急急急急求答案hibernate

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" org.hibernate.InvalidMappingException: Could not parse mapping document from resource com/xvxing/sshtest/entity/Classes.hbm.xml
at org.hibernate.cfg.Configuration.addResource(Configuration.java:523)
at org.hibernate.cfg.Configuration.parseMappingElement(Configuration.java:1511)
at org.hibernate.cfg.Configuration.parseSessionFactory(Configuration.java:1479)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1458)
at org.hibernate.cfg.Configuration.doConfigure(Configuration.java:1432)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1352)
at org.hibernate.cfg.Configuration.configure(Configuration.java:1338)
at com.xvxing.sshtest.util.ExportDB.main(ExportDB.java:9)
Caused by: org.hibernate.MappingException: must specify an identifier type: com.xvxing.sshtest.entity.Classes
at org.hibernate.cfg.HbmBinder.bindSimpleId(HbmBinder.java:396)
at org.hibernate.cfg.HbmBinder.bindRootPersistentClassCommonValues(HbmBinder.java:334)
at org.hibernate.cfg.HbmBinder.bindRootClass(HbmBinder.java:273)
at org.hibernate.cfg.HbmBinder.bindRoot(HbmBinder.java:144)
at org.hibernate.cfg.Configuration.add(Configuration.java:424)
at org.hibernate.cfg.Configuration.addInputStream(Configuration.java:465)
at org.hibernate.cfg.Configuration.addResource(Configuration.java:520)
... 7 more
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"><hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/sshtest</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">root</property>
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.show_sql">true</property>


<mapping resource="com/xvxing/sshtest/entity/Student.hbm.xml"/>
<mapping resource="com/xvxing/sshtest/entity/Classes.hbm.xml"/>

</session-factory>
</hibernate-configuration>
<?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>
<class name="com.xvxing.sshtest.entity.Classes" table="t_class">
<id>
<generator class="native" />
</id>
<property name="name" />
<set name="student" inverse="true">
<key column="classid" />
<one-to-many class="com.xvxing.sshtest.entity.Student"/>
</set>
</class>
</hibernate-mapping>
<?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>
<class name="com.xvxing.sshtest.entity.Student" table="t_student">
<id name="id">
<generator class="native"></generator>
</id>
<property name="name"/>
<many-to-one name="classes" column="classid"/>
</class>
</hibernate-mapping>
package com.xvxing.sshtest.entity;public class Student {

private int id;
private String name;
private Classes classes;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Classes getClasses() {
return classes;
}
public void setClasses(Classes classes) {
this.classes = classes;
}
}
package com.xvxing.sshtest.entity;import java.util.Set;public class Classes {
 
private int id;
private String name;
private Set student;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set getStudent() {
return student;
}
public void setStudent(Set student) {
this.student = student;
}
}