【Hibernate异常】StaleStateException

异常信息：
org.hibernate.StaleStateException: Batch update returned unexpected row count from update [0]; actual row count: 0; expected: 1

 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:63)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3281)
    at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:3183)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3525)
    at org.hibernate.action.internal.EntityUpdateAction.execute(EntityUpdateAction.java:159)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:465)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:351)
    at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:350)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:56)
    at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1258)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.flush(AbstractEntityManagerImpl.java:1335)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:298)
    at com.sun.proxy.$Proxy65.flush(Unknown Source)
    at org.springframework.data.jpa.repository.support.SimpleJpaRepository.flush(SimpleJpaRepository.java:555)
    at org.springframework.data.jpa.repository.support.SimpleJpaRepository.saveAndFlush(SimpleJpaRepository.java:523)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.executeMethodOn(RepositoryFactorySupport.java:504)
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.doInvoke(RepositoryFactorySupport.java:489)
    at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.invoke(RepositoryFactorySupport.java:461)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.data.projection.DefaultMethodInvokingMethodInterceptor.invoke(DefaultMethodInvokingMethodInterceptor.java:56)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:99)
    at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:282)
    at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:96)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.invoke(PersistenceExceptionTranslationInterceptor.java:136)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.data.jpa.repository.support.CrudMethodMetadataPostProcessor$CrudMethodMetadataPopulatingMethodInterceptor.invoke(CrudMethodMetadataPostProcessor.java:133)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.data.repository.core.support.SurroundingTransactionDetectorMethodInterceptor.invoke(SurroundingTransactionDetectorMethodInterceptor.java:57)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:213)

异常出现场景：
调用update(entity)方法的时候，查看数据，entity的ID不为空，而且在数据库中有该条数据。只在更新时有问题，新增时不会有问题。
代码片段：

public void saveOrUpdateClass(ClassVo data){
  // 省略无关代码
  Class class = data.getClass();
  List<Student> studentList = data.getStudentList();
  // 先删除再保存
  iStudentDao.deleteByClassId(class.getId());
  studentList.forEach(student -> {
    student.setClassId(class.getId());
  })
  iStudentDao.save(studentList);
}

根据网上查到的答案：
1.update的时候，实体的ID变了，或者ID为空；
2.更新数据时，显式地使用了id的setId()方法为其指定值，但实体类的id指定了主键生成策略（@GeneratedValue(strategy = GenerationType.AUTO)）；
3....
排除了n个可能后，终于找到了问题所在！

原因：
数据操作步骤是：新增——读取到网页——更新（第二次保存），而每次更新数据student都是全删全插操作。由于读取到页面的数据，重新传回后台时，原来的student数据是带有ID的，hibernate实际上会执行update方法，页update方法要求数据中有该条记录，否则就会报错。但是，我们之前执行了delete的方法，所以hibernate混乱了。。。。又要删除，然后又update，就报错了。

解决方法：
在save之前，把每个student的ID设为空或设置一个新的ID（必须是数据库中没有的）。