Not fully thread safe as it seems.
Not nice, but not yet causing severe problems.

Jul 06, 2015 11:03:40 AM hudson.model.listeners.RunListener report
WARNUNG: RunListener failed
java.lang.RuntimeException: Failed to serialize org.zeroturnaround.stats.ClusterStatisticsPlugin#statsData for class org.zeroturnaround.stats.ClusterStatisticsPlugin
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:214)
at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:182)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138)
at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:167)
at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:108)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:43)
at com.thoughtworks.xstream.core.TreeMarshaller.start(TreeMarshaller.java:82)
at com.thoughtworks.xstream.core.AbstractTreeMarshallingStrategy.marshal(AbstractTreeMarshallingStrategy.java:37)
at com.thoughtworks.xstream.XStream.marshal(XStream.java:1026)
at com.thoughtworks.xstream.XStream.marshal(XStream.java:1015)
at com.thoughtworks.xstream.XStream.toXML(XStream.java:988)
at hudson.XmlFile.write(XmlFile.java:178)
at hudson.Plugin.save(Plugin.java:255)
at org.zeroturnaround.stats.ClusterStatisticsPlugin.maybeSave(ClusterStatisticsPlugin.java:54)
at org.zeroturnaround.stats.StartCompleteListener.onCompleted(StartCompleteListener.java:50)
at hudson.model.listeners.RunListener.fireCompleted(RunListener.java:199)
at hudson.model.Run.execute(Run.java:1809)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Caused by: java.lang.RuntimeException: Failed to serialize org.zeroturnaround.stats.model.StatsData#runStats for class org.zeroturnaround.stats.model.StatsData
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:214)
at hudson.util.RobustReflectionConverter$2.visit(RobustReflectionConverter.java:182)
at com.thoughtworks.xstream.converters.reflection.PureJavaReflectionProvider.visitSerializableFields(PureJavaReflectionProvider.java:138)
at hudson.util.RobustReflectionConverter.doMarshal(RobustReflectionConverter.java:167)
at hudson.util.RobustReflectionConverter.marshal(RobustReflectionConverter.java:108)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84)
at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:223)
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:210)
... 21 more
Caused by: java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at com.thoughtworks.xstream.converters.collections.CollectionConverter.marshal(CollectionConverter.java:73)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller.convert(AbstractReferenceMarshaller.java:69)
at com.thoughtworks.xstream.core.TreeMarshaller.convertAnother(TreeMarshaller.java:58)
at com.thoughtworks.xstream.core.AbstractReferenceMarshaller$1.convertAnother(AbstractReferenceMarshaller.java:84)
at hudson.util.RobustReflectionConverter.marshallField(RobustReflectionConverter.java:223)
at hudson.util.RobustReflectionConverter$2.writeField(RobustReflectionConverter.java:210)
... 30 more