Thursday, March 27, 2008

I don't get Spring

Like CrazyBob, of guice fame, I also don't get Spring. Hammet also doesn't get it.
I mean what is the whole idea of wiring by xml? To decouple the wiring from the model.
But if you put the wiring.xml in the jar, so that it lands in the maven repository, to be used by others then it's not really decoupled.

Suppose I have 2 services, with their corresponding interfaces

class Producer implements IProducer { ... }

class Consumer implements IConsumer {
____Consumer(IProducer producer) { ... }

what do we put in the xml file ? Basically we repeat the code-lines.
[bean id="..." class="Producer][/bean]
[bean id="..." class="Consumer"]
____[constructor-arg ref="..." index="0"/]

So not only we write again the required dependency, but we do it in xml so that we don't get
the benefit of refactoring (any changes due to renaming are lost).
I've intentionally left the ids unspecified. What should we put in there? The name of the class or the name of the interface?
- If we put the name of the class, then the Consumer-owner must know which class implements the IProducer interface, so a large part of the benefit of using interfaces is lost.
- If we put the name of the interface, in a declarative way, 'I am an IConsumer and I need an IProducer' then we have really duplicated the constructor line and the class declaration line in XML.

And now what about setter-injection. When I look at a class, I look at the interface it implements and what services it needs (in the constructor) to fulfill that contract. But Spring favors 'Setter Injection'.

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