public class A{

         public static class B{
                    //...
         }
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping SYSTEM "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 18/Nov/2009 14:54:38 by Hibernate Tools 3.2.5.Beta -->
<hibernate-mapping>
    <class name="package.A" entity-name="A" table="A">
          <!--(properties...)-->
        <many-to-one name="B" class="package.A$B" column="B" fetch="join"/>   
    </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">
<!-- Generated 18/Nov/2009 14:54:38 by Hibernate Tools 3.2.5.Beta -->
<hibernate-mapping>
    <class name="package.A$B" entity-name="B" table="B">
   </class>   
</hibernate-mapping>

java.lang.ExceptionInInitializerError
   at package.Main.<clinit>(Main.java:31)
Caused by: org.hibernate.MappingException: An association from the table A refers to an unmapped class: package.A$B

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory>
      <property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
        <property name="connection.url">jdbc:hsqldb:mem:testdb</property>
        <property name="connection.username">sa</property>
        <property name="connection.password"></property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">2</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.HSQLDialect</property>

        <!-- Echo all executed SQL to stdout
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create</property>-->
       
      <!-- Mapping files -->
       <mapping resource="A$B.hbm.xml"/>   
      <mapping resource="A.hbm.xml"/>

</session-factory>
</hibernate-configuration>