Chapter 4. Build Script Basics

In Gradle the most basic building block is the task. The tasks for your build are defined in the build script. To try this out, create the following build script named build.gradle.

task hello << {
    println 'Hello world!'
}

In a command-line shell, enter into the containing directory and execute the build script by running gradle -q hello:

> gradle -q hello
Hello world!

What's going on here? This build file defines a single task, called hello, and adds an action to it. When you run gradle hello, Gradle executes the hello task, which in turn executes the action you've provided. The action is simply a closure containing some Groovy code to execute.

If you think this looks similar to Ant's targets, well, you are right. Gradle tasks are the equivalent to Ant targets. But as you will see, they are much more powerful. We have used a different terminology to Ant as we think the word task is more expressive than the word target. Unfortunately this introduces a terminology clash with Ant, as Ant calls its commands, such as javac or copy, tasks. So when we talk about tasks, we always mean Gradle tasks, which are the equivalent to Ant's targets. If we talk about Ant tasks (Ant commands), we explicitly say ant task.

Gradle's build scripts expose to you the full power of Groovy. As an appetizer, have a look at this:

task upper << {
    String someString = 'mY_nAmE'
    println "Original: " + someString 
    println "Upper case: " + someString.toUpperCase()
}

> gradle -q upper
Original: mY_nAmE
Upper case: MY_NAME

or

task count << {
    4.times { print "$it " }
}

> gradle -q count
0 1 2 3

As you probably have guessed, you can declare dependencies between your tasks.

task hello << {
    println 'Hello world!'
}
task intro(dependsOn: hello) << {
    println "I'm Gradle"
}

> gradle -q intro
Hello world!
I'm Gradle

To add a dependency, the corresponding task does not need to exist.

task taskX(dependsOn: 'taskY') << {
    println 'taskX'
}
task taskY << {
    println 'taskY'
}

> gradle -q taskX
taskY
taskX

The dependency of taskX to taskY is declared before taskY is defined. This is very important for multi-project builds. Task dependencies are discussed in more detail in Section 12.4, “Adding Dependencies to Tasks”.

Please notice, that you can't use a shortcut notation (see Section 4.6, “Shortcut notations”) when referring to task, which is not defined yet.

The power of Groovy be used for more than defining what a task does. For example, you can also use it to dynamically create tasks.

4.times { counter ->
    task "task_$counter" << {
        println "I'm task number $counter"
    }
}

> gradle -q task_1
I'm task number 1

Once tasks are created they can be accessed via an API. This is different to Ant. For example you can create additional dependencies.

4.times { counter ->
    task "task_$counter" << {
        println "I'm task number $counter"
    }
}
task_0.dependsOn task_2, task_3

> gradle -q task_0
I'm task number 2
I'm task number 3
I'm task number 0

Or you can add behavior to an existing task.

task hello << {
    println 'Hello Earth'
}
hello.doFirst {
    println 'Hello Venus'
}
hello.doLast {
    println 'Hello Mars'
}
hello << {
    println 'Hello Jupiter'
}

> gradle -q hello
Hello Venus
Hello Earth
Hello Mars
Hello Jupiter

The calls doFirst and doLast can be executed multiple times. They add an action to the beginning or the end of the task's actions list. When the task executes, the actions in the action list are executed in order. The << operator is simply an alias for doLast.

As you might have noticed in the previous examples, there is a convenient notation for accessing an existing task. Each task is available as a property of the build script:

task hello << {
    println 'Hello world!'
}
hello.doLast {
    println "Greetings from the $hello.name task."
}

> gradle -q hello
Hello world!
Greetings from the hello task.

This enables very readable code, especially when using the out of the box tasks provided by the plugins (e.g. compile).

You can assign arbitrary new properties to any task.

task myTask
myTask.myProperty = 'myCustomPropValue'

task showProps << {
    println myTask.myProperty
}

> gradle -q showProps
myCustomPropValue

Let's talk a little bit about Gradle's Ant integration. Ant can be divided into two layers. The first layer is the Ant language. It contains the syntax for the build.xml, the handling of the targets, special constructs like macrodefs, etc. Basically everything except the Ant tasks and types. Gradle does not offer any special integration for this first layer. Of course you can in your build script execute an Ant build as an external process. Your build script may contain statements like: "ant clean compile".execute(). ^[6]

The second layer of Ant is its wealth of Ant tasks and types, like javac, copy or jar. For this layer Gradle provides excellent integration simply by relying on Groovy. Groovy is shipped with the fantastic AntBuilder. Using Ant tasks from Gradle is as convenient and more powerful than using Ant tasks from a build.xml file. Let's look at an example:

task checksum << {
    File[] files = new File('../antChecksumFiles').listFiles()
    Arrays.sort(files)
    files.each { File file ->
        ant.checksum(file: file.canonicalPath, property: file.name)
        println "$file.name Checksum: ${ant.antProject.properties[file.name]}"
    }
}

> gradle -q checksum
agile_manifesto.html Checksum: 2dd24e01676046d8dedc2009a1a8f563
agile_principles.html Checksum: 659d204c8c7ccb5d633de0b0d26cd104
dylan_thomas.txt Checksum: 91040ca1cefcbfdc8016b1b3e51f23d3

In your build script, a property called ant is provided by Gradle. It is a reference to an instance of Groovy's AntBuilder. The AntBuilder is used the following way:

To learn more about the Ant Builder have a look in 'Groovy in Action' 8.4 or at the Groovy Wiki

Gradle scales in how you can organize your build logic. The first level of organizing your build logic for the example above, is extracting a method.

task checksum << {
    fileList('../antChecksumFiles').each { File file ->
        ant.checksum(file: file.canonicalPath, property: file.name)
        println "$file.name Checksum: ${ant.antProject.properties[file.name]}"
    }
}

task length << {
    fileList('../antChecksumFiles').each { File file ->
        ant.length(file: file.canonicalPath, property: file.name)
        println "$file.name Length: ${ant.antProject.properties[file.name]}"
    }
}

File[] fileList(String dir) {
    File[] files = new File(dir).listFiles()
    Arrays.sort(files)
    files
}

> gradle -q checksum
agile_manifesto.html Checksum: 2dd24e01676046d8dedc2009a1a8f563
agile_principles.html Checksum: 659d204c8c7ccb5d633de0b0d26cd104
dylan_thomas.txt Checksum: 91040ca1cefcbfdc8016b1b3e51f23d3

Later you will see that such methods can be shared among subprojects in multi-project builds. If your build logic becomes more complex, Gradle offers you other very convenient ways to organize it. We have devoted a whole chapter to this. See Chapter 25, Organizing Build Logic.

Gradle allows you to define one or more default tasks for your build.

defaultTasks 'clean', 'run'

task clean << {
    println 'Default Cleaning!'
}

task run << {
    println 'Default Running!'
}

task other << {
    println "I'm not a default task!"
}

> gradle -q
Default Cleaning!
Default Running!

This is equivalent to running gradle clean run. In a multi-project build every subproject can have its own specific default tasks. If a subproject does not specify default tasks, the default tasks of the parent project are used (if defined).

As we describe in full detail later (See Chapter 23, The Build Lifecycle) Gradle has a configuration phase and an execution phase. After the configuration phase Gradle knows all tasks that should be executed. Gradle offers you a hook to make use of this information. A use-case for this would be to check if the release task is part of the tasks to be executed. Depending on this you can assign different values to some variables.

build.taskGraph.whenReady {taskGraph ->
    if (taskGraph.hasTask(':release')) {
        version = '1.0'
    } else {
        version = '1.0-SNAPSHOT'
    }
}

task distribution << {
    println "We build the zip with version=$version"
}
task release(dependsOn: 'distribution') << {
    println 'We release now'
}

> gradle -q distribution
We build the zip with version=1.0-SNAPSHOT

Now let's try running the release task:

> gradle -q release
We build the zip with version=1.0
We release now

The important thing is, that the fact that the release task has been choosen, has an effect before the release task gets executed. Nor has the release task to be the primary task (i.e. the task passed to the gradle command).

This is not the end of the story for tasks. So far we have worked with simple tasks. Tasks will be revisited in Chapter 12, More about Tasks and when we look at the Java Plugin in Chapter 15, The Java Plugin.