My Hibernate version: 2.0.3

I have a problem when i designing my system.
I try to mapping two classes to one table. Both class don't know each other.
For example:
Class A{
long id;
int common_attr_1;
String common_attr_2;
String ext_a_attr_1;
}
Class B{
long id;
int common_attr_1;
String common_attr_2;
boolean ext_b_attr_1;
}
Above code ignored get/set methods.
The table contains all the fields of the two classes.
The mapping of them are:
<class name="A" table="T1">
<id column="id" length="22" name="id" type="long">
<generator class="native"/>
</id>
<property name="common_attr_1" type="integer"/>
<property name="common_attr_2" length="100" type="string"/>
<property name="ext_a_attr_1" length="100" type="string"/>
</class>
<class name="B" table="T1">
<id column="id" length="22" name="id" type="long">
<generator class="native"/>
</id>
<property name="common_attr_1" type="integer"/>
<property name="common_attr_2" length="100" type="string"/>
<property name="ext_b_attr_1" type="boolean"/>
</class>

above mapping file mayby has some minor mistake, but it should present my meaning.

My question is :
1. Is the practice (mapping two classes to one table) good practice of hibernate? Are there any pattern or best practice can guide me
how to design my system?
2. If hibernate can do this, How to resolve the problem that when i save class A, the ext_b_attr_1 is null.
When I load object as type B, it doesn't work because the ext_b_attr_1 is null.
3. If I add some relation to A or B, it make the problem more complex. When B has one relation to another class, Class A can't aware the
relation.
4. If above problem can be solved, are there any thing i should notice? Are there any thing i should not do?

Thank you for your help.

This is not possible, refactor your code to use a common superclass.

In this case if a class hierarchy is mapped to the same table do you have to add an additional discriminator column to the table to indicate to Hibernate which subclass is to be instantiated when a row is retrieved? (as per the docs for maping class hierarchies)

Thanks, Kevin

If you use table-per-class-hierarchy, yes you have to use a discriminator column.