Does hibernate a default optimistic locking for detached objects?

I have an application that does:

void deleteObj(id){
    MyObj obj = getObjById(id);
    if (obj == null) {
        throw new CustomException("doesn't exists");
    }
    em.remove(obj);//em is a javax.persistence.EntityManager
}

I haven't explicitly configure optimistic locking with version field.However, if two request are running in parallel, trying to delete the same object, then I get sometimes an HibernateOptimisticLockingFailureException and other times the "CustomException".

Is it normal to get HibernateOptimisticLockingFailureException without explicitly setting optimistic locking ? Does hibernate a default optimistic locking for detached objects ?

What are you doing to handle this HibernateOptimisticLockingFailureException ? Retry or inform to the user with a default message like "server busy" ?

First of all, HibernateOptimisticLockingFailureException is a result of Spring's persistence exception translation mechanism. It's thrown in response to StaleStateException, whose javadoc says:

Thrown when a version number or timestamp check failed, indicating that the Session contained stale data (when using long transactions with versioning). Also occurs if we try delete or update a row that does not exist.

From the common sense, optimistic lock exception occurs when data modification statement returns unexpected number of affected rows. It may be caused by mismatch of version value as well as by absence of the row at all.

To make sure that entity was actually removed you can try to flush the context by em.flush() right after removing and catch an exception thrown by it (note that it should be subclass of PersistenceException having StaleStateException as a cause).