Comments on: Open question on dependency injection in Clojure

By: Daniel Glauser

Daniel Glauser — Sun, 07 Mar 2010 02:26:22 +0000

Hi Alex,

I also agree with Chas, but I’d like to add one thing. I have felt for years that dependency injection and aspects were a poor man’s substitute for functional programming. Aspects are a little more powerful but I feel that if you have a language, libraries and framework that are built so they expect behavior to be passed around then you no longer need dependency injection or aspects. In fact, a lot of the patterns that we are used to in the OO space seem to be irrelevant when working in the functional paradigm.

Regards,
Daniel

By: Brenton Ashworth

Brenton Ashworth — Wed, 03 Mar 2010 04:17:09 +0000

I have been working exclusively in Java for past 10 years and love DI when I am using Java. I understand where these questions come from because I struggled with them at first. You are trying to figure out how to write Java programs in Clojure. The fact is, you don’t. In Clojure, you don’t create object graphs and therefore you don’t have to figure out how to mock everything. A large Clojure program will have data and functions and a small amount of explicit state management. There are very few situations where you need to do anything like mocking in Java and for that you can use Clojure’s binding form.

By: Jeff

Jeff — Wed, 03 Mar 2010 02:24:04 +0000

I am just learn Clojure and trying to wrap my brain around it. But this is a great blog post as I too see graphs of objects being injected into Java apps. Sometimes I wonder if the Clojure community actually knows modern Java development and practices, or left years ago and I are unaware of how DI is part and parcel of most JEE development now as is testing. And with testing comes mocking. So yes, I am a newbie, and yes perhaps the light will ‘turn on’ when I get it. But I also fear no one is writing real EE like apps in Clojure and it’s all just another brainy academic exercise. How do you mock out things (lot of things) in Clojure? Guess (I hope) I learn…

By: Patrick

Patrick — Tue, 02 Mar 2010 09:59:56 +0000

The difficulty, I think, is that dependency injection is not usually just about injecting or overriding single dependencies, it’s about binding graphs of values. In the simple case DI lookup seems like a map, where “dataSource” points to the implementation of the DataSource class. But in apps where I see, e.g. Spring being used, the DataSource implementation can itself get a connection pool injected, the pool gets a JDBC driver injected as well as individual properties for pool size, etc. The magic of DI is that if you visualize your (OO) app as a set of object graphs, you can, at runtime, replace either individual nodes or whole sub-graphs dynamically. That’s really what makes it so powerful. In Java this is more or less managable because class fields and class types offer a natural way to organize the mapping of the overrides. So for me the question would be how to override any of a set of functions used within a Clojure app no matter how deep those functions are defined, without making a mess of it.

By: Stuart Sierra

Stuart Sierra — Tue, 02 Mar 2010 01:02:08 +0000

I’m with Chas for the most part. Dependency Injection is a poor-man’s closure. I’ve been thinking recently that using global Vars for configuration is a bad idea in a multi-threaded environment; closures with explicitly-passed parameters are more robust and flexible.