Gradle Extra Properties

Internal

• Gradle Runtime and Project Configuration

Overview

An extra property is a property that can be declared on most model object instances available to the closure being executed. Extra properties allow defining variables that are used multiple times within the same script, such as a file that is referred from multiple locations of the build script. An extra property is equivalent to a user-defined variable.

Internally, extra properties are stored as key-value pairs in a map, associated with the model object instance. For settings.gradle, the available model objects are Settings and Gradle (ProjectDescriptor does not allow defining extra properties). Settings allows defining extra properties, but those properties cannot be easily accessed at configuration and execution phase, so the best candidate for defining extra properties during the configuration phase is Gradle.

For build.gradle, extra properties can be defined on any available object model instances, and the most common object to define properties on is the project instance. The the extra properties defined on the project instance are known as project properties.

They can be declared with a special syntax described below. Once the properties have been declared, they can be read and set like predefined properties. 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 sub-projects, but not vice-versa.

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.

To add properties, use an "ext" namespace closure:

ext {
  color = 'blue'
}

An equivalent assignment is:

project.ext.color = 'blue'

A benefit of using the closure over the other syntax is that multiple assignments can be declared compactly:

ext {
  color = 'blue'
  shape = 'square'
  size = 10
}

Note that the "ext" script block or project.ext syntax must be used only for the declaration of the property, for subsequent access the extra property can be looked up directly on the project instance.

project.ext.color = 'green'
project.color = 'red' // project.ext.color and project.color are equivalent after property initialization

task displayColor {
  doLast {
    println project.ext.color
    println project.color
    println color
  }
}

For the above example, ./gradlew displayColor will display "red".