MyCode:
try {
Configuration cfg = new Configuration().addClass(Employee.class);
SessionFactory sessions = cfg.buildSessionFactory(); // error here
new SchemaExport(cfg).create(true, true);
Session session = sessions.openSession();
} catch (Throwable ex) {
logger.debug("buildSessionFactory failed: " + ex);
throw new ExceptionInInitializerError(ex);
}

I really don't know why this happen.

When I first call the method:
Caused by: java.lang.ExceptionInInitializerError
at org.interactive.interactive.business.test.CompileTestMgrEJB.testHibernate(CompileTestMgrEJB.java:89)
at org.interactive.interactive.business.test.CompileTestMgr_bfazjd_EOImpl.testHibernate(CompileTestMgr_bfazjd_EOImpl.java:45)
... 24 more
Caused by: java.lang.IllegalAccessError
at net.sf.cglib.core.ClassEmitter.setTarget(ClassEmitter.java:47)
at net.sf.cglib.core.ClassEmitter.<init>(ClassEmitter.java:39)
at net.sf.cglib.core.KeyFactory$Generator.generateClass(KeyFactory.java:165)
at net.sf.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:215)
at net.sf.cglib.core.KeyFactory$Generator.create(KeyFactory.java:145)
at net.sf.cglib.core.KeyFactory.create(KeyFactory.java:117)
at net.sf.cglib.core.KeyFactory.create(KeyFactory.java:108)
at net.sf.cglib.core.KeyFactory.create(KeyFactory.java:104)
at net.sf.hibernate.impl.SessionFactoryImpl.<clinit>(SessionFactoryImpl.java:236)
at net.sf.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:791)
at org.interactive.interactive.business.test.CompileTestMgrEJB.testHibernate(CompileTestMgrEJB.java:84)
... 25 more

After first call:
Caused by: java.lang.ExceptionInInitializerError
at org.interactive.interactive.business.test.CompileTestMgrEJB.testHibernate(CompileTestMgrEJB.java:89)
at org.interactive.interactive.business.test.CompileTestMgr_bfazjd_EOImpl.testHibernate(CompileTestMgr_bfazjd_EOImpl.java:45)
... 24 more
Caused by: java.lang.NoClassDefFoundError
at net.sf.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:791)
at org.interactive.interactive.business.test.CompileTestMgrEJB.testHibernate(CompileTestMgrEJB.java:84)
... 25 more


<hibernate-configuration>
<session-factory name="java:/hibernate/HibernateFactory">
<property name="show_sql">true</property>
<property name="connection.driver_class">org.gjt.mm.mysql.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/sale</property>
<property name="connection.username">root</property>
<property name="connection.password"></property>
<property name="dialect">net.sf.hibernate.dialect.MySQLDialect</property>
<!-- <mapping resource="hibernatemaps/sale/Employee.hbm.xml" /> -->
<mapping resource="org/interactive/interactive/integration/sale/entity/Employee.hbm.xml" />
</session-factory>
</hibernate-configuration>

<hibernate-mapping>
<class name="org.interactive.interactive.integration.sale.entity.Employee" table="Employee">
<id name="employeeId" column="employee_id" type="long">
<generator class="assigned" />
</id>
<property name="name" column="name" />
<property name="password" column="password" />
</class>
</hibernate-mapping>

package org.interactive.interactive.integration.sale.entity;
public class Employee {
private Long employeeId;
private String name;
private String password;
get..
set...
}

Thanks for your help!

Hibernate version:
2.1.6
Name and version of the database you are using:
MySQL

This is version conflict, probably you use "new" cglib with "old" ASM libray.
Download latest cglib bundled with ASM (http://prdownloads.sourceforge.net/cglib/cglib-full-2.0.2.jar?download) and "clear" classpath, it must solve your problem.