IoC containers, Part 1

Mike Spille wrote an interesting essay about Inversion of Control containers, (with a followup here). He gives a checklist of desirable features in these containers, and surveys HiveMind, Spring, and PicoContainer. PicoContainer comes out worst, partly because it lacks some of the features from his checklist (and in fact, these omissions are deliberate on the part of the creators of PicoContainer).

I have a particular interest in IoC containers, because I developed one at work, and continue to maintain it and enhance it today. I call it the init framework (colleagues often call it components.xml, because its per-module configuration files have that name). Here, I'll just call it TIF.

In terms of generalities, TIF has a lot in common with the well-known IoC containers. It shares many ideas and some specific techniques, and the basic benefits are similar. But these similarities represent a case of convergent evolution. I began work on TIF a little over 2 years ago (the CVS logs say the first check-in was on 2002-07-01; the first lines of code must have been written a few days earlier). If those other IoC containers existed at that time, I wasn't aware of them.

(From the public CVS repositories, it seems that HiveMind originates in May 2003, PicoContainer from mid-2003, and Spring from August 2003, but it is possible that those dates are misleading: the projects may have been hosted elsewhere before moving to their current repositories. In any case, some of these ideas may well have been around earlier; I seem to remember being aware of Apache Avalon, though it was certainly not something I had any desire to imitate.)

So I find it interesting to compare TIF with the open source ones, and to consider why I made the design choices I did and why the authers of the other containers made similar or different choices. Since Mike's essay it perhaps the nearest thing I have seen to a survey of the IoC "market", it provides useful context for these comparisons.

In summary, here is how the comparison comes out:

  • Like HiveMind, TIF has a strong emphasis on XML-based configuration.
  • Like HiveMind and Spring, TIF has strong support for resolving dependencies both by name and by type.
  • Like PicoContainer, TIF supports only constructor-based injection, not setter-based injection. However, there is a twist, which I believe achieves many of the benefits of setter-based injection without raising the difficult questions of that approach.
  • Like PicoContainer, TIF des not support cyclic dependencies. It did support them at one point, but that support was removed, and I have no inclination to put it back.

These last two points stand out, because Mike's article, and the subsequent discussion, criticized PicoContainer for missing precisely these features. Mike's position is that these are examples where the creators of PicoContainer put their purist principles ahead of the pragmatic needs of developers who might use their container. I can't speak for the creators of PicoContainer, but my decisions had very pragmatic reasons behind them: If my container doesn't meet the needs of the real-world projects I work on, my colleagues simply won't use it!

In subsequent posts, I will consider each of these features in turn, and explain how TIF works the way it does. Finally, I plan to write another post to describe notable features of TIF not shared by other containers.

(These posts are partly a response to colleagues who have asked me why I wrote TIF when there are similar open-souce projects around, and why it doesn't support cyclical dependencies. I expect someone will eventually ask me why it doesn't support setter-based injection, and I will be able to point them here.)