尝试一一删除元素：
List<String> Mylist = a.getMyList();
em.getTransaction().begin();
for (String element:Mylist){
em.remove(element);
}
em.getTransaction().commit();
List<String> Mylist = a.getMyList();
listStirng.clear();
em.getTransaction().begin();
for (String str : Mylist) {
em.merge(str);
}
em.getTransaction().commit();
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清零编程群