[Springframework-developer] Reworked root/child bean definition concept

[<jue...@we...>]

I've just committed a fairly significant reworking of our root/child =
bean definition concept: A child can now inherit respectively override =
virtually any attributes from a parent, not just bean property values =
and the singleton flag. From the new ChildBeanDefinition javadoc:

 * <p>Will use the bean class of the parent if none specified, but can
 * also override it. In the latter case, the child bean class must be
 * compatible with the parent, i.e. accept the parent's property values
 * and constructor argument values, if any.
 *
 * <p>A child bean definition will inherit constructor argument values,
 * property values and method overrides from the parent, with the option
 * to add new values. If init method, destroy method and/or static =
factory
 * method are specified, they will override the corresponding parent =
settings.
 *
 * <p>The remaining settings will <i>always</i> be taken from the child =
definition:
 * depends on, autowire mode, dependency check, singleton, lazy init.

Everything that worked before still works; it should be fully backward =
compatible. Formerly, this mechanism has mainly been used for view =
definitions; there's just a much wider range of usage scenarios now, for =
example:

  <bean id=3D"inheritedTestBean" =
class=3D"org.springframework.beans.TestBean">
    <property name=3D"name"><value>parent</value></property>
    <property name=3D"age"><value>1</value></property>
  </bean>
=09
  <bean id=3D"inheritsWithDifferentClass" =
class=3D"org.springframework.beans.DerivedTestBean"
      parent=3D"inheritedTestBean" init-method=3D"initialize">
    <property name=3D"name"><value>override</value></property>
    <!-- age should inherit value of 1 from parent -->
  </bean>

The child class above uses a different bean class and init method, but =
still inherits the property values from the parent. Alternatively, the =
parent can even omit the bean class completely, simply specifying common =
properties for children:

  <bean id=3D"inheritedTestBeanWithoutClass">
    <property name=3D"name"><value>parent</value></property>
    <property name=3D"age"><value>1</value></property>
  </bean>

  <bean id=3D"inheritsWithClass" =
class=3D"org.springframework.beans.DerivedTestBean"
      parent=3D"inheritedTestBeanWithoutClass" =
init-method=3D"initialize">
    <property name=3D"name"><value>override</value></property>
    <!-- age should inherit value of 1 from parent -->
  </bean>

Of course, such a parent bean cannot get instantiated on its own: It's =
just a template that serves as parent definition for children. =
preInstantiateSingletons will ignore such beans without class and parent =
completely.

I'm aware that this is a significant change in concept, but I've =
experienced repeatedly that this is what users expect it work like - =
being surprised that all they could do was overriding bean property =
values. A colleague of mine has a current use case that I've changed =
this for.

If there any suggestions or objections, feel free to voice them - before =
1.1 RC1 :-)

Juergen


DI J=FCrgen H=F6ller
Senior System Architect
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