Java is a general-purpose programming language that matured over more than ten years. It provides a solid platform on which many third party libraries (of various quality and complexity of course) were developed. Maven is one of the several ways large Java projects can be described formally and built automatically. Maven has the ability to manage dependencies a lot better than the traditional way of bundling everything together in a large archive and aims at simplifying and unifying build process, although advanced configuration quickly drifts into XML nightmares. Then comes Eclipse, an integrated development environment. Although not perfect, very far from it, Eclipse has been a time saver for me, especially when comes time to search into large code bases and refactor code. Eclipse is designed for Java, and it has a plugin to integrate well with Maven, called M2Eclipse. We can safely state that Java, Maven and Eclipse play well together.
Then comes Groovy, a language built on top of Java. Source code in the Groovy language is compiled into byte-code like Java is, and the byte-code can run under the same virtual machine as regular Java programs, with the exception they need a set of runtime classes and the generated byte-code has some more indirections as compared to one generated by a traditional Java compiler. As a Java extension, we would expect Groovy to play well with Maven and Eclipse. Well in practice, I found out not to be exactly the case.
I experienced what follows with Eclipse Kepler, Groovy 2.2 and Maven 3. Things may have been better with older versions or with newer ones, that will have to be seen.
Groovy and Eclipse, almost well but…
First time you will try to write a Groovy program in Eclipse, you will notice that there is absolutely no IDE support for that language. You won’t be able to use any code assist and Eclipse will not allow to compile or run Groovy code for you. You will need to install an extension to get Groovy support. This is the Groovy Eclipse plugin. The plugin works relatively well, but it has a couple of annoying drawbacks.
First, code completion works in random erratic ways. I sometimes get tired and turn it off. For example, I had a variable of type String. I knew it was a String and the IDE had the ways to know, because I declared the type of the variable in my code. In Groovy, you can use variables without declaring the type. However, when I was trying to get proposed completions for to, I was getting toUpperCase() but not toLowerCase(). This was completely arbitrary.
When running a Groovy script, the arguments in the launch configuration get prepopulated with a list of standard stuff that you must not delete. If you want to pass your own arguments to your script, you have to append them at the end of what the Groovy extension inserted in the Arguments box and you need to be careful not to delete the predefined stuff if you completely replace your custom arguments.
Debugging Groovy code in Eclipse is like playing Russian roulette. Sometimes you can print the contents of a variable, sometimes you cannot; you don’t know when it will fail and why. Sometimes you can expand an object and see its fields, sometimes the + icon is not there and you cannot expand, again for no obvious reasons. Execution may step into closures or may not, you don’t know, at least I didn’t. You can work around by putting breakpoints in the closures, but when you go out the closure, you end up in strange places of the execution within internals of Groovy. Conditional breakpoints never worked, at all, so I had to constantly pollute my code with insane if (some condition) println(“Bla”) and be careful to remove all the junk after I’m done debugging.
Error messages are sometimes cryptic. If you are unlucky enough, you can even manage to get an Internal error from the Groovy Eclipse compiler! I was getting that in one of my classes and had to disable static type checking for that class to get rid of it.
On Monday, August 4th 2014, things went completely south after I upgraded my build to Groovy 2.3. Everything was working fine with Maven on the command line. Eclipse was compiling the code fine. I set up the project to use Groovy 2.3 and there was no issue. However, when running the project, I was getting the following runtime error.
Conflicting module versions. Module [groovy-all is loaded in version 2.2.1 and you are trying to load version 2.3.6
I looked at my POM file, analyzed Maven dependencies with both mvn dependency:tree and Eclipse, found no Groovy artifact except the 2.3.6 one, verified my PATH to make sure only Groovy 2.3 was on it, checked Eclipse preferences many many times, restarted Eclipse several times, to no avail. There seems to be something in the Groovy Eclipse plugin hard-coded for Groovy 2.2, even if the compiler is set to 2.3!
Any search on Google is yielding results about Grails and Spring, as if nobody is using Groovy alone anymore, only with other frameworks. Nobody else seems to be having the issue.
Maven + Groovy = fire hazard!
Maven relies on plugins to perform its tasks, so the ability to build something with Maven depends on the quality of the plugins. There is unfortunately no official well known, well tested and stable plugin to build Groovy stuff using Maven. The page Choosing your build tool gives a good idea of what is currently available.
First I read about GMaven, but I quickly learned it was not maintained anymore, so I didn’t try to use it. Then I read that the Groovy Eclipse Compiler was the recommended one. I was a bit reluctant, thinking this was a piece of hackery that would pull out a bunch of dependencies from Eclipse, resulting to an heavy weight solution. But this was in fact well isolated and just the compiler, no need to pull the whole Eclipse core!
Maven Eclipse compiler worked well a couple of months for me. However, yesterday, things went south all of a sudden. First, there were compilation errors in my project that would not show up into Eclipse but appeared when compiling with Maven. These were error messages related to the static type checking. After fixing these, compilation went well, but all of a sudden, at runtime, I was getting a ClassNotFondError about ShortTypeHandling. I read that this class was introduced by Groovy 2.3 while my project was using Groovy 2.2. Digging further, it seemed that the Groovy Eclipse Compiler was pulling Groovy 2.3, compiling code against it but the code was executed with Groovy 2.2. This should in principle not cause any problem, but it seems that in Groovy, byte-code is not fully compatible between versions!
I tried updating my dependency to the Groovy Eclipse Compiler in the hope that would fix the issue. However, that changed my ShortTypeHandling exception for stack overflows. It happened that the clone() method of one of my class was calling super.clone(), which is perfectly normal. But Groovy was making something nasty that was causing super.clone() to recursively call clone() of my subclass! This resulted to an infinite loop causing the stack overflow.
I found this issue to be even more intricate after I tried to compile my code on JDK8 and found it out to be working correctly. In other words, the JDK was affecting how Groovy Eclipse Compiler was building the byte-code!!! In JDK7, something would fluke the byte-code, causing the stack overflow errors, while in JDK8, everything would go fine!
I then tried updating the compiler once more, to the latest and greatest. Things compiled, but I was back at square one with the ShortTypeHandling exception! So no matter what I was trying, Maven was unable to build the project anymore.
I was about to give up on Maven and use a batch file to call Groovy directly, but that would have been a lot of fiddling with the class path. I was not happy at all about this solution.
Then I found out about the GMavenPlus plugin. I tried it and it worked like a charm! The plugin makes use of the Groovy artifact defined in the project’s dependencies rather than hard-coding a compiler for its own version of Groovy. It uses the official Groovy compiler API rather than its own, so things get compiled the same way as when using the Groovy Ant task or the standalone groovyc compiler. GMavenPlus saved my day yesterday, freeing me from a lot of hassle.
Is it worth it?
I’m not sure at all. I got several problems with Groovy that would deserve a separate post. The integration difficulties with Maven and Eclipse make me believe it is better just to use Java directly. JDK8 introduced lambda expressions that fulfill a part of what Groovy is trying to implement in its own special way. For projects that really need a true scripting language, there are already several of them, like Python, which is built from the basis for scripting.