JPA @Version字段不会递增

我是JPA和Hibernate的新手，我有一个乐观锁定的问题。我有一个类，它有一个@Version注释的字段。当我更新此类表示的实体时，版本计数器不会增加。这是我的代码：类：

@Entity
@Table (name = "Studenten")
public class Student implements Serializable{

private static final long serialVersionUID = 705252921575133272L;

@Version
private int version;
private int matrnr;
private String name;
private int semester;


public Student (){
    
}
public Student (int matrnr, String name){
    this.matrnr = matrnr;
    this.name = name;
}

public Student (int matrnr, String name, int semester){
    this(matrnr, name);
    this.semester = semester;
}

这是主要方法：

public static void main(String[] args) {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("VEDA_Vortrag");
    EntityManager em = emf.createEntityManager();
    
    EntityTransaction tx = em.getTransaction();
    
    try{
        tx.begin();
        Student s = em.find(Student.class, 195948);
        s.setSemester(1);
        em.persist(s);
        tx.commit();
    }catch(Exception e){
        if(tx != null && tx.isActive()){
            tx.rollback();
            System.out.println("Error in Transaction. Rollback!");
        }
    }
    finally{
        em.close();
        emf.close();
    }
}

这是控制台所说的：

Hibernate: 
select
    student0_.matrnr as matrnr0_0_,
    student0_.name as name0_0_,
    student0_.semester as semester0_0_,
    student0_.version as version0_0_ 
from
    Studenten student0_ 
where
    student0_.matrnr=?
Hibernate: 
    update
        Studenten 
    set
        name=?,
        semester=?,
        version=? 
    where
        matrnr=?

有人能告诉我出了什么问题吗？

编辑：好的，我已经尝试了一些东西。我已将锁定设置为LockModeType.OPTIMISTIC_FORCE_INCREMENT并收到此错误消息：

ERROR: HHH000099: an assertion failure occurred (this may indicate a bug in Hibernate, but is more likely due to unsafe use of the session): org.hibernate.AssertionFailure: cannot force version increment on non-versioned entity
Jun 20, 2014 9:00:52 AM org.hibernate.AssertionFailure <init>

因此，很明显，我有一个非版本化的实体。但是为什么？我的班级里有@Version注释。

编辑：我开始认为问题在于持久性.xml。在这里：

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
    version="2.0">
<persistence-unit name="VEDA_Vortrag">
    <provider>org.hibernate.ejb.HibernatePersistence</provider>
    <properties>
        <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/uni"/>
        <property name="javax.persistence.jdbc.user" value="*******"/>
        <property name="javax.persistence.jdbc.password" value="******"/>
        <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
        <property name="hibernate.show_sql" value="true"/>
        <property name="hibernate.format_sql" value="true"/>
    </properties>
</persistence-unit>

有什么问题吗？