"Convention over configuration" is a popular paradigm in developer circles these days. Instead of explicit instructions over every detail or action, you would follow the framework's conventions and get most of the work done implicitly and for free. Ruby on Rails made this idea really take off, and we see it in action in many frameworks. The popular build tool Maven also adopts convention over configuration, and in the process demonstrates how not to do it.
I've had a pretty rough time getting comfortable with Maven. This was a green field Java project: we were starting absolutely from scratch. There were no legacy build processes to support, so we were ready to follow Maven convention everywhere. Nevertheless, it has been a long, painful (one colleague said "character building") journey. Why did it have to be so hard?
(Before somebody uses the term "learning curve" here: do you know what exactly is a "learning curve"? I don't. What is measured on the X and Y axis, and why is a "steep learning curve" supposed to be a bad thing?)
A big part of the problem has to with discoverability. This is the obvious trade-off if you decide to go with convention over configuration: that which is explicit and discoverable is now implicit. The Java world began with explicitness, and the Java language itself is often painful in its verbosity. But in that verbosity, most people could eventually figure out what is going on. This is also true with Maven's explicit counterpart, Ant: you can follow an Ant script in all its ugly glory and figure out what it does. Maven by contrast drops you in a maze of twisty passages, all alike.
Consider how you would figure out an Ant script. You could do it the hard way, by reading it. Or if it's well designed you could type "ant -p" to get a list of targets. But in either case, the answer is in front of you in the XML somewhere. If you need help, most of Ant's tasks in common use are documented right in the manual. Maven? The conventions for directory structures are easy enough, but the actions are quite opaque. You just have to float in the Maven sea long enough to absorb things. For example, suppose you want to do a quick incremental build. How do you turn off unit tests?
Is it -Dmaven.tests.skip=true?
Is it -Dmaven.skip.test=true?
Is it -Dmaven.run.tests=false?
Let me try "mvn --help". Gee, that was helpful.
Maven has a huge constellation of plugins, and since it can load them on demand, the universe of possible actions, options and goals is endless. While having a huge feature set can be a good thing, the trouble is figuring out which of those plugins are relevant to what you are doing. Conventions are good if they are limited and obvious. Maven is neither. If you have to Google for help to do routine build tasks, maybe something is wrong.
Going our own way
Happy families are all alike; every unhappy family is unhappy in its own way
-- Leo Tolstoy
Tolstoy might as well be talking about Maven. One problem is that software builds have a lot of variability. Marrying a convention-based tool to a problem domain that has significant variability leads to a lot of teams that are unhappy in their own way. Consider the task of deploying a webapp. What command do you use? I hope one of these works: "mvn deploy", "mvn deploy:deploy", "mvn jboss:deploy", "mvn war:deploy", "mvn tomcat:deploy", "mvn tomcat:redeploy".
There is a certain magical quality to Maven, and I don't mean in a good way. Classic short fairy tales have a certain arbitrary quality to them. You need to know the right incantation at the right time and at the right place, or suffer a horrible end. Maven is something like that. Say "Open Sesame", and the cave will open. Kiss that particular frog and find wedded bliss. Type "sql:execute" in that particular directory, and your DB will get initialized.
There is one common complaint that I don't hold too strongly against Maven: its performance. This is because I've concluded that the key to developer productivity is to bypass the build tool as much as possible. I've documented this somewhat when I was using Ant, and it is all the more important for a banana slug like Maven. I know Maven's developers are working on its performance. Still, there is no way it can be as fast as a properly configured Eclipse setup that will compile and deploy any Java class as fast as I can save it. (This applies to editable, non-compiled resources too.) My approach to a Maven project is to understand the build well enough to escape Maven most of the time.
Maven's huge universe of possible invocations, the absence of discoverability, the lack of unified documentation and the difficulty in figuring out the relevant action in a particular context means it's generally hard to figure out what to type after the "mvn" on the command line. Convention over configuration works really well if you can know the convention. If the convention is neither limited nor discoverable, you're lost.
The way I see it, you have a certain "complexity quota" in any project. Once you exceed that complexity quota, working within that project becomes a burden because it becomes impossible to keep all the relevant instructions in your head at once. Every convention you adopt adds to the required knowledge of the project, increasing the likelihood of exceeding that complexity quota. The decision to adopt Maven must trade off the consumption of that complexity quota that could have been allocated elsewhere.
Still, Maven's popularity demonstrates that it has a lot of strong points. And I do appreciate its power. In theory, everything sounds great. "Tell me what you want", Maven says soothingly, "and I will take care of you". Unfortunately, trying to tell Maven what I want often finds me tongue-tied, and what Maven ultimately decides to do is often not what I want. I hope this pain was ultimately worth it.