Java JPA异常：对象：不是已知的实体类型

我是JPA的新手，我遇到了一些问题。我读到persistence.xml可能有问题

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="GreekTravelPU" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>entities.Room</class>
<class>entities.Country</class>
<class>entities.User</class>
<class>entities.RoomType</class>
<class>entities.Role</class>
<class>entities.Photo</class>
<class>entities.Availability</class>
<class>entities.Message</class>
<class>entities.Location</class>
<class>entities.Booking</class>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
  <property name="javax.persistence.jdbc.url"     value="jdbc:mysql://localhost:3306/greektraveldb?zeroDateTimeBehavior=convertToNull"/>
  <property name="javax.persistence.jdbc.user" value="psilos"/>
  <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
  <property name="javax.persistence.jdbc.password" value="psilos"/>
  <property name="javax.persistence.schema-generation.database.action" value="create"/>
</properties>
</persistence-unit>
</persistence>

实际代码是

@Override public void update(User user) { 
    EntityManager em = EntityManagerHelper.getEntityManager(); 
    try { 
        EntityTransaction entityTrasacrion = em.getTransaction(); 
        entityTrasacrion.begin(); 
        em.merge(em); 
        entityTrasacrion.commit(); 
    } catch (RuntimeException e) { 
        throw e; 
    } finally { 
        EntityManagerHelper.closeEntityManager(); 
    } 
}

---

您正在将实体管理器与实体管理器合并！！！哎呀

em.merge(em);

需要

em.merge(user);

---

您正在将实体管理器与实体管理器合并！！！哎呀

em.merge(em);

需要

em.merge(user);

---

我们还需要导致异常的代码。

@Override public void update（User User）{EntityManager em=EntityManagerHelper.getEntityManager（）；try{EntityTransaction entityTrasacrion=em.getTransaction（）；entityTrasacrion.begin（）；em.merge（em）；entityTrasacrion.commit（）；}catch（运行时异常e）{throw e；}最后{entitymanager.closeEntityManager（）；}}

我想更新。在此链接中查找您是否尝试过此方法？

manager.merge（美工）-这有什么问题吗？我们还需要导致异常的代码。
@覆盖公共无效更新（用户用户）{EntityManager em=EntityManager RHELPER.getEntityManager（）；尝试{EntityTransaction entityTrasacrion=em.getTransaction（）；entityTrasacrion.begin（）；em.merge（em）；entityTrasacrion.commit（）；}catch（运行时异常e）{throw e；}最后{entitymanager.closeEntityManager（）；}}
我想更新。在此链接中查找您是否尝试过此方法？
manager.merge（美工）-怎么了？