Gradle Feature Spotlight: Continuous Build

Earlier this year the Gradle project released version 2.5 with a heap of new features and improvements. One of the most touted of those features is an incubating feature (read: beta) named Continuous Build which automatically re-executes tasks after a file change. Rubyists may recognize that this functionality is similar to what the guard gem provides.

What makes "continuous build" special is that it makes use of the existing build data present in your Gradle build. Using a task’s inputs the continuous build feature can automatically "watch" the appropriate files to re-execute your task, or your tasks dependent tasks, automatically as they change!

For users JRuby/Gradle this means that upgrading to Gradle 2.5 or later, and ensuring your build.gradle declares task inputs and continuous build will "just work!" Consider the following example for running RSpec tests:

build.gradle

buildscript {
    repositories { jcenter() }
    dependencies {
        classpath "com.github.jruby-gradle:jruby-gradle-plugin:1.0.3" (1)
    }
}
apply plugin: 'com.github.jruby-gradle.base' (2)

dependencies {
    jrubyExec 'rubygems:rspec:3.3.0' (3)
}

import com.github.jrubygradle.JRubyExec

task spec(type: JRubyExec) {
    group 'JRuby'
    description 'Execute the RSpecs in JRuby'
    script 'rspec'
    inputs.source fileTree('spec').include('**/*.rb'), fileTree('lib').include('**/*.rb') (4)
}

1	Specify our dependency on the JRuby/Gradle base plugin
2	Apply the plugin to our current project
3	Define our RSpec gem dependency
4	Set our task inputs to the `.rb` files in `spec/` and in `lib/`

Using the build.gradle above, I can auto-execute my tests whenever a Ruby file inside of the spec/ (my tests) or lib/ (my code under test) with the following command:

% ./gradlew -t spec

Here’s some example output from my example project:

example-project git:(master) % ./gradlew -t spec
Continuous build is an incubating feature.
:spec

Randomized with seed 37453
..............................................

Finished in 0.52 seconds (files took 3.82 seconds to load)
46 examples, 0 failures

Randomized with seed 37453


BUILD SUCCESSFUL

Total time: 8.77 secs

Waiting for changes to input files of tasks... (ctrl-d to exit)

At this point the Gradle process is patiently waiting until I write my most recent changes, then it kicks off the same task:

Change detected, executing build...

:spec

Randomized with seed 64935
..............................................

Finished in 0.502 seconds (files took 3.5 seconds to load)
46 examples, 0 failures

Randomized with seed 64935


BUILD SUCCESSFUL

Total time: 7.341 secs

Waiting for changes to input files of tasks... (ctrl-d to exit)

Pretty neat huh?

What makes this functionality exceptionally powerful for JRuby/Gradle users is that it respects the task inputs but also the task dependency graph. If my spec task declares a dependency on the compileJava task, whenever my Java source code changes, that will trigger a re-execution of compileJava and in turn spec!

So when you’re authoring tasks, even if you’re not this feature right now, be sure to declare task inputs. That build metadata can unlock lots of interesting functionality as Gradle continues to improve!

Continuous build is one of many examples of powerful Gradle functionality which can easily used in JRuby/Gradle, I hope you find it useful!