- The Groovy Programming Language

In a series of articles labeled “” I’ll be discussing modern alternatives to the Java programming language for use with the Java Platform. This is the first installment of the series - “The Groovy Programming Language”.


We all know and love the Java platform(note the emphasis on platform) for a couple of obvious reasons. Most notably it features a huge high quality standard library and a legendary execution environment - the Java Virtual Machine(JVM). The JVM is known to posses the following qualities:

  • Runs on all major platforms
  • Enterprise ready
  • Extremely stable
  • Extremely fast
  • Highly customizable - many aspects of its work can easily be adjusted such as GC settings, heap settings, etc.

There is a terrific community around the Java platform that has contributed an immense amount of high quality software vital to the success of Java. Hibernate, Spring, Eclipse, NetBeans, the myriad of Apache projects are all community efforts.

So far so good, but it’s not all rainbows and unicorns in the land of Java. The Java programming language is a common source of gripe amongst many developers for various reasons. I’ll list here some of the more notable of them:

  • It’s not a pure OOP language(there is a difference between primitive and reference data types)
  • It uses static typing(highly subjective topic, but commonly brought up)
  • It doesn’t have support for closures(which instantly kills half of functional’s programming ideas such as higher order functions)
  • It has limited meta programming capabilities(compared to Ruby and Lisp for instance)
  • There is no concept of top-level procedures(outside of class definitions)(this makes Java unsuitable for creating “script” programs)
  • Its syntax is too verbose
  • It’s not suitable for the creation of DSLs
  • Its development is sluggish and restrained by the corporate promise of backward compatibility
  • It’s an imperative language
  • Its parallel programming model is revolving mostly around locks and threads

Naturally there are some languages that alleviate some(most) of Java’s problems such as:

  • Ruby
  • Python
  • Lisp
  • Erlang
  • Haskell

They all, however, lack(with their default implementations at least) an execution environment that can match the JVM. They also lack Java’s immense amount of libraries currently available. When enthusiasts started porting existing languages to the JVM this came as no surprise

  • it was only logical. A good language get a good execution environment. For instance some folks ported Ruby to Java which resulted in JRuby, Python also got a Java port called Jython. Though a lot of good has come from such ports they was also a price to pay. In the case of Ruby and Python some libraries are not written in Ruby/Python, but in C for performance reasons, which naturally leads to problems when you factor in a Java implementation of the language. There is also the fact that the JVM(currently) doesn’t have support for dynamic memory dispatching which is feature vital for dynamically typed languages to get a decent performance. For this reason Jython is notoriously slow. The JRuby team did a much better work, circumvented a lot of the JVM limitations and actually managed to create a Ruby implementation that in some scenarios beats the performance of the standard MRI Ruby 1.9 with it’s custom YARV virtual machine implemented in C. JRuby will be the object of a further discussion down the road. Some of the languages that were ported to the JVM suffer from another problem as well - it’s not straightforward and efficient in them to reuse existing Java libraries directly.

I’d like to lead this discussion in another direction, however - programming languages that were implemented from scratch with the JVM as their execution environment. While there are many of those three stand out and have gathered a significant momentum in recent years. They will be the subject of this post and the following two. Without further adieu I’d like you to meet Groovy, Scala and Clojure.

We’ll begin our discussion with Groovy…


Before I start I’d like to point out that my knowledge of Groovy is limited compared to my knowledge of Scala and Clojure. Despite this I decided to share my thoughts on the language because it’s certainly one of the most popular JVM languages out there. If I’ve made any errors in the article I didn’t mean to insult anyone with my ignorance and I’ll certainly be glad to fix them when someone points them out.

A little bit of Groovy history

James Strachan first talked about the development of Groovy in his blog in August

  1. Several versions were released between 2004 and 2006. After the JCP(yep, you read that correct - Groovy is actually a Java standard which is great or scary depending on your point of view) standardization process began, the version numbering was changed and a version called “1.0” was released on January 2, 2007. After various betas and release candidates numbered 1.1, on December 7, 2007, Groovy 1.1 Final was released and immediately rebranded as Groovy 1.5 as a reflection of the many changes that were made.

In July 2009, Strachan wrote on his blog that “I can honestly say if someone had shown me the Programming in Scala book by Martin Odersky, Lex Spoon & Bill Venners back in 2003 I’d probably have never created Groovy.” Strachan left the project silently a year before the Groovy 1.0 release in 2007. Leadership of the project was assumed by Guillaume Laforge (Project Manager and JSR-241 Spec Lead). Under his guidance Groovy thrived and his risen to be arguably the most widely used JVM language apart from Java.

Currently the development of Groovy proceeds with a very fast pace and the latest major update 1.8.0 was released just a couple of days ago.

Installation & Getting started

Platform independent installation These instructions describe how to install a binary distribution of Groovy.

  • download a binary distribution of Groovy and unpack it into some file on your local file system
  • set your GROOVY_HOME environment variable to the directory you unpacked the distribution
  • add GROOVY_HOME/bin to your PATH environment variable
  • set your JAVA_HOME environment variable to point to your JDK. On OS X this is /Library/Java/Home, on other unixes its often /usr/java etc. If you’ve already installed tools like Ant or Maven you’ve probably already done this step.

For instance here’s the relevant information of my shell’s configuration:

export JAVA_HOME=/usr/java/latest
export GROOVY_HOME=/opt/groovy-1.8.0

You should now have Groovy installed properly. You can test this by typing the following in a command shell:

$ groovysh

Which should create an interactive groovy shell where you can type Groovy statements. Or to run the Swing interactive console type:

$ groovyConsole

To run a specific Groovy script type:

$ groovy SomeScript.groovy

Linux installation

Most Linux distributions provide Groovy through their integrated package management system. On Debian(and derivatives like Ubuntu) you can install it like this:

$ sudo apt-get install groovy

On Red Hat systems the magic incantation looks like this:

$ sudo yum install groovy

Personally I’d prefer the platform-independent installation method, since some distribution package Groovy in a non-standard manner which confuses IDEs for instance.

Windows installation

Groovy features a native Windows installer.

Meet Groovy

“Groovy is like a super version of Java. It can leverage Java’s enterprise capabilities but also has cool productivity features like closures, builders and dynamic typing. If you are a developer, tester or script guru, you have to love Groovy.”

–Praise for Groovy,

Groovy is:

  • is one of the two standard languages for the JVM(the other is Java of course)
  • is an agile and dynamic language for the Java Virtual Machine
  • builds upon the strengths of Java but has additional power features inspired by languages like Python, Ruby and Smalltalk
  • makes modern programming features available to Java developers with almost-zero learning curve
  • supports Domain-Specific Languages and other compact syntax so your code becomes easy to read and maintain
  • makes writing shell and build scripts easy with its powerful processing primitives, OO abilities and an Ant DSL
  • increases developer productivity by reducing scaffolding code when developing web, GUI, database or console applications
  • simplifies testing by supporting unit testing and mocking out-of-the-box
  • seamlessly integrates with all existing Java classes and libraries
  • compiles straight to Java bytecode so you can use it anywhere you can use Java

Some of Groovy’s most compelling features are:

  • Pure OOP language
  • Mostly Java compatible syntax
  • Optional typing
  • No need to wait for a future version of Java to get:
    • Closures
    • Attributes
    • Smart switch
  • Duck typing
  • BigInteger based arithmetic
    • This deserves some special explanation because of a rather strange design decision in Groovy. Groovy will create a BigIntiger out of a large enough number literal, but it won’t promote the result of an integer operation into BigInteger - the result will actually overflow, in contrast to the semantics of most other dynamically typed languages. Multiply 1000 * 1000000000 and you will end up with -727379968 in Groovy.
  • SQL, XML & Swing improvements
  • Unified data access API

A core idea, guiding the design of Groovy, is making it easy to use for existing Java developers. Groovy’s designers have gone so far in that direction that the Groovy compiler will happily compile most Java source files without the need for any modifications. Groovy, however, builds heavily upon the standard Java’s syntax and you’ll do well to get a grip of Groovy’s core idioms. Groovy’s syntax in a nutshell:

// old school Java code, but also valid Groovy code
System.out.println("Hello, world!");

// idiomatic Groovy
println "Hello, world!"

// dynamic variable definition
def name = "Bozhidar"

// GString featuring string interpolation
println "Hello, $name"  // => "Hello, Bozhidar"

// statically typed variable
String songName = "Coding in the Name of"

println "Now playing - $songName"

String multiline = """this is a multiline
string. There is not need to embed
newline characters in it"""

println multiline

// method definition
def greet(name) {
    println "Hello, $name!"

// method invocation
greet "Bozhidar"

// safe dereferencing
def showSize(list) {
    println "List size is: ${list?.size}"

showSize([1, 2, 3])
// this is the important part

// a list
def beers = ["Zagorka", "Bolyarka", "Shumensko", "Ariana"]

// list access
println "My favourite beer is ${beers[1]}"

beers.each { beer -> println beer }

// imports can appear anywhere and support the creation of aliases
import static java.util.Calendar.getInstance as now
import java.sql.Date as SDate

println now()
// java.util package is automatically imported in Groovy so this is java.util.Date
println new Date()
println new SDate(2011, 5, 5)

// language support for regular expressions
if ("Hello, Groovy" =~ /\w+,\s\w+/) {
    println "It matches"

// range filtering with higher-order functions
(1..10).findAll { n -> n % 2 == 0}.each { n -> println n }

// map
def capitols = [Bulgaria: "Sofia", USA: "Washington", England:"London", France:"Paris"]

println capitols["Bulgaria"] // => Sofia
println capitols["France"]  // => Paris

// class definition
class Person {
    def name
    def age

    Person(name, age) { = name
        this.age = age

    String toString() {
        return "Name {$name}, age {$age}"

def me = new Person("Bozhidar", 26)
println me

From the brief overview you might have noticed that like Ruby and Python Groovy has language support for commonly used data structures such as lists, maps, ranges and regular expressions:

  • List - def number = [1, 2, 3]
  • Map - def countries = [BG: “Bulgaria”, DE: “Germany]
  • Range - def range = 1..1000
  • Regular expressions - def whitespace = /\s+/

You might have noticed another interesting feature in Groovy - the ability to combine static type(like in Java) with dynamic typing(like in Ruby and Python). The keyword def is used to introduce dynamically typed variables in Groovy.

Groovy comes with its very own standard library(GDK) which builds upon the JDK(for instance the File and String classes are enhanced in Groovy) and offer some new features like the Groovy’s famous builders.

OOP in Groovy

Groovy is a pure object oriented language. Everything is an object, operations are methods, etc. Compared the Java the picture looks right about this way:

  • Similar capabilities to Java
    • Define classes, interfaces, enums, annotations
  • Differences to Java
    • Classes (and interfaces etc.) public by default
    • Methods public by default
    • Property support within classes (auto-setters/getters)
    • Duck typing

Java developers should feel mostly at home.

SQL and XML handling

Groovy offers some nice improvements over JDBC and JAXP for handling database queries and XML parsing.

Groovy removes a lot of boilerplate when dealing with SQL queries compared to the native JDBC API:

import groovy.sql.Sql
sql = Sql.newInstance("jdbc:mysql://host/db", "username", "password", "com.mysql.jdbc.Driver")
sql.eachRow("select * from tableName", { println + " -- ${it.firstName} --"} )

This code is written for a connection to a MySQL database. You will need to adjust all the parameters to newInstance to connect to your database, especially username and password. Finally the third line calls the eachRow method of sql, passing in two arguments, the first being the query string, the second being a closure to print out some values. Notice that in the closure the fields of “it” are accessed in two different ways. The first is as a simple field reference, accessing the id field of it. The second is the included Groovy expression mentioned above.

So the output from a row might look like:

1 -- Bozhidar --
2 -- Maya --
3 -- Kate --
4 -- Valentine --

XML processing is common enough task in computing and Groovy’s developers tried to make it simple and straightforward as possible. Let’s parse the following file:

        <author firstname="Frank" lastname="Herbert"/>
        <title>Dune Messiah</title>
        <author firstname="Frank" lastname="Herbert"/>
        <title>Children of Dune</title>
        <author firstname="Frank" lastname="Herbert"/>
        <title>A Game of Thrones</title>
        <author firstname="George" lastname="Martin"/>

All the code we need to write is:

def books = new XmlSlurper().parse("books.xml") {
    println "Title = ${it.title}, Author: ${} ${}"

Ruby and Python developers probably aren’t particularly impressed, but I can only imagine the look on the faces of Java developers that are generally required to write huge amount of boilerplate code when dealing with XML.


Groovy has special syntax support for List and Maps. This is great because it gives a concise representation of the actual object being defined, so its easier to keep track of what a program or script is doing. But what about programs which contain arbitrary nested tree structures. Surely, they are the hardest ones to keep track of what is going on. Isn’t that an area where syntactic help will be most beneficial?

The answer is definitely yes and Groovy comes to the party with its builder concept. You can use it for DOM-like APIs or Ant tasks or Jelly tags or Swing widgets or whatever. Each may have their own particular factory mechanism to create the tree of objects - however they can share the same builder syntax to define them - in a concise alternative to XML or lengthy programming code.

One use for builders is the generation of markup:

import groovy.xml.*

def page = new MarkupBuilder()
page.html {
    head { title 'Hello, Groovy!' }
    body {
        div {
            3.times {
                p "Groovy power!"


    <title>Hello, Groovy!</title>
      <p>Groovy power!</p>
      <p>Groovy power!</p>
      <p>Groovy power!</p>

Builders can also be used to create Swing GUIs. Here’s a very small example:

import java.awt.FlowLayout

builder = new groovy.swing.SwingBuilder()
langs = ["Groovy", "Scala", "Clojure"]
gui = builder.frame(size: [290, 100], title: 'Groovy Swing') {
    panel(layout: new FlowLayout()) {
        panel(layout: new FlowLayout()) {
            for (lang in langs) {
                radioButton(text: lang)
        button(text: 'Perform Magic', actionPerformed: {
            builder.optionPane(message: "Feel the power of Groovy!").
                    createDialog(null, 'Message').show()
        button(text: 'Quit',
                actionPerformed: {System.exit(0)})

Run the code to see the resulting GUI!

Groovy tooling

  • IDE
    • IntelliJ IDEA - the ultimate Groovy IDE. It’s excellent Groovy support is part of its open source Community Edition.
    • Eclipse - the most popular Java IDE has an actively maintained Groovy plugin
    • NetBeans - Like IDEA NetBeans features built-in Groovy support
  • Groovy distribution
    • groovysh - A Groovy REPL for exploratory programming
    • groovyConsole - A GUI groovy shell with extended capabilities, that is handy for the development and testing of small Groovy scripts
    • groovyc - the Groovy compiler
  • Build tools

Killer apps

  • Grails
    • Groovy port of Ruby on Rails
    • Leverages the best Java technologies
      • Hibernate
      • Spring
      • Tomcat
  • Gradle - powerful build tool, considered by many vastly superior to Maven. Several high profile projects(such as Hibernate) already migrated their builds to Gradle.
  • Griffon - a Grails like application framework for developing desktop applications

Common use cases

Groovy is a general purpose language, but it’s used for some tasks more often than for others. It’s extremely suitable for:

  • web application development(usually with Grails)
  • scripting(although you have to factor in the cold startup time of the JVM before you start writing all your scripts in Groovy)
  • tests development - it’s a common practice in many Java projects to have the tests written in Groovy
  • GUI development(usually with Griffon)
  • Rapid prototyping - you’d do a quick app prototype in Groovy as a proof on concept and then you’d create a Java application based on it
  • Exploratory programming - the groovysh is a great way to test class capabilities, methods and ideas with almost zero overhead - no annoying compile/run cycles to slow you down

When performance is critical you’d probably want to avoid Groovy. According to some benchmarks around the Internet(like this one) Groovy is much slower than Java for certain tasks. I, however, haven’t read any new benchmarks on the subject and have no idea how reliable the old ones are and how relevant they are to the current Groovy version.

Future prospects

With so many languages being created all the time developers naturally ask themselves the same question over and over again - should I waste my time learning this language? Related questions seem to be:

  • Will it endure the test of time?
  • Does it have a vibrant and committed community around it?
  • Can I find professional support?
  • Does it integrate well with out current infrastructure?
  • Does it have good tooling?

After all most of the currently popular languages like Java, C# and PHP are nothing spectacular on their own, but have a combination of factors that worked in their favour to get them to the top - solid companies behind them, many deployment options and just the right amount of beefing up/simplifying C/C++ make existing developers transition to the new languages a relatively easy and painless experience.

SpringSource(the company responsible for the creation of the popular Spring framework, now a division of VMWare) employs most of the core Groovy developers and offers both Groovy and Grails support. The fact that a company such as this one believes in the technology is very important whey you’re trying to sell using Groovy in your current company. And if you’re existing infrastructure is built around Java - well, you have next to nothing to worry about, except maybe will Java 7 deliver the promised speed improvement for dynamic languages implemented on top of it.

Presently the Groovy community is vast and rapidly growing. The language itself - constantly evolving.

NetBeans and IntelliJ have built-in Groovy support, which is a big testament to the language’s popularity as well.

In a sentence I don’t see Groovy disappearing or dying anytime soon even if its original creator has lost faith in it.

Groovy resources


Groovy is a language aiming to bring dynamic productivity to Java developer without introducing them to a steep learning curve. The language is beautifully architectured and integrates seamlessly with the existing Java libraries and infrastructure. My biggest gripe with Groovy was the lack of advanced support for parallel and concurrent programming. A few days before I wrote this article, however, Groovy 1.8.0 was released and it features the excellent library for parallel programming GPars. Groovy’s performance is not stellar at this point, but I guess this will be improved upon in Java 7.

With its easy to grasp Java-like syntax Groovy is a solid contender for the attention of Java developers. A growing number of Groovy related job offerings is a sign of Groovy’s acceptance as a industrial strength tool.

Some people criticize Groovy for the lack of innovation and claim that it’s simply an amalgam of ideas borrowed from other languages. I don’t see nothing bad with that approach as long as the features are tastefully combined. Groovy might not be the most elegant language out there, but it’s one of the most practical ones and will help you get the job done.

P.S. Coming up next is a discussion of the Scala programming language.