我为Hibernate / JPA配置问题感到困惑，该问题阻止了我的带有JPA注释的实体自动注册：

java.lang.IllegalArgumentException: Unknown entity: com.example.crm.server.model.Language
    at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:671)
    at com.example.crm.server.model.Language.persist(Language.java:64)
    at com.example.crm.server.LanguageTest.testPersistAndRemove(LanguageTest.java:32)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

在我的实体课程中，我有：

@Entity
@Table(name="Languages")
public class Language implements Serializable
{
    @Id
    private Long id;
    private String name;
    // etc...
}

在MySQL中，语言表如下所示：

+-------------+----------+------+-----+---------+-------+
| Field       | Type     | Null | Key | Default | Extra |
+-------------+----------+------+-----+---------+-------+
| Language_ID | int(11)  | NO   | PRI | NULL    |       | 
| Name        | char(18) | YES  |     | NULL    |       | 
+-------------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

我的persistence.xml看起来像：

<?xml version="1.0" encoding="UTF-8"?>
<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_1_0.xsd"
             version="1.0">

    <persistence-unit name="crm">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>

        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost/crm"/>
            <property name="hibernate.connection.username" value="crmuser"/>
            <property name="hibernate.connection.password" value="mypass"/>
            <property name="hibernate.c3p0.min_size" value="5"/>
            <property name="hibernate.c3p0.max_size" value="20"/>
            <property name="hibernate.c3p0.idleTestPeriod" value="30"/>
            <property name="hibernate.c3p0.timeout" value="0"/>
            <property name="hibernate.c3p0.max_statements" value="0"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.query.jpaql_strict_compliance" value="false"/>
            <property name="hibernate.validator.apply_to_ddl" value="false"/>
            <property name="hibernate.validator.autoregister_listeners" value="false"/>
            <property name="hibernate.archive.autodetection" value="class, hbm"/>
            <property name="hibernate.hbm2ddl.auto" value="create"/>
        </properties>
    </persistence-unit>

</persistence>

编辑 ：这是我获取EntityManager并保持的方式：

public void persist()
{
    EntityManager em = entityManager();
    try
    {
        em.getTransaction().begin();
        em.persist(this);
        em.getTransaction().commit();
    }
    finally
    {
        em.close();
    }
}

public static EntityManager entityManager()
{
    return EMF.get().createEntityManager();
}