Intentionally setting a Spring bean to null

Question

I'm using Spring to inject JMS connection factory into my Java application. Since this factory is only required within the production environment, not while I'm developing though, I put the bean definition into a separate XML which I include into my main applicationContext.xml. In production environments this extra file contains the regular bean definition. In my local dev environment I'd like this bean to be null. Trying to simply remove the bean definition all-toghether obviously caused an error when Spring came across a reference ID it didn't know.

So I tried creating a factory bean that would simply return null. If I do this, Spring (2.5.x) complains that the factory returned null although based on the Spring API doc of the FactoryBean interface I expected this to work (see Spring API doc).

The XML looks something like this:

<bean id="jmsConnectionFactoryFactory" class="de.airlinesim.jms.NullJmsConnectionFactoryFactory" />

<bean id="jmsConnectionFactory" factory-bean="jmsConnectionFactoryFactory" factory-method="getObject"/>

What would be the "correct" way of doing this?

Answer 1

I'm pretty sure that Spring won't allow you to associate null with a bean id or alias. You can handle this by setting properties to null.

Here's how you did this in Spring 2.5

<bean class="ExampleBean">
    <property name="email"><null/></property>
</bean>

In Spring 3.0, you should also be able to use the Spring expression language (SpEL); e.g.

<bean class="ExampleBean">
    <property name="email" value="#{ null }"/>
</bean>

or any SpEL expression that evaluates to null.

And if you are using a placeholder configurator you could possibly even do this:

<bean class="ExampleBean">
    <property name="email" value="#{ ${some.prop} }`"/>
</bean>

where some.prop could be defined in a property file as:

some.prop=null

or

some.prop=some.bean.id

Answer 2

factory-bean/factory-method doesn't work with null, but a custom FactoryBean implementation works fine:

public class NullFactoryBean implements FactoryBean<Void> {

    public Void getObject() throws Exception {
        return null;
    }

    public Class<? extends Void> getObjectType() {
        return null;
    }

    public boolean isSingleton() {
        return true;
    }
}

<bean id="jmsConnectionFactory" class = "com.sample.NullFactoryBean" />

Answer 3

For anyone coming to this question, keep in mind that simply setting the @Autowired annotation as optional will do the trick (i.e. Spring will leave the reference null if no qualifying bean is found).

@Autowired(required = false)
private SomeClass someBean

Note that you would have to do this everywhere the bean is referenced, which may be a bigger hassle than creating a null-factory as mentioned above.