请教大家一个关于hibernate继承的问题

父类： Animal

package com.qchu.hiber.pojo;

public class Animal {
private int id;
private String name;
private String age;
//getter  setter
}

第一个子类：Dog

package com.qchu.hiber.pojo;

public class Dog extends Animal {
private String crawl;
        //getter setter
}

第二个子类：Bird

package com.qchu.hiber.pojo;

public class Bird extends Animal {
private String fly;
        // getter setter
}


这是我的映射文件：Animal.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping
package="com.qchu.hiber.pojo">

<class name="Animal" discriminator-value="0">

<id name="id">
<generator class="native"/>
</id>

<property name="name"/>
<property name="age"/>

<discriminator column="type" type="int"/>

<subclass name="Dog" discriminator-value="1">
<property name="crawl"/>
</subclass>

<subclass name="Bird" discriminator-value="2">
<property name="fly"/>
</subclass>

</class>

</hibernate-mapping>

在保存Dog对象的时候，总是报错：

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Exception in thread "main" java.lang.ExceptionInInitializerError
at com.qchu.hiber.demo.AnimalDemo.main(AnimalDemo.java:26)
Caused by: org.hibernate.InvalidMappingException: Could not parse mapping document from resource com/qchu/hiber/pojo/Animal.hbm.xml
at org.hibernate.cfg.Configuration.addResource(Configuration.java:569)
at org.hibernate.cfg.Configuration.parseMappingElement(Configuration.java:1587)
at org.hibernate.cfg.Configuration.parseSessionFactory(Configuration.java:1555)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1534)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1508)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1428)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1414)
at com.qchu.hiber.util.HibernateUtils.<clinit>(HibernateUtils.java:16)
... 1 more
Caused by: org.hibernate.InvalidMappingException: Could not parse mapping document from invalid mapping
at org.hibernate.cfg.Configuration.addInputStream(Configuration.java:502)
at org.hibernate.cfg.Configuration.addResource(Configuration.java:566)
... 8 more
Caused by: org.xml.sax.SAXParseException; lineNumber: 27; columnNumber: 10; The content of element type "class" must match "(meta*,subselect?,cache?,synchronize*,comment?,tuplizer*,(id|composite-id),discriminator?,natural-id?,(version|timestamp)?,(property|many-to-one|one-to-one|component|dynamic-component|properties|any|map|set|list|bag|idbag|array|primitive-array)*,((join*,subclass*)|joined-subclass*|union-subclass*),loader?,sql-insert?,sql-update?,sql-delete?,filter*,resultset*,(query|sql-query)*)".
at org.apache.xerces.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
at org.apache.xerces.util.ErrorHandlerWrapper.error(Unknown Source)
at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.dtd.XMLDTDValidator.handleEndElement(Unknown Source)
at org.apache.xerces.impl.dtd.XMLDTDValidator.endElement(Unknown Source)
at org.apache.xerces.impl.XMLNSDocumentScannerImpl.scanEndElement(Unknown Source)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
at org.dom4j.io.SAXReader.read(SAXReader.java:465)
at org.hibernate.cfg.Configuration.addInputStream(Configuration.java:499)
... 9 more

有人遇见过这个问题吗？