java.util.ConcurrentModificationException
 at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
 at java.util.ArrayList$Itr.next(ArrayList.java:851)
 at java.util.Collections$UnmodifiableCollection$1.next(Collections.java:1042)
 at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:587)
 at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:463)
 at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:337)
 at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
 at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1435)
 at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:491)
 at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3201)
 at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2411)
 at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:467)
 at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:146)
 at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$100(JdbcResourceLocalTransactionCoordinatorImpl.java:38)
 at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:220)
 at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:68)
 at org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:517)
 at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:761)
 at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:730)
 at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:504)
 at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:292)
 at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
 at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:673)