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Re: Dependency injection in Scala?

by Alex Boisvert-3 :: Rate this Message:

Hi Matt,

I agree with you Scala by itself (as it is today) it's not an ideal solution, but from my experience it beats using Spring/Guice. The main advantages I see are,

1) No "impedance mismatch" between Java and Scala idioms; you can integrate code that follows different dependency-injection approaches
(constructor-injection, setter-injection, service lookup, ..., cake pattern, mix of Java and Scala)
2) Stronger typing (no XML or annotations)
3) No additional framework to learn
4) You have the full power of Scala at your disposal so you have more options for the structuring of configuration itself

I'm also hopeful that the situation will improve with additional language support such as virtual classes, first-class properties, ideas from Sean McDirmid's SuperGlue language, etc.

alex

On 2/1/08, Matt Hellige <matt@...> wrote:

I agree with other replies, but in a more guarded way. I think the
need for a DI framework is somewhat reduced in Scala, as compared to
Java. But I definitely don't believe that the need is completely
eliminated. There are many things that Spring does that are not easy
to achieve without a framework. So I do advise that you look into the
patterns that people mentioned, but I also expect that people will
continue to use Spring/Guice/etc. with Scala and that eventually we
will see a new approach to DI that is a more natural fit with Scala's
naming conventions and language features.

Matt

On Feb 1, 2008 11:28 AM, Alex Boisvert <boisvert@...> wrote:
>
> On 2/1/08, Stepan Koltsov <stepan.koltsov@...> wrote:
>
>
> > Is there any way to write code in Scala as in Java+Spring? I. e. with
> > dependency injection of components.
> >
> > It is hard to write large applications without DI.
> >
> > Should DI be done in pure Scala some way, or there is any framework,
> > or Spring itself should be used?
>
> Hi Stepan,
>
> I see two sides to this story,
> -How to structure code to make it more configurable? (parametric)
> -How to wire parts together and provide configuration
>
> Scala's type system and concise syntax help you on both aspects compared to
> Java.  I've posted a small example yesterday [1].
>
> Similarly to Java and other OO programming languages without open classes,
> you want to reduce as much as possible static linking inside your
> application.  This typically implies two practices.  First, using object
> factories instead of calling constructors directly; and second, minimizing
> the use of singleton objects.  These practices apply to any part or behavior
> that you want to be able to reconfigure.  It's okay to rely on static
> linking for things that you don't expect (or want to) change.   The "Cake
> Pattern" paper [2] discusses some of these considerations.
>
> In practice, I've found that using Scala scripts (using the interpreter) was
> a better solution than Spring to wire pieces together and inject
> configuration parameter inside my applications.   There are some issues with
> using Spring since Scala conventions for class naming, method naming
> (JavaBeans-style setters/getters) and overall application structuring tend
> to differ from Java.  Some simple things work but you'll run into issues
> with most complex scenarios.   As far as I know, nobody has tried to resolve
> these issues or is working on a Scala dependency-injection framework.
>
> Hope this helps,
> alex
>
> [1]
> http://www.nabble.com/Cake-pattern-and-dependency-injection-td15199303.html#a15208024
>  [2] http://lamp.epfl.ch/~odersky/papers/ScalableComponent.pdf

--
Matt Hellige / matt@...
http://matt.immute.net

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