Spring bean definition null value

[wjlabcwjl]

public class A{
  private B b;
}

public class B{
}

<bean id="aBean" class="A">
<property name="b">  ???  </property>
</bean>

When use new A() to create A, b is null.
How to create an instance of A with b = null when using context.getBean("aBean")?

In the bean definition, I have tried <property name="b"><null/></property>, but this don't work.