Chapter 13. Writing Build Scripts

This chapter looks at some of the details of writing a build script.

13.1. The Gradle build language

Gradle provides a domain specific language, or DSL, for describing builds. This build language is based on Groovy, with some additions to make it easier to describe a build.

A build script can contain any Groovy language element. [5] Gradle assumes that each build script is encoded using UTF-8.

13.2. The Project API

In the tutorial in Chapter 7, Java Quickstart we used, for example, the apply() method. Where does this method come from? We said earlier that the build script defines a project in Gradle. For each project in the build, Gradle creates an object of type Project and associates this Project object with the build script. As the build script executes, it configures this Project object:

Getting help writing build scripts

Don't forget that your build script is simply Groovy code that drives the Gradle API. And the Project interface is your starting point for accessing everything in the Gradle API. So, if you're wondering what 'tags' are available in your build script, you can start with the documentation for the Project interface.

  • Any method you call in your build script which is not defined in the build script, is delegated to the Project object.

  • Any property you access in your build script, which is not defined in the build script, is delegated to the Project object.

Let's try this out and try to access the name property of the Project object.

Example 13.1. Accessing property of the Project object


println name

Output of gradle -q check

> gradle -q check

Both println statements print out the same property. The first uses auto-delegation to the Project object, for properties not defined in the build script. The other statement uses the project property available to any build script, which returns the associated Project object. Only if you define a property or a method which has the same name as a member of the Project object, would you need to use the project property.

13.2.1. Standard project properties

The Project object provides some standard properties, which are available in your build script. The following table lists a few of the commonly used ones.

Table 13.1. Project Properties

Name Type Default Value
project Project The Project instance
name String The name of the project directory.
path String The absolute path of the project.
description String A description for the project.
projectDir File The directory containing the build script.
buildDir File projectDir/build
group Object unspecified
version Object unspecified
ant AntBuilder An AntBuilder instance

13.3. The Script API

When Gradle executes a script, it compiles the script into a class which implements Script. This means that all of the properties and methods declared by the Script interface are available in your script.

13.4. Declaring variables

There are two kinds of variables that can be declared in a build script: local variables and extra properties.

13.4.1. Local variables

Local variables are declared with the def keyword. They are only visible in the scope where they have been declared. Local variables are a feature of the underlying Groovy language.

Example 13.2. Using local variables


def dest = "dest"

task copy(type: Copy) {
    from "source"
    into dest

13.4.2. Extra properties

All enhanced objects in Gradle's domain model can hold extra user-defined properties. This includes, but is not limited to, projects, tasks, and source sets. Extra properties can be added, read and set via the owning object's ext property. Alternatively, an ext block can be used to add multiple properties at once.

Example 13.3. Using extra properties


apply plugin: "java"

ext {
    springVersion = "3.1.0.RELEASE"
    emailNotification = ""

sourceSets.all { ext.purpose = null }

sourceSets {
    main {
        purpose = "production"
    test {
        purpose = "test"
    plugin {
        purpose = "production"

task printProperties << {
    println springVersion
    println emailNotification
    sourceSets.matching { it.purpose == "production" }.each { println }

Output of gradle -q printProperties

> gradle -q printProperties

In this example, an ext block adds two extra properties to the project object. Additionally, a property named purpose is added to each source set by setting ext.purpose to null (null is a permissible value). Once the properties have been added, they can be read and set like predefined properties.

By requiring special syntax for adding a property, Gradle can fail fast when an attempt is made to set a (predefined or extra) property but the property is misspelled or does not exist. Extra properties can be accessed from anywhere their owning object can be accessed, giving them a wider scope than local variables. Extra properties on a project are visible from its subprojects.

For further details on extra properties and their API, see the ExtraPropertiesExtension class in the API documentation.

13.5. Some Groovy basics

Groovy provides plenty of features for creating DSLs, and the Gradle build language takes advantage of these. Understanding how the build language works will help you when you write your build script, and in particular, when you start to write custom plugins and tasks.

13.5.1. Groovy JDK

Groovy adds lots of useful methods to the standard Java classes. For example, Iterable gets an each method, which iterates over the elements of the Iterable:

Example 13.4. Groovy JDK methods


// Iterable gets an each() method
configurations.runtime.each { File f -> println f }

Have a look at for more details.

13.5.2. Property accessors

Groovy automatically converts a property reference into a call to the appropriate getter or setter method.

Example 13.5. Property accessors


// Using a getter method
println project.buildDir
println getProject().getBuildDir()

// Using a setter method
project.buildDir = 'target'

13.5.3. Optional parentheses on method calls

Parentheses are optional for method calls.

Example 13.6. Method call without parentheses


test.systemProperty 'some.prop', 'value'
test.systemProperty('some.prop', 'value')

13.5.4. List and map literals

Groovy provides some shortcuts for defining List and Map instances. Both kinds of literals are straightforward, but map literals have some interesting twists.

For instance, the “apply” method (where you typically apply plugins) actually takes a map parameter. However, when you have a line like “apply plugin:'java'”, you aren't actually using a map literal, you're actually using “named parameters”, which have almost exactly the same syntax as a map literal (without the wrapping brackets). That named parameter list gets converted to a map when the method is called, but it doesn't start out as a map.

Example 13.7. List and map literals


// List literal
test.includes = ['org/gradle/api/**', 'org/gradle/internal/**']

List<String> list = new ArrayList<String>()
test.includes = list

// Map literal.
Map<String, String> map = [key1:'value1', key2: 'value2']

// Groovy will coerce named arguments
// into a single map argument
apply plugin: 'java'

13.5.5. Closures as the last parameter in a method

The Gradle DSL uses closures in many places. You can find out more about closures here. When the last parameter of a method is a closure, you can place the closure after the method call:

Example 13.8. Closure as method parameter


repositories {
    println "in a closure"
repositories() { println "in a closure" }
repositories({ println "in a closure" })

13.5.6. Closure delegate

Each closure has a delegate object, which Groovy uses to look up variable and method references which are not local variables or parameters of the closure. Gradle uses this for configuration closures, where the delegate object is set to the object to be configured.

Example 13.9. Closure delegates


dependencies {
    assert delegate == project.dependencies

[5] Any language element except for statement labels.