Gradle Variables and Properties

External

• https://docs.gradle.org/current/userguide/build_environment.html#sec:gradle_configuration_properties

Internal

• Gradle Concepts
• Project and Build Script
• gradle.properties

Overview

Configuration scripts can declare variables during the initialization and configuration phases, and the values of those variables can be accessed and updated during the execution phase and used to drive build logic. There are two types of variables that can be declared: local variables and extra properties.

Properties can be displayed with:

gradle properties

The command displays project properties, and the properties added by various plugins.

Gradle Properties

Also see:

gradle.properties

Local Variables

Local variables, which are a feature of the underlying Groovy language, are declared with the "def" keyword. They are only visible in the scope where they have been declared. The scope include all closures declared in scope.

def myVariable = "something"
...
println myVariable

Extra Properties

An extra property is a property that can be declared on most model object instances available to the closure being executed. In case of a settings script, the available model objects are Settings, Gradle, ProjectDescriptor, etc., but of those ProjectDescriptor does not allow defining extra properties, and Settings, even if allows defining extra properties, cannot be easily accessed at configuration and execution phase, so the best candidate for defining extra properties is Gradle. For build scripts, extra properties can be defined on any available object model instances. They can be declared with a special syntax described below. Once the properties have been added, 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.

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.

Declaring and Using Extra Properties

The extra properties are declared using the "ext" DSL keyword, applied to the model object we intend to declare extra properties for. However, "ext" is not necessary to access the property once declared, as shown in the examples below:

Initialization Phase

The property is defined in settings.gradle:

...
gradle.ext.color = "blue"
...

The property can be accessed anywhere in a build or settings script:

println "gradle is $gradle.color"

Extra Properties and Multi-Project Builds

The extra properties are not available and throw an execution error if they are accessed before are defined. That is why a multi-project build that declares extra properties in sub-projects' build.gradle and attempts to use them in the root project build.gradle will fail at the configuration phase:

BUILD FAILED in 3s

...

* Where:
Build file '.../root-project/build.gradle' line: 17

* What went wrong:
A problem occurred evaluating root project 'root-project'.
> Could not get unknown property 'blah' for project ':sub-project-A' of type org.gradle.api.Project.

The error occurs even "blah" was declared in sub-project-A's build.gradle, because the build file was not evaluated at the time the root project file is executed. If we log build.gradle evaluation, we get something like:

exiting root project build.gradle ....
entering sub-project-A build.gradle ....
exiting sub-project-A build.gradle ....

which shows that the sub-project build script is executed after the root project build script is executed.

At Configuration Phase

The property can be declared in a build script as follows:

project.ext.isApi = true

The property can be accessed anywhere in a build script:

println isApi

Declaring multiple extra properties in a closure:

...
ext {
    log4jVersion = "2.1.7"
    emailNotification = "build@example.com"
}
...

Auto-Delegation

Properties not defined in the build script are auto-delegating to the Project instance. For example:

name

and

project.name

are equivalent. Normally, using "name" is sufficient. In case you define a property or a method which has the same name as a member of the Project object, you will need to use the project property.

Predefined Properties

Project Coordinates

name

A String containing the name of the project directory. It automatically becomes the project's name coordinate. See Gradle Project Coordinates.

group

See Gradle Project Coordinates - Group.

version

See Gradle Project Coordinates - Version.

Other Properties

project

The Project instance.

projectDir

The File instance corresponding to the directory containing the build script.

path

A String containing the absolute path of the project. The path of the root project is ":"

description

A String containing the description for the project.

buildDir

A File representing <projectDir>/build.

Plugin Properties

Plugins add specific properties. For a list of properties added by various plugins, consult the plugin pages.