New Hibernate Mapping file wizard add inner classes fields to root class fields and look for getter/setter in inner class.

public class B {
public static class Id implements Serializable

private Id id;
...
}
will create mapping:

<hibernate-mapping>
<class name="pack.B" table="B">
<id name="id" type="pack.B$Id" access="field">
<column name="ID" />
<generator class="assigned" />
</id>
<property name="categoryId" type="java.lang.Long" access="field">
<column name="CATEGORYID" />
</property>
<property name="itemId" type="java.lang.Long" access="field">
<column name="ITEMID" />
</property>
...