Java 接口的XML映射

这是XML文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="de.pentos.vrkarriere.model">
    <!-- <class name="IBill" abstract="true" table="Bills"> -->
    <class name="Bill" table="Bills">
        <id name="id">
            <generator class="identity" />
        </id>
        <property name="created" not-null="true" update="false" insert="false" generated="insert">
            <column name="created" sql-type="timestamp" default="CURRENT_TIMESTAMP"></column>
        </property>
        <property name="modified" not-null="true" update="false" insert="false" generated="always">
            <column name="modified" sql-type="timestamp" default="CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP"></column>
        </property>
        <property name="fileName" not-null="true" />
    </class>

</hibernate-mapping>

错误是

java.lang.IllegalArgumentException: Unknown entity: de.pentos.vrkarriere.model.Bill
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:1149)
    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$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:294)
    at com.sun.proxy.$Proxy79.persist(Unknown Source)
    at de.pentos.vrkarriere.storage.mysql.BillDAO.create(BillDAO.java:21)

它应该将bill检测为实现IBill接口的bill类。
我错过了什么

java.lang.IllegalArgumentException: Unknown entity: de.pentos.vrkarriere.model.Bill
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:1149)
    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$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:294)
    at com.sun.proxy.$Proxy79.persist(Unknown Source)
    at de.pentos.vrkarriere.storage.mysql.BillDAO.create(BillDAO.java:21)