How do I override a Spring bean definition yet still reference the overridden bean?

Question

I'm attempting to implement a delegate Service provider by overriding the bean definition for the original service with my delegate Service. However, as the name would imply, the delegate Service needs a reference to the original service to delegate calls to.

I'm having trouble figuring out how to override the bean definition while using the original bean def without running into a circular reference issue.

For example:

<!-- Original service def in spring-context.xml -->
<bean id="service" class="com.mycompany.Service"/>

<!-- Overridden definition in spring-plugin-context.xml -->
<bean id="service" class="com.mycompany.DelegatedService"/>
    <constructor-arg ref="service"/>
</bean>

Is this possible?

Answer 1

The short answer to your question is that you cannot have two bean definitions with the same name. If you try, one will hide the other, and only one definition will be usable.

Your question's example seems to suggest that you're trying to wrap the original service bean in a proxy object, with the wrapper performing some before-and-after work around calls to the service. One way to achieve this, without defining two service beans, and without modifying the original service bean, is to use a Spring AutoProxyCreator, probably a BeanNameAutoProxyCreator.

This allows you to list a bean (or beans) that are to be automatically proxied. You specify the interceptors you want to be applied to invocations on the target bean. You would implement these interceptors to do the work you need to do.

Spring would automatically create a delegating proxy for you, which would have the bean id service as before, but with your additional functionality.

Answer 2

You can create proxies and interceptors. So now the bean named service will become a proxy to the original service which needs to be renamed to something else. So the changes will be limited to the Spring XML only and not be propagated to your java code.

<bean id="personTarget" class="com.mycompany.PersonImpl">
    <property name="name"><value>Tony</value></property>
    <property name="age"><value>51</value></property>
</bean>

<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
    <property name="someProperty"><value>Custom string property value</value></property>
</bean>

<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor">
</bean>

<bean id="person" 
    class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="proxyInterfaces"><value>com.mycompany.Person</value></property>

    <property name="target"><ref local="personTarget"/></property>
    <property name="interceptorNames">
        <list>
            <value>myAdvisor</value>
            <value>debugInterceptor</value>
        </list>
    </property>
</bean>