ElementArboStructure non serializable

java.io.NotSerializableException: com.univ.objetspartages.om.ElementArboStructure
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347) ~[na:1.7.0_80]
        at java.util.HashMap.writeObject(HashMap.java:1129) ~[na:1.7.0_80]
        at sun.reflect.GeneratedMethodAccessor94.invoke(Unknown Source) ~[na:na]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_80]
        at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_80]
        at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:440) ~[na:1.7.0_80]
        at net.sf.ehcache.Element.writeObject(Element.java:851) ~[ehcache-core-2.7.1.jar:2.7.1]
        at sun.reflect.GeneratedMethodAccessor107.invoke(Unknown Source) ~[na:na]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_80]
        at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_80]
        at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347) ~[na:1.7.0_80]
        at net.sf.ehcache.distribution.LegacyEventMessage.writeObject(LegacyEventMessage.java:117) ~[ehcache-core-2.7.1.jar:2.7.1]
        at sun.reflect.GeneratedMethodAccessor104.invoke(Unknown Source) ~[na:na]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_80]
        at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_80]
        at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177) ~[na:1.7.0_80]
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347) ~[na:1.7.0_80]
        at org.jgroups.util.Util.objectToByteBuffer(Util.java:562) ~[jgroups-3.4.1.Final.jar:3.4.1.Final]
        at net.sf.ehcache.distribution.jgroups.JGroupsCachePeer.sendData(JGroupsCachePeer.java:201) [ehcache-jgroupsreplication-1.7.jar:na]
        at net.sf.ehcache.distribution.jgroups.JGroupsCachePeer.flushQueue(JGroupsCachePeer.java:244) [ehcache-jgroupsreplication-1.7.jar:na]
        at net.sf.ehcache.distribution.jgroups.JGroupsCachePeer.access$200(JGroupsCachePeer.java:48) [ehcache-jgroupsreplication-1.7.jar:na]
        at net.sf.ehcache.distribution.jgroups.JGroupsCachePeer$AsyncTimerTask.run(JGroupsCachePeer.java:265) [ehcache-jgroupsreplication-1.7.jar:na]
        at java.util.TimerThread.mainLoop(Timer.java:555) [na:1.7.0_80]