Archive for category Kobalt

Announcing Kobalt 1.0

I’m happy to announce the release of Kobalt 1.0.

Kobalt has been stable for more than a year now but it’s finally reached a point where all the features that I wanted to incorporate in a first release are now present. The most recent one is incremental Kotlin compilation from the Kotlin compiler itself.

Kobalt already supports incremental builds at the task level (it can determine if a task needs to run and completely skip it if nothing needs to be done) but with Kotlin incremental compilation, build times are dramatically reduced for situations where you have only modified a few files:

===== kobalt-plugin-api:compile
  Kotlin 1.1.0 compiling 163 files
  Actual files that needed to be compiled: 1

Here is a quick summary of Kobalt’s features:

  • A clean, minimal syntax for build files in Kotlin. Each project is clearly delineated along with plug-in configurations.
  • Full support for build files in IDEA since they are valid Kotlin class files.
  • Parallel builds. Projects that don’t depend on each other get built in parallel. This is performed with the same DynamicGraph algorithm that TestNG uses, which guarantees the optimal parallelism for your builds.
  • Sensible defaults to avoid boilerplate. Kobalt uses Maven Central and JCenter by default, ships with a Kotlin, Java and Groovy plug-ins, instant support for Bintray and Github uploads, etc…
  • Automatic detection of newer dependencies found in the repos.
  • Seamless self upgrades. Kobalt will let you know when newer versions are available and will let you upgrade with a simple command.
  • An intuitive and fully statically typed plug-in architecture, inspired from Eclipse and IDEA’s.
  • … and many more.

To get started, download the distribution, go read the documentation and join us on Slack to ask questions, report issues or suggest new features.

The Kobalt diaries: Parallel builds

I’ve always wondered why the traditional build systems we use on the JVM (Maven, Gradle) don’t support parallel builds (edit: this is not quite true, see the comments), so now that I developed my own build system, I decided to find out if there was any particular difficulty to this problem that I had overlooked. This turned out to be easier than I anticipated and with interesting benefits. Here is a preliminary result of my findings.

Defining parallel builds

While most builds are sequential by nature (task B can’t usually run until task A has completed), projects that contain multiple sub projects can greatly benefit from the performance boost of parallel builds. This speed up will obviously not apply to single projects or subprojects that have direct dependencies on each other (although this is not quite true as we will see below). If you’re curious, the engine that powers Kobalt’s parallel builds is an improved version of TestNG’s own DynamicGraphExecutor, which I described in details in this post.

In order to measure the efficacy of parallel builds, I needed a more substantial project, so I picked ktor, Kotlin’s web framework developed by JetBrains. ktor is interesting because it contains more than twenty sub projects with various dependencies with each other. Here is a dependency graph:

You can see that core is the main project that everybody else depends on. After that, the dependencies open up and we have the potential to build some of these projects in parallel: locations, samples, etc…

I started by converting the ktor build from Maven to Kobalt. Right now, ktor is made of about twenty-five pom.xml files that add up to a thousand lines of build files. Kobalt’s build file (Build.kt) is one file of about two hundred lines, and you can find it here. The fact that this build file is a pure Kotlin file allows to completely eliminate the redundancies and maximize the reuse of boiler plate code that most sub projects define.

Extracting the dependencies from Build.kt is trivial thanks to Kobalt’s convenient syntax to define dependencies:

$ grep project kobalt/src/Build.kt
val core = project {
val features = project(core) {
val tomcat = project(core, servlet) {

We see the core project at the top of the dependency graph. Then features depends on core, tomcat depends on both core and servlet and so on…

So without further ado, let’s launch the build in parallel mode and let’s see what happens:

$ ./kobaltw --parallel assemble

At the end of a parallel build, Kobalt optionally displays a summary of the way it scheduled the various tasks. Here is what we get after building the project from scratch:

?  Time (sec) ? Thread 39           ? Thread 40           ? Thread 41           ? Thread 42           ?
?  0          ? core                ?                     ?                     ?                     ?
?  45         ? core (45)           ?                     ?                     ?                     ?
?  45         ?                     ? ktor-locations      ?                     ?                     ?
?  45         ?                     ?                     ? ktor-netty          ?                     ?
?  45         ?                     ?                     ?                     ? ktor-samples        ?
?  45         ? ktor-hosts          ?                     ?                     ?                     ?
?  45         ?                     ?                     ?                     ?                     ?
?  45         ? ktor-hosts (0)      ?                     ?                     ?                     ?
?  45         ? ktor-servlet        ?                     ?                     ?                     ?
?  45         ?                     ?                     ?                     ?                     ?
?  45         ?                     ?                     ?                     ?                     ?
?  45         ?                     ?                     ?                     ? ktor-samples (0)    ?
?  45         ?                     ?                     ?                     ? ktor-freemarker     ?
?  49         ?                     ?                     ?                     ? ktor-freemarker (3) ?
PARALLEL BUILD SUCCESSFUL (68 seconds, sequential build would have taken 97 seconds)

The “Time” column on the left describes at what time (in seconds) each task was scheduled. Each project appears twice: when they start and when they finish (and when they do, the time they took is appended in parentheses).

Analyzing the thread scheduling

As you can see, core is scheduled first and since all the other projects depend on it, Kobalt can’t build anything else until that project completes, so the other four threads remain idle during that time. When that build completes forty-five seconds later, Kobalt now determines that quite a few projects are now eligible to build, so they get scheduled on all the idle threads: ktor-locations, ktor-netty, etc… The first to complete is ktor-hosts and Kobalt immediately schedules the next one on that thread.

Finally, Kobalt gives you the complete time of the build and also how long a sequential build would have taken, calculated by simply adding all the individual project times. It’s an approximation (maybe these projects would have been built faster if they weren’t competing with other build threads) but in my experience, it’s very close to what you actually get if you perform the same build sequentially.

Obviously, the gain with parallel build is highly dependent on the size of your projects. For example, if project C depends on projects A and B and these two projects are very small, the gain in parallelizing that build will be marginal. However, if A and B are both large projects, you could see your total build time divided by two. Another big factor in the gain you can expect is whether you use an SSD. Since all these parallel builds are generating a lot of files in various directories concurrently, I suspect that a physical hard drive will be much slower than an SSD (I haven’t tested, I only have SSD’s around).

Taking things further

When project B depends on project A, it certainly looks like you can’t start building B until A has completed, right? Actually, that’s not quite true. It’s possible to parallelize (or more accurately, vectorize) such builds too. For example, suppose you launch ./kobaltw test on these two projects:

$ ./kobalw test
----- A:compile
----- A:assemble
----- A:test
----- B:compile
----- B:assemble
----- B:test

But the dependency of B on A is not on the full build: B certainly doesn’t need to wait for A to have run its tests before it can start building. In this case, B is ready to build as soon as A is done assembling (i.e. created A.jar). So here, we could envision having threads scheduled at the task level, and not at the project level. So what we could really do is:

?  Time (sec) ? Thread 39           ? Thread 40           ?
?             ? A:compile           ?                     ?
?             ? A:assemble          ?                     ?
?             ? A:test              ? B:compile           ?
?             ?                     ? B:assemble          ?
?             ?                     ? B:test              ?

As you can see above, Kobalt schedules B:compile as soon as A:assemble has completed while starting A:test on a separate thread, resulting in a clear reduction in build time.

This task-based approach can improve build times significantly since tests (especially functional tests) can take minutes to run.

Wrapping up

I started implementing parallel builds mostly as a curiosity and with very low expectations but I ended up being very surprised to see how well they work and how they improve my build times, even when just considering project-based concurrency and not task-based concurrency. I’d be curious to hear back from Kobalt users on how well this new feature performs on their own projects.

And if you haven’t tried Kobalt yet, it’s extremely easy to get started.

The Kobalt diaries: Automatic Android SDK management

The dreaded SDK Manager

Android has always had a weird dependency mechanism. On the JVM (and therefore, Android), we have this great Maven repository system which is leveraged by several tools (Gradle and Kobalt on Android) and which allows us to add dependencies with simple additions to our build files. This is extremely powerful and it has undoubtedly been instrumental in increasing the JVM’s popularity. This system works for pretty much any type of applications and dependencies.

Except for Android libraries.

The Android support libraries (and I’m using “support” loosely here to include all such libraries and not just the one that Google calls “support”) are not available on any of the Maven repositories. They are not even available on Google’s own repository. Instead, you need to use a special tool to download them, and once you do that, they land on your local drive as a local Maven repository, which you then need to declare to Gradle so you can finally add the dependencies you need.

I suspect the reason why these libraries are not available in a straight Maven repo is that you need to accept licenses before you can use them, but regardless, this separate download management makes building Android applications more painful, especially for build servers (Travis, Jenkins) which need to be configured specifically for these builds.

The graphical tool used to download this repository, called “SDK Manager”, is also a command tool called "android" that can be invoked without the GUI, and inspired by Jake Wharton’s sdk-manager-plugin, I set out to implement automatic SDK management for Kobalt.

Once I became more comfortable with the esoteric command line syntax used by the "android" tool, adding it to the Kobalt Android plug-in was trivial, and as a result, Kobalt is now able to completely install a full Android SDK from scratch.

In other words, all you need in your project is a simple Android configuration in your Build.kt file:

    android {
        compileSdkVersion = "23"
        buildToolsVersion = "23.0.1"
        // ...
    dependencies {

The Kobalt Android plug-in will then automatically download everything you need to create an apk from this simple build file:

  • If $ANDROID_HOME is specified, use it and make sure a valid SDK is there. If that environment variable is not specified, install the SDK in a default location (~/.android-sdk).
  • If no Build Tools are installed, install them.
  • Then go through all the Google and Android dependencies for that project and install them as needed.
  • And a few other things…

A typical run on a clean machine with nothing installed will look like this:

$ ./kobaltw assemble
Android SDK not found at /home/travis/.android/android-sdk-linux, downloading it
Couldn't find /home/travis/.android/android-sdk-linux/build-tools/23.0.1, downloading it
Couldn't find /home/travis/.android/android-sdk-linux/platform-tools, downloading it
Couldn't find /home/travis/.android/android-sdk-linux/platforms/android-23, downloading it
Couldn't find Maven repository for extra-android-m2repository, downloading it
Couldn't find Maven repository for extra-google-m2repository, downloading it
          | Building androidFlavors |
------ androidFlavors:clean
------ androidFlavors:generateR
------ androidFlavors:compile
  Java compiling 4 files
------ androidFlavors:proguard
------ androidFlavors:generateDex
------ androidFlavors:signApk
Created androidFlavors/kobaltBuild/outputs/apk/androidFlavors.apk

Obviously, these downloads will not happen again unless you modify the dependencies in your build file.

I’m hopeful that Google will eventually make these support libraries available on a real remote Maven repository so we don’t have to jump through these hoops any more, but until then, Kobalt has you covered.

This feature is available in the latest kobalt-android plug-in as follows:

val p = plugins("com.beust:kobalt-android:0.81")

The Kobalt diaries: testing

Kobalt automatically detects how to run your tests based on the test dependencies that you declared:

dependenciesTest {

By default, Kobalt supports TestNG, JUnit and Spek. You can also configure how your tests run
with the test{} directive:

test {
    args("-excludegroups", "broken", "src/test/resources/testng.xml")

The full list of configuration parameters can be found in the TestConfig class.

Additionally, you can define multiple test configurations, each with a different name. Each
configuration will create an additional task named "test" followed by the name of
that configuration. For example:

test {
    args("-excludegroups", "broken", "src/test/resources/testng.xml")
test {
    name = "All"

The first configuration has no name, so it will be launched with the task "test",
while the second one can be run with the task "testAll".

The full series of articles on Kobalt can be found here.

The Kobalt diaries: templates

The latest addition to Kobalt is templates, also known as “archetypes” in Maven.

Templates are actions performed by plug-ins that create a set of files in your project. They are typically used when uou are beginning a brand new project and you want some default files to be created. Of course, nothing stops you from invoking templates even if you already have an existing project since templates can generate pretty much any kind of files. Here is how they work in Kobalt.

You can get a list of available templates with the --listTemplates parameter:

$ kobaltw --listTemplates
Available templates
  Plug-in: Kobalt
    "java"              Generate a simple Java project
    "kotlin"            Generate a simple Kotlin project
    "kobalt-plugin"     Generate a sample Kobalt plug-in project

You invoke a template with the --init parameter. Let’s call the "kobalt-plugin" template:

$ ./kobaltw --init kobalt-plugin
Template "kobalt-plugin" installed
Build this project with `./kobaltw assemble`
$ ./kobaltw assemble
          | Building kobalt-line-count |
----- kobalt-line-count:compile
----- kobalt-line-count:assemble
  Created .\kobaltBuild\libs\kobalt-line-count-0.18.jar
  Created .\kobaltBuild\libs\kobalt-line-count-0.18-sources.jar
  Wrote .\kobaltBuild\libs\kobalt-line-count-0.18.pom

The template was correctly installed, then it provided instructions on what to do next, which we followed, and now we have a fully working project. This one is particular since it’s a Kobalt plug-in and I’ll get back to it shortly. But before that, let’s drill a bit deeper into templates.

Templates would be pretty useless if they were limited to the default Kobalt distribution, so of course, you can invoke templates on plug-ins. Even plug-ins that you haven’t downloaded yet! Kobalt can download plug-ins from any Maven repository and run them.

To illustrate this, let’s see what templates the Kobalt Android plug-in offers:

$ kobaltw --listTemplates --plugins com.beust:kobalt-android:
Available templates
  Plug-in: Kobalt
    "java"              Generate a simple Java project
    "kotlin"            Generate a simple Kotlin project
    "kobaltPlugin"      Generate a sample Kobalt plug-in project
  Plug-in: Android
    "androidJava"       Generate a simple Android Java project
    "androidKotlin"     Generate a simple Kotlin Java project

Several things happened here. First of all, we are invoking the same --listTemplates command as earlier but this time, there is a new --plugins parameter. You pass this parameter a list of Maven id’s representing the Kobalt plug-ins you want Kobalt to install. This is similar to declaring these plug-ins in your build file, except that typically, when you run a template, you don’t have a build file yet. So this is an easy way to install plug-ins without requiring a build file.

Finally, notice that the Maven id used above, "com.beust:kobalt-android:" doesn’t have a version number and instead, ends with a colon. This is how you ask Kobalt to locate the latest version of the plug-in for you.

Kobalt responded by determining that the latest version of the Kobalt Android plug-in is 0.40, downloading it, installing it and then asking it what templates it provides. The Kobalt Android plug-in provides two templates, both of them creating a full-blown Android application, one written in Kotlin and one in Java. Let’s install the Kotlin one:

$ kobaltw --plugins com.beust:kobalt-android: --init androidKotlin
Template "androidKotlin" installed
Build this project with `./kobaltw assemble`
$ find .
$ ./kobaltw assemble
          ? Building kobalt-demo ?
----- kobalt-demo:generateR
----- kobalt-demo:compile
----- kobalt-demo:proguard
----- kobalt-demo:generateDex
----- kobalt-demo:signApk
Created kobaltBuild\outputs\apk\kobalt-demo.apk
----- kobalt-demo:assemble

We now have a complete Android application written in Kotlin.

Let’s go back to the template we built at the beginning of this article: the Kobalt plug-in called "kobalt-line-count-0.18.jar". It’s a valid Kobalt plug-in so how do we test it? We could upload it to JCenter and then invoke it with the --plugins parameter, but Kobalt provides another handy command line parameter to test such plug-ins locally: --pluginJarFiles. This parameter is similar to --plugins in that it installs a plug-in, except that it does so from a local jar file and not a remote Maven id.

Let’s install this plug-in and see which tasks are then available to us:

$ ./kobaltw --pluginJarFiles kobaltBuild/libs/kobalt-line-count-0.18.jar --tasks
List of tasks
  ----- kobalt-line-count -----
    dynamicTask         Dynamic task
    lineCount           Count the lines

As you can see, Kobalt has installed the kobalt-line-count plug-in, which then added its own tasks to Kobalt’s default ones. The Kobalt plug-in template appears to work fine. From this point on, you can edit it and create your own Kobalt plug-in.

Speaking of plug-in development, how hard is it to add templates to your Kobalt plug-in? Not hard at all! All you need to do is to implement the ITemplateContributor interface:

interface ITemplateContributor {
    val templates: List<ITemplate>

Each template provides a name, a description and a function that actually generates the files for the current project. Feel free to browse how Kobalt’s Android plug-in implements its templates.

The full series of articles on Kobalt can be found here.

The Kobalt Diaries: Incremental Tasks

One of the recent additions to Kobalt is incremental tasks. This is the ability for each build task to be able to check whether it should run or not based on whether something has changed compared to the previous run. Here are a few quick outlines of how this feature works in Kobalt.


Kobalt’s incremental task architecture is based on checksums. You implement an incremental task by giving Kobalt a way to compute an input checksum and an output checksum. When the time comes to run your task, Kobalt will ask for your input checksum and it will compare it to that of the previous run. If they are different, your task is invoked. If they are identical, Kobalt then compares the two output checksums. Again, if they are different, your task is run, otherwise it’s skipped. Finally, Kobalt updates the output checksum on successul completion of your task.

This mechanism is extremely general and straightforward to implement for plug-in developers, who remain in full control of how exhaustive their checksum should be. You could decide to stick to the default MD5 checksums of the files and directories that are of interest to your task, or if you want to be faster, only check the timestamps of your file and return a checksum reflecting whether Kobalt should run you or not. And of course, checksums don’t even have to map to files at all: if your task needs to perform a costly download, it could first check a few HTTP headers and again, return a checksum indicating whether your task should run.

Having said that, build systems tend to run tasks that have files for inputs and outputs, so it seems logical to think about an incremental resolution that would be based not on checksums (which can be expensive to compute) but on file analyses. While a checksum can tell you “One of these N files has been modified”, it can’t tell you exactly which one, and such information can open the door to further incremental work (see below for more details).

One approach for file-based tasks could be for the build system to store the list of files along with some other data (timestamp or checksum) and then pass the relevant information to the task itself. The complication here is that file change resolution implies knowing the following three pieces of information:

  • Which files were modified.
  • Which files were added.
  • Which files were removed.

The downside is obviously that there is more bookkeeping required to preserve this information around between builds but the clear benefit is that if a task ends up being invoked, it can perform its own incremental work on just the files that need to be processed, whereas the checksum approach forces the task to perform its work on the entire set of inputs.


Incremental tasks are not very different from regular tasks. An incremental task returns an IncrementalTaskInfo instance which is defined as follows:

class IncrementalTaskInfo(
	val inputChecksum: String?,
    val outputChecksum: () -> String?,
    val task: (Project) -> TaskResult)

The last parameter is the task itself and the first two are the input and output checksums of your task. Your task simply uses the @IncrementalTask annotation instead of the regular @Task and it needs to return an instance of that class:

@IncrementalTask(name = "compile", description = "Compile the source files")
fun taskCompile(project: Project) = IncrementalTaskInfo(/* ... */)

Most of Kobalt’s own tasks are now incremental (wherever that makes sense) including the Android plug-in. Here are a few timings showing incremental builds in action:


TaskFirst runSecond run
kobalt-wrapper:compile627 ms22 ms
kobalt-wrapper:assemble9 ms9 ms
kobalt-plugin-api:compile10983 ms54 ms
kobalt-plugin-api:assemble1763 ms154 ms
kobalt:compile11758 ms11 ms
kobalt:assemble42333 ms2130 ms
70 seconds2 seconds

Android (u2020)

TaskFirst runSecond run
u2020:generateRInternalDebug32350 ms1652 ms
u2020:compileInternalDebug3629 ms24 ms
u2020:retrolambdaInternalDebug668 ms473 ms
u2020:generateDexInternalDebug6130 ms55 ms
u2020:signApkInternalDebug449 ms404 ms
u2020:assembleInternalDebug0 ms0 ms
43 seconds2 seconds

Wrapping up

At the moment, Kobalt only supports checksum-based incremental tasks since that approach subsumes all the other approaches but I’m not ruling out adding input-specific incremental tasks in the future if there’s interest. In the meantime, checksums are working very well and pretty efficiently, even on large directories and/or large files.

If you are curious to try it yourself, please download Kobalt and report back!

The full series of articles on Kobalt can be found here.

The Kobalt diaries: profiles

When I started thinking about how profiles should work in Kobalt, I realized that the simplest approach I’d like to see in a build tool is defining a boolean variable and having if statements in my build file. So that’s exactly how Kobalt’s profiles are implemented.

You start by defining boolean values initialized to false in your build file:

  val experimental = false
  val premium = false

Then you use this variable wherever you need it in your build file:

  val p = javaProject {
      name = if (experimental) "project-exp" else "project"
      version = "1.3"

Finally, you invoke ./kobaltw with the --profiles parameter followed by the profiles you want to activate, separated by a comma:

  ./kobaltw -profiles experimental,premium assemble

Keep in mind that since your build file is a real Kotlin source file,
you can use these profile variables pretty much anywhere, e.g.:

dependencies {
    if (experimental)

And that’s it.

The full series of articles on Kobalt can be found here.

The Kobalt diaries: it’s the little things

When I embarked on the ridiculously ambitious goal of writing a build tool, I had plans to tackle both big problems and small problems. My previous (and probably future) blog post cover the big problems such as performance, plug-in architecture and DSL syntax, but in this post, I’m going to cover a few little things that I was quite happy to finally be able to get from my build tool.

I’ve always found it a hassle to keep up with the latest versions of the dependencies of my build, especially since it’s its job to tell me. Therefore, Kobalt has a handy --checkVersions parameter that will check to see if it can find any new version of your dependencies:

$ ./kobaltw --checkVersions
New versions found:

Another convenient switch is --resolve, which looks up a dependency and gives you some information about it, such as which Maven repo it is found in and its own dependency tree. You can also use an id without a version (e.g. org.testng:testng:) to ask Kobalt to find the most recent version of that artifact:

$ ./kobaltw --resolve org.testng:testng:
?                                     org.testng:testng:                           ?
?  ?
? junit:junit:4.10
?      ? org.hamcrest:hamcrest-core:1.1
? com.beust:jcommander:1.48
? org.apache.ant:ant:1.7.0
?      ? org.apache.ant:ant-launcher:1.7.0
? org.yaml:snakeyaml:1.15
? org.beanshell:bsh:2.0b4

Finally, I’ve always been bugged by what I consider a glaring omission of the Gradle Android plug-in: not being able to run my applications. The plug-in generates tasks for the various variants of your application (assembleDevDebug, assembleDevRelease, installDevDebug, etc…) but strikingly, no "run" task. I’m happy to report that Kobalt’s Android plug-in supports exactly that. To see it in action, clone the Kobalt example and follow the instructions at the bottom of the README:

$ ./kobaltw runFreeDebug // build, install and launch that variant
$ ./kobaltw runFreeRelease // build, install and launch that variant

I’ve made a lot of improvements to the Android plug-in lately, but that will be the topic for another post.

The full series of articles on Kobalt can be found here.

The Kobalt diaries: annotation processing

I recently added apt support to Kobalt, which is a requirement for a build system these days, and something interesting happened.

First of all, the feature itself in Kobalt: pretty straightforward. The apt plug-in adds a new dependency directive similar to compile:

dependencies {

The processing tool can be further configured (output directory, arguments, etc…) with a separate apt directive:

apt {
    outputDir = "generated/sources/apt"

In order to test this new feature, I decided to implement a simple annotation processor project and I went for a Version class generator. As I wrote this processor, I realized that it was actually something I could definitely use in my other projects.

Of course, you can always simply hard code the version number of your application in a source file but that version number is typically something that’s useful outside of your code: you might need it in your build file, or when you generate your artifacts, or maybe other projects need to refer to it. Therefore, it often makes sense to isolate that version number in a property file and have every entity that needs it read it from that property file.

This is how version-processor was born. It’s pretty simple really: all you need to do is annotate one of your classes with @Version and a file is created, which you can then refer to. That version number can either be hardcoded or specified in a properties file. Head over to the project’s main page for the details.

And of course, it’s built with Kobalt and if you are curious, here is the processor’s build file:

val processor = javaProject {
    name = "version-processor"
    group = "com.beust"
    artifactId = name
    version = "0.2"
    directory = "processor"
    assemble {
        mavenJars {}
    jcenter {
        publish = true

Happy version generating!

The full series of articles on Kobalt can be found here.

The Kobalt diaries: Android

A lot of work has gone into Kobalt since I announced its alpha:

I’m plannning to post more detailed updates as things progress but today, I’d like to briefly show a major milestone: the first Android APK generated by Kobalt.

I picked the Code path intro app as a test application. I first built it with Gradle to get a feel for it and the apk was generated in about 27 seconds. Then I generated a Build.kt file with ./kobaltw --init, added a few Android related directives to reach the following (complete) build file:

import com.beust.kobalt.*
val p = javaProject {
    name = "intro_android_demo"
    group = "com.example"
    artifactId = name
    version = "0.1"
    dependencies {
    sourceDirectories {
    android {
        applicationId = name
        buildToolsVersion = "21.1.2"

Then I launched the build with ./kobaltw assemble, and…

Less than five seconds to generate, compile it, compile the code, run aapt, generate classes.dex and finally, generated the apk. If you are curious, you can check out the full log.

Admittedly, Kobalt doesn’t yet handle build types and flavors nor manifest merging, but the example app I’m building here doesn’t use those either so I don’t expect the build time to increase much. There is a lot more to be done before Kobalt’s Android plug-in is ready for more users, but this is a pretty encouraging result.