Instantiation of objects with dependencies as needed

[kenward]

I'm trying to use spring to instantiate an object with dependencies as the need arises (based on certain runtime conditions).

something along the lines of

getEvent()
{
if event is of type A
instantiate AHandler using spring
else
instantiate BHandler using spring
}

Is there a better option than having my class implement ApplicationContextAware?

-Jeff

[slkf]

From what it looks like, you are trying to use a handler to deal with some event. Unfortunately, there really is no way to 'lazy' initialize a dependency in this manner without being context aware. You can refer to the reference manual to see exactly how Spring does support lazy initialization (Section 3.3.4 for version 2.5.6, and section 3.3.3 - topic 4 for version 3.0.0).

The best way you can achieve this is to build an event handler map in the Spring configuration, and inject this map into your bean. Depending on the type of event, you will look up the respective handler in the map. Just a simple map will do the trick, no need for anything special:

<bean
    id="yourBean"
    class="what.ever">
    <property name="handlerMap">
        <map>
            <entry key="eventCode1" value-ref="handler1" />
            <entry key="eventCode1" value-ref="handler2" />
            ....
        </map>
    </property>
</bean>

<bean
    id="handler1"
    class="what.ever.Handler" />

<bean
    id="handler2"
    class="what.ever.Handler" />

...

You can read more about 'collection' injection in the reference manuals (section 3.3.2.4 for version 2.5.6, and section 3.4 'Dependencies and configuration in detail' > 'Collections' for version 3.0.0).

And as a reminder about performance tuning,

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. - Donald E. Knuth

For more details on premature optimization, I refer you to Joshua Bloch's Effective Java 2nd Edition Item 55 "Optimize judiciously" (Effective Java 2nd Edition).

Give this technique a try, and let me know if it works for you!

- SLKF