为什么下面的代码导致org.datanucleus.exceptions.NucleusUserException：对象管理器已关闭？似乎在query.getResultList()处抛出异常.

public final void removeUserTokens(final String username) {
    final Query query = entityManager.createQuery(
        "SELECT p FROM PersistentLogin p WHERE username = :username");
    query.setParameter("username",username);

    for (Object token : query.getResultList()) {
        entityManager.remove(token);
    }
}          
@H_502_8@
例外：

org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed
 at org.datanucleus.ObjectManagerImpl.assertIsOpen(ObjectManagerImpl.java:3876)
 at org.datanucleus.ObjectManagerImpl.getFetchPlan(ObjectManagerImpl.java:376)
 at org.datanucleus.store.query.Query.getFetchPlan(Query.java:497)
 at org.datanucleus.store.appengine.query.DatastoreQuery$6.apply(DatastoreQuery.java:611)
 at org.datanucleus.store.appengine.query.DatastoreQuery$6.apply(DatastoreQuery.java:610)
 at org.datanucleus.store.appengine.query.LazyResult.resolveNext(LazyResult.java:94)
 at org.datanucleus.store.appengine.query.LazyResult$LazyAbstractListIterator.computeNext(LazyResult.java:215)
 at org.datanucleus.store.appengine.query.AbstractIterator.tryToComputeNext(AbstractIterator.java:132)
 at org.datanucleus.store.appengine.query.AbstractIterator.hasNext(AbstractIterator.java:127)
 at org.datanucleus.store.appengine.query.LazyResult$AbstractListIterator.hasNext(LazyResult.java:169)
 at com.mystuff.service.auth.PersistentTokenRepositoryImpl.removeUserTokens(PersistentTokenRepositoryImpl.java:90)
@H_502_8@
编辑：我增加了datanucleus的日志级别,这就是我所看到的.

FINE: Object Manager "org.datanucleus.ObjectManagerImpl@5d8d3d6c" opened for datastore "org.datanucleus.store.appengine.DatastoreManager@2447e380"
Feb 25,2010 7:21:38 AM org.datanucleus.ObjectManagerImpl initialiseLevel1Cache
FINE: Level 1 Cache of type "weak" initialised
Feb 25,2010 7:21:38 AM org.datanucleus.JDOClassLoaderResolver classForName
FINE: Class "java.lang.PersistentLogin" was not found in the CLASSPATH [Class resolver called from org.datanucleus.util.Imports.resolveClassDeclaration (line=177)]
Feb 25,2010 7:21:38 AM org.datanucleus.ObjectManagerImpl disconnectSMCache
FINE: Level 1 Cache cleared
Feb 25,2010 7:21:38 AM org.datanucleus.ObjectManagerImpl postClose
FINE: Object Manager "org.datanucleus.ObjectManagerImpl@5d8d3d6c" closed
Feb 25,2010 7:21:38 AM com.google.apphosting.utils.jetty.JettyLogger warn
WARNING: /j_spring_security_logout
Object Manager has been closed
org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed
 at org.datanucleus.ObjectManagerImpl.assertIsOpen(ObjectManagerImpl.java:3876)
 at org.datanucleus.ObjectManagerImpl.getFetchPlan(ObjectManagerImpl.java:376)
@H_502_8@