GAE ConcurrentModificationException while commiting - google-app-engine

Question

I'm testing a GAE application with JPA deployed on localhost with jetty provided by the SDK (version 1.7.1).
At every commit the framework raise a ConcurrentModificationException, but the object persist in the datastore.
There's something I'm doing wrong? Tnx in advance
appengine-api-1.0-sdk-1.7.1
datanucleus-api-jpa-3.1.2
datanucleus-core-3.1.2
datanucleus-appengine-2.1.0-final
Transaction failed to commit
org.datanucleus.transaction.NucleusTransactionException: Transaction failed to commit
at org.datanucleus.TransactionImpl.commit(TransactionImpl.java:365)
at it.gustaff.core.PlaceRService.createPlace(PlaceRService.java:91)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.google.appengine.tools.development.agent.runtime.Runtime.invoke(Runtime.java:115)
at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)
at com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$TypeOutInvoker._dispatch(AbstractResourceMethodDispatchProvider.java:185)
at com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75)
at com.sun.jersey.server.impl.uri.rules.HttpMethodRule.accept(HttpMethodRule.java:288)
at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
at com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:108)
at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
at com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1469)
at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1400)
at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1349)
at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1339)
at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537)
at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:708)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:35)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:60)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:94)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:370)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.google.appengine.datanucleus.DatastoreConnectionFactoryImpl$DatastoreManagedConnection.close(DatastoreConnectionFactoryImpl.java:146)
at org.datanucleus.store.connection.ConnectionManagerImpl$2.transactionCommitted(ConnectionManagerImpl.java:296)
at org.datanucleus.TransactionImpl.internalPostCommit(TransactionImpl.java:577)
at org.datanucleus.TransactionImpl.commit(TransactionImpl.java:354)
... 51 more
<persistence-unit name="appengine-transactions-optional">
<provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
<class>it.gustaff.data.Place</class>
<properties>
<property name="datanucleus.NontransactionalRead" value="true"/>
<property name="datanucleus.NontransactionalWrite" value="true"/>
<property name="datanucleus.singletonEMFForName" value="true"/>
<property name="datanucleus.ConnectionURL" value="appengine"/>
</properties>
</persistence-unit>
public Place createPlace(Place p){
EntityManager entityManager = EMF.get().createEntityManager();
entityManager.getTransaction().begin();
//avoid to persist a Place with an Id already set
p.setId(null);
entityManager.persist(p);
ExecutionContext ec = (ExecutionContext) entityManager.getDelegate();
ec.getTransaction().commit();
entityManager.close();
return p;
}
#Entity
public class Place {
private String name;
private String description;
private String address;
//omitted other getter/setter
private com.google.appengine.api.datastore.Key id;
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
public Key getId() {
return id;
}
public void setId(Key id) {
this.id = id;
}
}

Answer 1

You don't quote your entity and what p.setId(null) is doing. That aside something you can try (to find where the real ConcurrentModificationException is happening) is (something like) replace the entityManager.getTransaction().commit() with this
ExecutionContext ec = (ExecutionContext)entityManager.getDelegate();
ec.getTransaction().commit();
If this throws a NucleusTransactionException then print out the stack trace with the cause and add it to your question. The cause is where the real problem is, and if its a problem in GAE then it can be fixed, otherwise if its a problem in your code then you know where it is.