After starting JBoss and deploying an application, it works fine. But if I re-deploy the application without making any changes, I got the following exception while running the application:

18:08:59,519 ERROR [STDERR] Caused by: org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of package.Student.id
18:08:59,519 ERROR [STDERR] at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
18:08:59,519 ERROR [STDERR] at org.hibernate.tuple.entity.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:183)
18:08:59,519 ERROR [STDERR] at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3524)
18:08:59,529 ERROR [STDERR] at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:3240)
18:08:59,529 ERROR [STDERR] at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:181)
18:08:59,529 ERROR [STDERR] at org.hibernate.engine.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:218)
18:08:59,529 ERROR [STDERR] at org.hibernate.type.EntityType.getIdentifier(EntityType.java:108)
18:08:59,529 ERROR [STDERR] at org.hibernate.type.ManyToOneType.nullSafeSet(ManyToOneType.java:87)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.bindPositionalParameters(Loader.java:1513)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1575)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.doQuery(Loader.java:661)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:224)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.doList(Loader.java:2144)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2028)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.Loader.list(Loader.java:2023)
18:08:59,529 ERROR [STDERR] at org.hibernate.loader.criteria.CriteriaLoader.list(CriteriaLoader.java:95)
18:08:59,529 ERROR [STDERR] at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1569)
18:08:59,529 ERROR [STDERR] at org.hibernate.impl.CriteriaImpl.list(CriteriaImpl.java:283)
18:08:59,539 ERROR [STDERR] at org.hibernate.impl.CriteriaImpl.uniqueResult(CriteriaImpl.java:305)

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if I restart JBoss, everything works fine.
What could be the problem? Thanks for help.

Dave