkNotifier

Development IDE & Build
1

First, sorry for the topic title, but Discourse won’t let me use “long words” in the title, so I had to split the exception up.

After rebasing my work branch on latest main, I get these during startup. I can’t be sure that they haven’t been there before, but I can’t remember seeing them.

I’ve looked through the latest commits to main, but nothing obvious pops up. Still, ConcurrentModificationException isn’t something to ignore, as it reveals thread unsafe code, so I’m posting it here.

19:12:16.123 [ERROR] [e.thing.internal.ChannelLinkNotifier] - bundle org.openhab.core.thing:5.0.0.202502200852 (156)[org.openhab.core.thing.internal.ChannelLinkNotifier(251)] : Error during instantiation of the implementation object
java.lang.reflect.InvocationTargetException: null
	at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:74) ~[?:?]
	at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502) ~[?:?]
	at java.lang.reflect.Constructor.newInstance(Constructor.java:486) ~[?:?]
	at org.apache.felix.scr.impl.inject.internal.ComponentConstructorImpl.newInstance(ComponentConstructorImpl.java:326) ~[bundleFile:?]
	at org.apache.felix.scr.impl.manager.SingleComponentManager.createImplementationObject(SingleComponentManager.java:286) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.SingleComponentManager.createComponent(SingleComponentManager.java:115) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.SingleComponentManager.getService(SingleComponentManager.java:1002) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.SingleComponentManager.getServiceInternal(SingleComponentManager.java:975) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:776) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1274) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1225) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184) [bundleFile:?]
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116) [bundleFile:?]
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500) [org.eclipse.osgi-3.18.0.jar:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1274) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1225) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184) [bundleFile:?]
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116) [bundleFile:?]
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500) [org.eclipse.osgi-3.18.0.jar:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.enableInternal(AbstractComponentManager.java:674) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.enable(AbstractComponentManager.java:437) [bundleFile:?]
	at org.apache.felix.scr.impl.manager.ConfigurableComponentHolder.enableComponents(ConfigurableComponentHolder.java:671) [bundleFile:?]
	at org.apache.felix.scr.impl.BundleComponentActivator.initialEnable(BundleComponentActivator.java:310) [bundleFile:?]
	at org.apache.felix.scr.impl.Activator.loadComponents(Activator.java:593) [bundleFile:?]
	at org.apache.felix.scr.impl.Activator.access$200(Activator.java:74) [bundleFile:?]
	at org.apache.felix.scr.impl.Activator$ScrExtension.start(Activator.java:460) [bundleFile:?]
	at org.apache.felix.scr.impl.AbstractExtender.createExtension(AbstractExtender.java:196) [bundleFile:?]
	at org.apache.felix.scr.impl.AbstractExtender.modifiedBundle(AbstractExtender.java:169) [bundleFile:?]
	at org.apache.felix.scr.impl.AbstractExtender.modifiedBundle(AbstractExtender.java:49) [bundleFile:?]
	at org.osgi.util.tracker.BundleTracker$Tracked.customizerModified(BundleTracker.java:488) [org.eclipse.osgi-3.18.0.jar:?]
	at org.osgi.util.tracker.BundleTracker$Tracked.customizerModified(BundleTracker.java:1) [org.eclipse.osgi-3.18.0.jar:?]
	at org.osgi.util.tracker.AbstractTracked.track(AbstractTracked.java:232) [org.eclipse.osgi-3.18.0.jar:?]
	at org.osgi.util.tracker.BundleTracker$Tracked.bundleChanged(BundleTracker.java:450) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:949) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEventPrivileged(EquinoxEventPublisher.java:229) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:138) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:130) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.EquinoxContainerAdaptor.publishModuleEvent(EquinoxContainerAdaptor.java:217) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.container.Module.publishEvent(Module.java:499) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.container.Module.start(Module.java:486) [org.eclipse.osgi-3.18.0.jar:?]
	at org.eclipse.osgi.internal.framework.EquinoxBundle.start(EquinoxBundle.java:445) [org.eclipse.osgi-3.18.0.jar:?]
	at aQute.launcher.Launcher.start(Launcher.java:699) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.Launcher.startBundles(Launcher.java:679) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.Launcher.activate(Launcher.java:585) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.Launcher.launch(Launcher.java:403) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.Launcher.run(Launcher.java:185) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.Launcher.main(Launcher.java:161) [biz.aQute.launcher-7.0.0.jar:?]
	at aQute.launcher.pre.EmbeddedLauncher.executeWithRunPath(EmbeddedLauncher.java:170) [biz.aQute.launcher.pre.jar:?]
	at aQute.launcher.pre.EmbeddedLauncher.findAndExecute(EmbeddedLauncher.java:135) [biz.aQute.launcher.pre.jar:?]
	at aQute.launcher.pre.EmbeddedLauncher.main(EmbeddedLauncher.java:52) [biz.aQute.launcher.pre.jar:?]
Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1720) ~[?:?]
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509) ~[?:?]
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?]
	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151) ~[?:?]
	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174) ~[?:?]
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:?]
	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596) ~[?:?]
	at org.openhab.core.thing.internal.ChannelLinkNotifier.<init>(ChannelLinkNotifier.java:58) ~[?:?]
	at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62) ~[?:?]
	... 85 more
2

Please report a bug, especially if you can reproduce this error in your environment.

Guilty line is this one: openhab-core/bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/ChannelLinkNotifier.java at 41720b4f0c025f8884064be0b18d48ab656ead3a · openhab/openhab-core · GitHub

I believe we can solve it by creating copy of the links:

- itemChannelLinkRegistry.stream()
+ new ArrayList(itemChannelLinkRegistry.getAll()).stream()

In second case we iterate over completely new collection which is a view over links at the moment of the call. A bit awkward, but its gonna work.

3

I haven’t tried to consistently reproduce it, but I see it regularly in my log (during startup). It feels to me like it’s time sensitive.

I haven’t dug into it though, so which I think I would have to do to file an issue. I’m not sure I can find a consistently reproducable case.

I don’t think your suggested fix will work or is the problem, because a copy is already being made. If you look at AbstractRegistry.stream():

    @Override
    public Stream<E> stream() {
        return getAll().stream();
    }

But, this explains everything. I had been making some experiment that has been causing this when I think about it. So, this isn’t a bug in main at all. I’m so sorry, thank you for making me realize what this is about.

4

Well, it will make a bit difference because fix which I mentioned creates new list, rather than using one returned by registry. It removes the concurrent modification exception for sure.

5

My point was that itemChannelLinkRegistry.stream() already returns a stream of a copy of the collection. I forgot to show getAll() to make this clear:

    @Override
    public Collection<E> getAll() {
        elementReadLock.lock();
        try {
            return new HashSet<>(elements);
        } finally {
            elementReadLock.unlock();
        }
    }

My “experiment” modified these methods which is why the ConcurrentModificationException could happen in the first place.

6

My bad, you’re right. I thought that there was a side modification happening from another thread to source collection, but that’s apparently not a case. Looking at the stream statement, it is innocent and does not have any side effects. Could it be a variant of https://bugs.openjdk.org/browse/JDK-8251333?
Given above JDK issue, could it be that collection is empty?

7

No, as I said, this is entirely my fault. I had done some tweaks/experients that caused this to happen, I just didn’t realize the correlation until you wrote what you did. It’s a non-issue, I’ve reversed the “experiment” anyway, so this won’t happen again.

8

I have seen ConcurrentModificationException in my logs as well, that’s why I keep digging. :smiley:

9

Ah, I see, but those can origin many, many places. I’ve seen code in Core that isn’t thread-safe, so I’ve no problem imagining that those do occur from time to time - but you should probably start with the exact stack trace that produced it. This particular incident was my fault :wink: