IvyとMaven2の比較

Maven2と比較してどうなの？とは度々聞かれることである。ここに我々の意見を述べておこう。

ただ、明らかにこの手の比較はバイアスされる（ここはIvyのサイトだよ）けれども、できる限りフェアにいこう。 このページに何か抜けがあったり間違いがあったりしたら知らせてほしい。 またcodehausのMaven2機能比較ページを見てもよいだろう。これは別の視点を与えてくれる。

さらに、この話にはさまざまなディスカッションがなされている。その中でも among which the one triggered by spring contemplating about switching to maven is may be the more interesting.

しかしここでは、Maven2とIvyの大きな違いと思うところを指摘しよう。

木とリンゴを比較する

まず最初に、もっとも重要な違いというのは、それらが同じ種類のツールではないということだ。 Maven2はソフトウェアプロジェクト管理と統合化初ツールであるのに対し、Ivyは依存関係管理ツールであり、それはもっともポピュラーなビルド管理ツールであるAntと高度に統合されている。 したがって、もう少し面白い比較というのは、Ant+IvyとMaven2になるかと思うが、これはこのページの範囲を大きく超えてしまう。 ここでは依存関係管理にだけ集中しよう。

Different concepts Ivy heavily relies on a unique concept called configuration. In ivy, a module configuration is a way to use or to see the module. For instance, you can have a test and runtime configuration in your module. But you can also have a mysql and an oracle configuration. Or an hibernate and a jdbc configuration. In each configuration you can declare what artifacts (jar, war, ...) are required. And in each configuration, you can declare your dependencies on other modules, and describe which configuration of the dependency you need. This is called configuration mapping, and it is a very flexible way to answer to a lot of problems we face very often in software development.

Maven2 on its side has something called the scope. You can declare a dependency as being part of the test scope, or the buildtime scope. Then depending on this scope you will get the dependency artifact (only one artifact per module in maven2) with its dependencies depending on their scope. Scopes are predefined in maven2 and you can't change that. No way to create an oracle scope. No way to indicate you need what has been declared to be needed in the runtime scope of your dependency in your compile one. Everything here is written in the marble.

And this leads to some kind of troubles... as Matt Raible stated in his blog talking about maven2 dependencies:

• There are a *lot* of unnecessary dependencies downloaded for many libraries. For example, Hibernate downloads a bunch of JBoss JARs and the Display Tag downloads all the various web framework JARs. I found myself excluding almost as many dependencies as I added.

The problem is that hibernate can be used with several cache implementations, several connection pool implementation, ... And this can't be managed with scopes, wheres Ivy configurations offers an elegant solution to this kind of problem. For instance, in ivy, assuming hibernate as an ivy file like this one, then you can declare a dependency like that:

<dependency org="hibernate" name="hibernate" rev="2.1.8" conf="default->proxool,oscache"/>

to get hibernate with its proxool and oscache implemetations, and like that:

<dependency org="hibernate" name="hibernate" rev="2.1.8" conf="default->dbcp,swarmcache"/>

to get hibernate with dbcp and swarmcache.

Documentation An important thing to be able to use a tool is its amount of documentation. With Ivy, even if they are written in broken english (would you have preferred well written french :-)), the reference documentation is extensive and covers all the features including many examples. We also provide some official tutorials which are maintained with the new versions of Ivy. And since we consider documentation so important, we also provide online versions of documentation per Ivy version since Ivy 2.0.0-alpha2.

With maven2, it's a bit difficult to clearly know what can be considered as dependency management documentation, but we didn't managed to find much: some small introductory guides, short entries in the pom reference guide, and not really much more. Even in the maven book you can get for free on mergere website, the insight about dependency management is still light in our point of view.

Conflict management Conflict management are an important part of dependency management, cause when dealing with transitive dependencies you often have to face conflicts. In this area, Ivy let you do whatever you want: use one conflict manager in one module, another one elsewhere, decide which revision you will get, ... You can even plug your own conflict manager if you need to.

With maven2, conflict management is quite simple: the principle is to get the nearest definition. So if your module depends on foo 1.0, none of your dependencies will ever manage to get foo 1.1 without a change in your own dependency declaration. It may be ok in some cases, it may not in others...

Flexibility In Ivy many things can be configured, and many others can be plugged in: dependency resolvers, conflict manager, module descriptor parser, latest revision strategy, ...

Maven2 also offers repository pluggability, but not much more as far as we know. Moreover, repository configuration is much less flexible than with Ivy: no repository chaining, no way to split metadata and artifacts in multiple repositories, ...

Public Repositories Maven2 comes out of the box configured to use maven2 repository, which contains a lot of modules (both artifacts and module descriptors). The only problem some may face is that module descriptors are not always checked, so some are not really well written. Ivy being compatible with maven 2 metadata, the default public repository used is also the maven 2 repository, which is fine for a good out of the box experience.

However, we don't recommend to use such a public repository for an enterprise build system, and as such Ivy provides features and documentation to build your own enterprise repository based (or not) on data available in the public repository.