05 June 2014

Tags: groovy android swift gr8conf

Yesterday ended GR8Conf Europe, a Groovy language conference in Copenhagen that was once again a successful event. I want to thank the crew again for this, and of course the Groovy language community which is so friendly and always helpful. This year turned out to be very special for me, because just two days after the announcement by Apple of the Swift language, I was talking about running Groovy on Android! As Guillaume Laforge noticed, there are a lot of similarities between the Swift language and Groovy language.

The timing is almost perfect, because Android users will want to have a language which is as modern as Swift is, but running on Android. And I see no better candidate than Groovy language here.

The presentation

Should you be interested in the slides, you can find them below. Since it was presented at GR8Conf Europe, it gives a bit of history of the changes needed to have the runtime working on Android too:

For those of you who are new to Groovy, you have to know that this language is derived from Java, meaning that the learning curve is almost 0, but it also removes a lot of its verbosity and adds a lot of features to it, such as closures (similar to lambdas in Java 8, but Java 8 is not available for Android developers), builders, runtime and compile-time metaprogramming.

As an example of how Groovylang can be used to reduce the verbosity of Java on Android, I will take a simple example: asynchronous tasks. Asynchronous tasks are required as soon as a task takes too much time to be executed on the UI thread. That is the case, by default, for any network based operations, in order to guarantee that the UI remains snappy even if network is slow or unavailable. The problem is that those asynchronous tasks are incredibly verbose. You have to write a lot of code, that I would tend to name "inner class hell", just for this. Let’s imagine that you need to parse a JSON feed, then update the UI accordingly. Then you would have to write something like this (no kidding):

public class FeedActivity {
    TextView mTextView;


    void updateFeed() {
    	new FeedTask().execute("https://path/to/feed");

    class FeedTask extends AsyncTask<String, Void, String> {
        protected String doInBackground(String... params) {
            DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
            HttpPost httppost = new HttpPost(params[0]);

            InputStream inputStream = null;
            String result = null;
            try {
                HttpResponse response = httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();

                inputStream = entity.getContent();
                // json is UTF-8 by default
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
                StringBuilder sb = new StringBuilder();

                String line = null;
                while ((line = reader.readLine()) != null) {
                result = sb.toString();
            } catch (Exception e) {
                // Oops
            } finally {
                try {
                    if (inputStream != null) {
                } catch (Exception squish) {
            StringBuilder speakers = null;
            try {
                JSONObject jObject = new JSONObject(result);
                JSONArray jArray = jObject.getJSONArray("speakers");
                speakers = new StringBuilder();
                for (int i = 0; i < jArray.length(); i++) {
                    speakers.append(" ");
            } catch (JSONException e) {
                // do something?
            return speakers.toString();

        protected void onPostExecute(String s) {

So now, let’s see the equivalent in Groovy (no kidding either):

public class FeedActivity {
    TextView mTextView


    void updateFeed() {
    	Fluent.async {
            def json = new JsonSlurper().parse([:], new URL('https://path/to/feed'), 'utf-8')
            json.speakers.join(' ')
        } then {
	    mTextView.text = it

I think you can start to see the advantage of using Groovy in your own Android projects. Of course, the Fluent class that I’m using here is a support class which I implemented in my first Android project (which is open sourced btw), but it’s really simple and gives an example of how Android users could benefit from Groovification of their APIs.

Feeling the pain

This is actually a key point of my talk: I hadn’t written any Android application before this talk, and I definitely wanted to be able to write an application in Groovy on Android. Why? Because it’s been some time already that I use Groovy everyday, and there’s no turning back. I wanted to feel the pain of the Java developers on Android, so that I can write better tools for them. And that’s actually where you, as a user, come in action: there are so many Groovy libraries out there whose sole objective is to ease the pain, make things that shouldn’t be complicated a breeze. This is exactly the point. My example with Fluent is one example of simplification of usages of Android APIs, but you have many more to invent, especially because you must have been as annoyed as I did by all those asynchronous tasks, XML files (think of builders!), callback hell, etc…​

The beginning of a new era

In upcoming posts, I will try to demonstrate that we’re just at the beginning. Some people are already asking for a {swift} alternative for Android. It’s there guys, you have it, so spread the word and let’s make it happen! I am convinced that it is the beginning of a new era for Android and Groovy. Google already switched their main build system to Gradle, which is, by the way, using Groovy, so I think it’s time to move over and show your love!

All you need to get started is explained in the slide deck above, and you can find the source code of the sample android application on GitHub:

Update 3: it’s even simpler now

Groovy 2.4.0 has been released now and building an Android application in Groovy is even simpler. All you have to do is to apply a Gradle plugin. Instructions can be found here. You absolutely don’t have to build Groovy from sources or add tasks to your Gradle build as described originally in this post! Just use the plugin!
Information below is outdated. Please use the Gradle plugin !.

Update 2: build instructions with the Gradle plugin

You can now use a Gradle plugin to integrate Groovy with Android. The plugin can be found here: groovy-android-gradle-plugin. As of version 0.2, the plugin supports the Android plugin 0.10+.

Update: build instructions

If you want to try it by yourself, here’s how you can do it. First of all, official support for Android will be in Groovy 2.4. Before the first beta, you’ll have to build it from sources, and here is the quickest way:

git clone https://github.com/melix/groovy-core.git --branch master
cd groovy-core
./gradlew -PskipIndy=true install

Then you can clone the sample application:

cd ..
git clone https://github.com/melix/gr8confagenda.git

This contains a project that you can open using Android Studio.

If you want to use the Groovy language in your own Android project, a requirement is that it is using Gradle. If so, you can update your build.gradle file as is:

android {
   packagingOptions {
        // workaround for https://stackoverflow.com/questions/20673625/android-gradle-plugin-0-7-0-duplicate-files-during-packaging-of-apk
        exclude 'META-INF/LICENSE.txt'
        exclude 'META-INF/groovy-release-info.properties'

repositories {

dependencies {
    compile 'org.codehaus.groovy:groovy:2.4.0-SNAPSHOT:grooid'
    // the following dependency is necessary if you want JSON support
    compile ('org.codehaus.groovy:groovy-json:2.4.0-SNAPSHOT') {
        transitive = false

// add support for Groovy to existing configurations
android.applicationVariants.all {
    task "groovy${name}Compile"(type: GroovyCompile) {
        source = javaCompile.source + fileTree('src/main/java').include('**/*.groovy')
        destinationDir = javaCompile.destinationDir
        classpath = javaCompile.classpath
        groovyClasspath = classpath
        sourceCompatibility = '1.6'
        targetCompatibility = '1.6'
        doFirst {
            def runtimeJars = plugins.findPlugin(com.android.build.gradle.AppPlugin).runtimeJars
            classpath = files(runtimeJars) + classpath
    javaCompile.enabled = false

And that’s all! Now, one option for you is to write support libraries and make them available to the community. Enjoy!