https://issues.apache.org/bugzilla/show_bug.cgi?id=54086
Priority: P2
Bug ID: 54086
Assignee: [email protected]
Summary: ConcurrentModificationException in NioReceiver on
shutdown
Severity: normal
Classification: Unclassified
OS: Solaris
Reporter: [email protected]
Hardware: Sun
Status: NEW
Version: 7.0.30
Component: Cluster
Product: Tomcat 7
Solaris 10 x86, jdk 1.7.
We use tomcat clustering for session replication with 4 nodes and sometimes 8
nodes. We get a ConcurrentModificationException occasionally on shutdown. I
have been unable to reliably reproduce the exception. In the log, I see "Unable
to close cluster receiver selector." with the exception below:
java.util.ConcurrentModificationException
java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
java.util.HashMap$KeyIterator.next(HashMap.java:928)
java.util.Collections$UnmodifiableCollection$1.next(Collections.java:1067)
org.apache.catalina.tribes.transport.nio.NioReceiver.closeSelector(NioReceiver.java:382)
org.apache.catalina.tribes.transport.nio.NioReceiver.stopListening(NioReceiver.java:365)
org.apache.catalina.tribes.transport.nio.NioReceiver.stop(NioReceiver.java:86)
org.apache.catalina.tribes.group.ChannelCoordinator.internalStop(ChannelCoordinator.java:203)
org.apache.catalina.tribes.group.ChannelCoordinator.stop(ChannelCoordinator.java:115)
org.apache.catalina.tribes.group.ChannelInterceptorBase.stop(ChannelInterceptorBase.java:178)
org.apache.catalina.tribes.group.ChannelInterceptorBase.stop(ChannelInterceptorBase.java:178)
org.apache.catalina.tribes.group.ChannelInterceptorBase.stop(ChannelInterceptorBase.java:178)
org.apache.catalina.tribes.group.interceptors.MessageDispatchInterceptor.stop(MessageDispatchInterceptor.java:172)
org.apache.catalina.tribes.group.ChannelInterceptorBase.stop(ChannelInterceptorBase.java:178)
org.apache.catalina.tribes.group.ChannelInterceptorBase.stop(ChannelInterceptorBase.java:178)
org.apache.catalina.tribes.group.GroupChannel.stop(GroupChannel.java:438)
org.apache.catalina.ha.tcp.SimpleTcpCluster.stopInternal(SimpleTcpCluster.java:744)
org.apache.catalina.util.LifecycleBase.stop(LifecycleBase.java:232)
org.apache.catalina.core.ContainerBase.stopInternal(ContainerBase.java:1199)
org.apache.catalina.util.LifecycleBase.stop(LifecycleBase.java:232)
org.apache.catalina.core.StandardService.stopInternal(StandardService.java:502)
org.apache.catalina.util.LifecycleBase.stop(LifecycleBase.java:232)
org.apache.catalina.core.StandardServer.stopInternal(StandardServer.java:753)
org.apache.catalina.util.LifecycleBase.stop(LifecycleBase.java:232)
org.apache.catalina.startup.Catalina.stop(Catalina.java:751)
org.apache.catalina.startup.Catalina.start(Catalina.java:713)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:601)
org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:322)
org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:451)
I looked at the code briefly and noticed the use of a SelectionKey Iterator. I
have not dug deep enough to find any issue in the NioReceiver code, but I did
find this potentially relevant text in the Selector javadocs
(http://docs.oracle.com/javase/7/docs/api/java/nio/channels/Selector.html):
"A selector's key and selected-key sets are not, in general, safe for use by
multiple concurrent threads. If such a thread might modify one of these sets
directly then access should be controlled by synchronizing on the set itself.
The iterators returned by these sets' iterator methods are fail-fast: If the
set is modified after the iterator is created, in any way except by invoking
the iterator's own remove method, then a ConcurrentModificationException will
be thrown."
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