I am having a problem with hibernate lazy collection initialization. When I call some method on a lazy initialized collection, size() or add(), it always throws out the LazyInitializationException. I set the "lazy" attribute in Set value "true" and "inverse" attribute "true" and "cascade" attribute "save-update". I debug into the Hibernate code and found the exception is throw on the statement "session.afterLoad();" in the "doQueryAndInitializeNonLazyCollections" method in Loader class. But the inner exception seems to be "ClassCastException" which is caught in the "initialize" method in "PersistentCollection" class. I am just confused about this.

You have to have an open Session if you want to access an unitialized collection.

Said as christian ,you should keep session open if you want to access an uninitialized collection.