今天继续学习JPA中实体的最后一种状态：移除态。还是先把图晒出来：
按照名称进行猜想，移除态应该就是对象从数据库中删除以后，对象变成了不受持久化上下文管理的移除态。不过呢，这里有一点需要注意的是：
什么样的对象才会进入进入移除态呢？
变成移除状态的对象是调用persist()方法后，是像persist一样参数对象受到持久化上下文管理呢？还是像merge方法一样返回的对象受到持久化上下文管理呢？
这里还有一个显而易见的问题，移除的对象再次进入持久化以后，猜测提交事务以后肯定是发出一条insert的SQL语句。有没有可能会发出update语句呢？
针对上面的三个问题，我们会一个一个通过代码来解决：
第一种情况:直接删除Transient状态的对象
public void removedToPersist() {
Teacher teacher = new Teacher("new_email@gmail.com");
logger.info("teacher: {} is transient state",teacher);
EntityManager entityManager = entityManagerFactory.createEntityManager();
EntityTransaction tx = entityManager.getTransaction();
tx.begin();
entityManager.remove(teacher);
logger.info("teacher: {} is removed state",teacher);
Long persistedId = teacher.getId();
tx.commit();
entityManager.close();
}日志信息如下：
teacher: Student{id=null, email='new_email@gmail.com'} is transient state
Handling transient entity in delete processing
teacher: Student{id=null, email='new_email@gmail.com'} is persistent state可以看出这种情况下，没有发出任何SQL，也不应该发出任何SQL
第二种情况:直接删除Persistent状态的对象：
public void removedToPersist() {
Teacher teacher = new Teacher("new_email@gmail.com");
logger.info("teacher: {} is transient state",teacher);
EntityManager entityManager = entityManagerFactory.createEntityManager();
EntityTransaction tx = entityManager.getTransaction();
tx.begin();
entityManager.persist(teacher);
logger.info("teacher: {} is persist state",teacher);
Long persistedId = teacher.getId();
tx.commit();
entityManager.close();
entityManager = entityManagerFactory.createEntityManager();
tx = entityManager.getTransaction();
tx.begin();
teacher = entityManager.find(Teacher.class, persistedId);
entityManager.remove(teacher);
tx.commit();
entityManager.close();
logger.info("teacher: {} is removed state",teacher);
}teacher: Student{id=null, email='new_email@gmail.com'} is transient state
Hibernate:
call next value for hibernate_sequence
teacher: Student{id=1, email='new_email@gmail.com'} is persist state
Hibernate:
insert
into
teachers
(email, id)
values
(?, ?)
Hibernate:
select
teacher0_.id as id1_0_0_,
teacher0_.email as email2_0_0_
from
teachers teacher0_
where
teacher0_.id=?
Hibernate:
delete
from
teachers
where
id=?
teacher: Student{id=1, email='new_email@gmail.com'} is removed state从上面的代码和日志信息可以看出，当处于持久态的对象执行删除的时候，对象可以正常删除。
如果对象处于游离态执行删除会怎么样呢？我们来试试：
我们将上面的
teacher = entityManager.find(Teacher.class, persistedId);修改为:
logger.info("delete detached state teacher：{}",teacher);执行之后，打印的日志如下：
teacher: Student{id=null, email='new_email@gmail.com'} is transient state
Hibernate:
call next value for hibernate_sequence
teacher: Student{id=1, email='new_email@gmail.com'} is persist state
Hibernate:
insert
into
teachers
(email, id)
values
(?, ?)
delete detached state teacher：Student{id=1, email='new_email@gmail.com'}
java.lang.IllegalArgumentException: Removing a detached instance com.jpa.demo.model.Teacher#1因此，这里需要注意的是：删除游离态的对象会抛出一个异常：Removing a detached instance
NOTE: Only persistent objects can be removed in JPA, any attempt to delete a dettached object will cause
java.lang.IllegalArgumentException: Removing a detached instance.
entityManager的移除方法签名如下:
public void remove(Object entity);因此我们猜测，当移除态的对象按照最上面的状态迁移图再次调用persist()方法后，应该是参数对象被持久化上下文管理，我们不妨用代码试试：
这次的代码如下：
public void removedToPersist() {
Teacher teacher = new Teacher("new_email@gmail.com");
logger.info("teacher: {} is transient state",teacher);
EntityManager entityManager = entityManagerFactory.createEntityManager();
EntityTransaction tx = entityManager.getTransaction();
tx.begin();
entityManager.persist(teacher);
logger.info("teacher: {} is persist state",teacher);
Long persistedId = teacher.getId();
tx.commit();
entityManager.close();
entityManager = entityManagerFactory.createEntityManager();
tx = entityManager.getTransaction();
tx.begin();
teacher = entityManager.find(Teacher.class, persistedId);
entityManager.remove(teacher);
entityManager.persist(teacher);
tx.commit();
entityManager.close();
logger.info("teacher: {} is removed state",teacher);
// entityManager = entityManagerFactory.createEntityManager();
// tx = entityManager.getTransaction();
// tx.begin();
// entityManager.persist(teacher);
// logger.info("teacher: {} is persist state",teacher);
// tx.commit();
// entityManager.close();
}我们的猜想应该最先是一条insert语句，然后是一条查询语句，然后是一条delete语句，最后是一条insert语句。哈哈，看看日志信息吧：
teacher: Student{id=null, email='new_email@gmail.com'} is transient state
Hibernate:
call next value for hibernate_sequence
teacher: Student{id=1, email='new_email@gmail.com'} is persist state
Hibernate:
insert
into
teachers
(email, id)
values
(?, ?)
Hibernate:
select
teacher0_.id as id1_0_0_,
teacher0_.email as email2_0_0_
from
teachers teacher0_
where
teacher0_.id=?
teacher: Student{id=1, email='new_email@gmail.com'} is removed state哈哈，有意思，既没有删除，也么有插入，大家理解了吧，有些东西还真不是想当然的。
这里再补充一个知识点：
detached状态的对象，调用persist()方法会怎么样呢？
public void removedToPersist() {
Teacher teacher = new Teacher("new_email@gmail.com");
logger.info("teacher: {} is transient state",teacher);
EntityManager entityManager = entityManagerFactory.createEntityManager();
EntityTransaction tx = entityManager.getTransaction();
tx.begin();
entityManager.persist(teacher);
logger.info("teacher: {} is persist state",teacher);
Long persistedId = teacher.getId();
tx.commit();
entityManager.close();
entityManager = entityManagerFactory.createEntityManager();
tx = entityManager.getTransaction();
tx.begin();
teacher = entityManager.find(Teacher.class, persistedId);
entityManager.remove(teacher);
tx.commit();
entityManager.close();
logger.info("teacher: {} is removed state",teacher);
entityManager = entityManagerFactory.createEntityManager();
tx = entityManager.getTransaction();
tx.begin();
entityManager.persist(teacher);
logger.info("teacher: {} is persist state",teacher);
tx.commit();
entityManager.close();
}我们知道在最上面的状态迁移图中，detached可以通过merge方法进入persist状态，结果detached状态通过调用persist()方法后，让人吃惊：
teacher: Student{id=null, email='new_email@gmail.com'} is transient state
Hibernate:
call next value for hibernate_sequence
teacher: Student{id=1, email='new_email@gmail.com'} is persist state
Hibernate:
insert
into
teachers
(email, id)
values
(?, ?)
Hibernate:
select
teacher0_.id as id1_0_0_,
teacher0_.email as email2_0_0_
from
teachers teacher0_
where
teacher0_.id=?
Hibernate:
delete
from
teachers
where
id=?
[INFO] 2021-12-13 23:11:29 method: com.jpa.demo.dao.EntityStatusDemoDao.removedToPersist(EntityStatusDemoDao.java:133)----teacher: Student{id=1, email='new_email@gmail.com'} is removed state
javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: com.jpa.demo.model.Teacher结果抛出了一个异常：PersistenceException
也就是说，detached态时不能通过persist()方法进入持久化状态的。至于为什么这样设计，我也暂时还没有想明白，如果有知道的欢迎给我留言，不甚感谢。