public class A {
    private Long id;
    private Set<B> bs = new HashSet<B>();
}

public class B {
    private Long id;
    private A a;
}

<hibernate-mapping>
  <class name="A">
    <id name="id" access="field">
      <column name="aId" />
      <generator class="identity" />
    </id>
    <set name="bs" access="field" table="A_B">
      <key column="aId" />
      <many-to-many column="bId" class="B" unique="true" />
    </set>
  </class>
</hibernate-mapping>

<hibernate-mapping>
  <class name="B">
    <id name="id" access="field">
      <column name="bId" />
      <generator class="identity" />
    </id>   
    <join table="A_B" inverse="true" optional="true">
      <key column="bId" />
      <many-to-one name="a" column="aId" not-null="true" />
    </join>
    <joined-subclass name="...." table="....">
      ....
    </joined-subclass>
  </class>
</hibernate-mapping>