May 24, 2015

Rapid System Level Development in Groovy

Filed under: ResearchAndDevelopment — Ryan Wilcox @ 2:53 pm

Introduction: Setting the stage

Lately I’ve found myself turning to Groovy for — oddly enough — system level development tasks. This is an unexpected turn of events, and seemmingly mad choice of technologies to say the very least.

Why Groovy?:

  1. I can’t assume OS (so Unix command line tools are out). One of my recent tasks involved something pretty complex, so shell script was out anyway.
  2. I can’t assume Ruby or Python is installed on any of these machines, but I can assume the JVM is installed.
  3. Groovy is a not bad high level language that I’ve also been using for other larger (non system level) programs.
  4. Since I’m on the JVM I can make executable jars, bundling up Groovy and all other dependancies into an easy to run program.

That last point is the real kicker. I want these programs to be easy to run. Even with that, as much as three days ago I wouldn’t have imagined doing programming like this in Groovy.

But this article isn’t about my poor choices in system languages: it’s about a workflow for small Groovy tools, from inception to ending up with an executable jar.

“But, but, what about?”

But, but, what about Go?“. I hear you, and I almost wrote my scripts in Go. Especially with the new easy cross-compilation stuff coming in Go 1.5. I expect to write tools like this in Go in the latter half of 2015 (or: whenever Go 1.5 is released + probably a couple months). I don’t have the patience to learn how cross compilation works today (Go 1.4).

But, but what… executable jars with Groovy?! Aren’t your jars huge?” Yeah, about 6MB a pop. I’ll admit this feels pretty outrageous for like a 50 line Groovy program. I’m also typing this on a machine with 3/4rds of a TB of storage… so 6MB is not a dealbreaker for me at current scale. But it does make me sad.

But, but what about JVM startup costs?” Yup, this is somwhat of a problem, even in my situation. Especially when in rapid development mode. This is another place where I almost wish I was writing in Go (cheap startup and compile times).

But this article is about rapid development in Groovy: going from an idea to releasing an embedable jar – maybe for systems programming, maybe for other things.

Fast, initial development of small Groovy scripts

As a newcomer to the Groovy scene I’ve Googled for this information, and found a couple of disjointed (and in some cases bitrotted) pieces on how to do these things (primarily packaging executable jars created from Groovy source). I hope another person (newcomer or otherwise) finds it useful.

Create your maven structure (which we’ll promptly ignore)

$ mvn archetype:generate -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false -DgroupId=com.wilcoxd -DartifactId=RapidDev (with values appropriate for your groupId and artifactId)

Dig into the generate src/main/java/com/wilcoxd and:

  1. make a new Groovy file
  2. Open the new Groovy file in your editor
  3. Add package com.wilcoxd as the first line of the file. Substitute com.wilcoxd with the groupId you specified in the mvn archetype:generate command.

While semantically, you should rename your java folder to groovy, that doesn’t seem to work with packaging process to create the executable jar. Just leave it be (I guess).

Rapidly develop your Groovy project (with two tricks)

The nice thing about Groovy is that you can write your Groovy program just like you would expect to write a Ruby or Python or Javascript: just type code into a file and Groovy will Figure It Out(TM).

Trick One: develop running your script directly

  1. cd into /src/main/groovy/com/wilcoxd/
  2. Write your script in the body of your .groovy file.
  3. Occasionally pop out to the command line and run groovy RapidDev.groovy (or whatever your script is called)

Groovy does a fair bit of work to execute your (even unstructured!) code. There’s some magic here that I don’t fully understand, but whatever.

$ vi RapidDev.groovy
.... type type type...

$ cat RapidDev.groovy
package com.wilcoxd

println "hey world"

$ groovy RapidDev.groovy
hey world

Crazy talk! No class needed, I don’t even need a public static void main style function!

Trick Two: dependency “management” with @Grab

If you find yourself needing a third party module, use @Grab to get it.

We’ll set things up with Maven, properly, later. Right now we’re concentrating on getting our program working, and turns out we need to make a RESTful API request (or whatever). We just need a third party module.

$ cat RapidDev.groovy
package com.wilcoxd

@Grab(group='com.github.groovy-wslite', module='groovy-wslite', version='1.1.0')

println("hello world!!!")

@Grab pulls in your dependancies even without Maven. I don’t want to introduce Maven here, because then I have to build and run via Maven (I guess?? Again, newbie in JVM land here…). Magic-ing them in with @Grab is probably good enough.

I’m sure Grab is not sustainable for long term programs. In fact, this isn’t a long term proposition: in fact we’re going to remove comment out grab the second we get this script done.

... iterate: type type, pop out and run, type type type...

$ groovy RapidDev.groovy

... IT WORKED! ...

We’re done coding! Now time to Set up pom.xml!

Yay, we’re done. Our rapid, iterative development cycle let us quickly explore a concept and get a couple dozen or a couple hundred lines of maybe unstructured code out. Whatever, we built a thing! Development in the small is nicer, sometimes, than development in the large: different rules apply.

But now we need to set up pom.xml, so it builds a jar for us.

Specify your main class property

Add this as a child of the <project> in your pom.xml:


Adjust the value of start-class as appropriate for your class / artifact ID from the mvn artifact:generate part of this.

Add Groovy and other third party modules you @Grabed into your <dependancies> section

Something like this (with a translated dependency, @Grab syntax to Maven syntax for the wslite module we previously grabbed above), to the <dependencies> section:


Once you have this in your pom, comment out the @Grab declaration in your source file

Add build plugin dependancies (another child of <project>):




            <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">

Know your Groovy code will be groovy, unstructured and all

As mentioned before, Groovy goes to some magic to implicitly wrap a class around unstructured code. In fact, it will use the name of the file as the name of the class (so name your files like you would name Java classes!).

In our example, we’ve been editing RapidDev.groovy, which Groovy will wrap up in a class RapidDev declaration… or something. That package com.wilcoxd means Groovy will actually wrap our unstructured code into a class com.wilcoxd.RapidDev… which is a fine name and what we specified in our pom’s start-class property.


With a simple mvn package we can bundle our Groovy script up to an executable jar. A java -jar target/RapidDev-1.0-SNAPSHOT.jar runs it.

Which is awesome! I can take this and run it on any system with the JVM! I can write my “complex” systems level program once and run anywhere! I can reach deep into the Java ecosystem for spare parts to make my development easier, and still have a rapid development cycle one expects out of Python or Ruby.

Pretty neat!