spring bean abstract="true"

Abstract beans in Spring are somewhat different from abstract classes. In fact, abstract bean in Spring doesn't even have to be mapped to any class. Take this as an example:

<bean id="dao" abstract="true">
<property name="dataSource" ref="dataSource"/>
<property name="someHelper" ref="someHelper"/>
</bean>

<bean id="fooDao" class="FooDao" parent="dao">
<property name="fooHelper" ref="fooHelper"/>
</bean>
<bean id="barDao" class="BarDao" parent="dao">
<property name="barHelper" ref="barHelper"/>
</bean>

And classes:

public class FooDao {
private DataSource dataSource;
private SomeHelper someHelper;
private FooHelper fooHelper;

//setters
}

public class BarDao {
private DataSource dataSource;
private SomeHelper someHelper;
private BarHelper barHelper;

//setters
}

Note that 

FooDao

 and 

BarDao

 do
not have any parent (abstract or not) base class in common. Parent abstract bean definition is used only to group common
properties, so you avoid repetition in XML.

On the other hand introducing abstract 

Dao

 class
that both 

FooDao

 and 

BarDao

 inherit
from would be a good idea:

public abstract Dao {
protected DataSource dataSource;
protected SomeHelper someHelper;

//setters
}

public class FooDao extends Dao {
private FooHelper fooHelper;

//setters
}

public class BarDao extends Dao {
private BarHelper barHelper;

//setters
}

But still 

dao

 bean
doesn't have to define a class. Treat abstract beans as a way to reduce duplication in XML when several concrete beans have same dependencies.