Java 如果使用save，为什么saveOrUpdate在执行时不在数据库中插入注释

我对冬眠的奇怪行为有疑问

我有以下的守则：

Prepod prepod = new Prepod();
        prepod.setId(1l);
        Student student = new Student();
        student.setAge(12l);
        student.setName("name");
        student.setId(1l);
        prepod.getStudents().add(student);
        student.getPrepods().add(prepod);

        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();

        session.saveOrUpdate(prepod);
        session.saveOrUpdate(student);


        session.getTransaction().commit();

hibernate告诉我：

Hibernate: update Student set age=?, name=? where id=?
Exception in thread "main" org.hibernate.StaleStateException: Batch update returned unexpected row count from update [0]; actual row count: 0; expected: 1
    at org.hibernate.jdbc.Expectations$BasicExpectation.checkBatched(Expectations.java:81)
    at org.hibernate.jdbc.Expectations$BasicExpectation.verifyOutcome(Expectations.java:73)
    at org.hibernate.engine.jdbc.batch.internal.NonBatchingBatch.addToBatch(NonBatchingBatch.java:57)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3016)
    at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:2918)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3247)
    at org.hibernate.action.internal.EntityUpdateAction.execute(EntityUpdateAction.java:140)
    at org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:362)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:354)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:276)
    at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:326)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:52)
    at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1214)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:403)
    at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:175)
    at logic.Main.main(Main.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

但是如果我使用

    session.save(prepod);
    session.save(student);

我明白了：

Hibernate: select max(id) from prepod
Hibernate: select max(id) from Student
Hibernate: insert into prepod (id) values (?)
Hibernate: insert into Student (age, name, id) values (?, ?, ?)
Hibernate: insert into prepod_Student (prepods_id, students_id) values (?, ?)

这对我来说是个好行为

你能解释一下为什么它如此有效吗

---

在我认为saveOrUpdate自动选择save或update之前。

问题在于区分对象是否存在于数据库中

对象Prepod和Student的ID显然设置为generator=increment，这就是为什么我们看到selectmaxid语句。最有可能的是，未保存的值保留为默认值：0

---

这意味着Hibernate会将prepd实例视为新实例/临时实例，以防ID==0。但在代码中，您设置了id:student.setAge12l

因此，从Hibernate的角度来看，对象不再是瞬态的。它似乎是现有的、已经持久化的对象

因此，如果调用SaveOrUpdate，Hibernate将指示使用UPDATE语句，因为对象似乎不是瞬态的

---

如何解决？不要自行设置ID。只需依靠生成器=增量。在这种情况下，对象将是瞬态的，默认id==0，Hibernate将按预期工作

，但在代码中设置id:student.setAge12l；-这里出错