hib test case.zip I am attaching a minimal test case that reproduces this problem, or one that is very similar to it. It only occurs when the entity classes have bytecode instrumentation.

I suspect what is happening is that, upon persisting a parent object and a set of child objects, the propagation of the parent ID to the children is triggering bytecode that updates the parent's set of children, which is perhaps the same set the propagation logic is iterating over. The error occurs after successfully setting the parent ID to the first child, during the attempt to iterate to the next child.

WildFly Full 10.1.0.Final
Hibernate 5.0.10 / 5.2.10

Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
at org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:437)
at org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:363)
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:326)
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:162)
at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:111)
at org.hibernate.event.internal.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:456)
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:278)
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:178)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
at org.hibernate.jpa.event.internal.core.JpaPersistEventListener.saveWithGeneratedId(JpaPersistEventListener.java:67)
at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:189)
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:132)
at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:58)
at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:775)
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:748)
at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:753)
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:1146)
at org.jboss.as.jpa.container.AbstractEntityManager.persist(AbstractEntityManager.java:580)
at hibtest.Test2.init(Test2.java:61)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.jboss.as.ee.component.ManagedReferenceLifecycleMethodInterceptor.processInvocation(ManagedReferenceLifecycleMethodInterceptor.java:96)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.weld.ejb.Jsr299BindingsInterceptor.doLifecycleInterception(Jsr299BindingsInterceptor.java:114)
at org.jboss.as.weld.ejb.Jsr299BindingsInterceptor.processInvocation(Jsr299BindingsInterceptor.java:103)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.invocation.InterceptorContext$Invocation.proceed(InterceptorContext.java:437)
at org.jboss.weld.ejb.AbstractEJBRequestScopeActivationInterceptor.aroundInvoke(AbstractEJBRequestScopeActivationInterceptor.java:73)
at org.jboss.as.weld.ejb.EjbRequestScopeActivationInterceptor.processInvocation(EjbRequestScopeActivationInterceptor.java:83)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.weld.injection.WeldInjectionInterceptor.processInvocation(WeldInjectionInterceptor.java:53)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.ee.component.ManagedReferenceFieldInjectionInterceptorFactory$ManagedReferenceFieldInjectionInterceptor.processInvocation(ManagedReferenceFieldInjectionInterceptorFactory.java:107)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.ee.component.AroundConstructInterceptorFactory$1.processInvocation(AroundConstructInterceptorFactory.java:28)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.weld.injection.WeldInterceptorInjectionInterceptor.processInvocation(WeldInterceptorInjectionInterceptor.java:56)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.weld.ejb.Jsr299BindingsCreateInterceptor.processInvocation(Jsr299BindingsCreateInterceptor.java:100)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.ee.component.NamespaceContextInterceptor.processInvocation(NamespaceContextInterceptor.java:50)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:340)
at org.jboss.as.ejb3.tx.CMTTxInterceptor.invokeInOurTx(CMTTxInterceptor.java:275)
... 29 more