Initialization Scripts

Initialization scripts are scripts that run before the build script is executed. They allow you to customize the build environment or configure settings early in the build.

Initialization scripts can be useful for setting up common configurations, such as repositories, plugins, or custom tasks, across multiple projects.

Using an init script

Initialization scripts, also called init scripts, are similar to other scripts in Gradle. Initialization scripts run before the build starts.

They are useful for various purposes:

Setting up enterprise-wide configurations (e.g., custom plugin locations)
Configuring properties based on the environment (e.g., developer’s machine vs. CI server)
Providing user-specific information (e.g., authentication credentials)
Defining machine-specific details (e.g., JDK locations)
Registering build listeners (e.g., external tools that wish to listen to Gradle events might find this helpful)
Registering loggers (e.g., customize how Gradle logs the events that it generates)

One main limitation of init scripts is that they cannot access classes in the buildSrc project.

Invoking an init script

There are several ways to invoke an init script (in order of priority):

Specify a file on the command line with the option -I or --init-script followed by the path to the script.

The command line option can appear more than once, each time adding another init script. The build will fail if any files specified on the command line do not exist.
Put a file called init.gradle(.kts) in the $GRADLE_USER_HOME/ directory.
Put a file called init.gradle(.kts) in the $GRADLE_USER_HOME/init.d/ directory.
Put a file called init.gradle(.kts) in the $GRADLE_HOME/init.d/ directory.

This lets you package a custom Gradle distribution containing custom build logic and plugins. You can combine this with the Gradle wrapper to make custom logic available to all builds in your enterprise.

If more than one init script is found, they will all be executed in the order specified above.

Scripts in a given directory are executed in alphabetical order. For example, a tool can specify an init script on the command line and another in the home directory to define the environment. Both scripts will run when Gradle is executed.

Writing an init script

Like a Gradle build script, an init script is a Groovy or Kotlin script. Each init script has a Gradle instance associated with it. Any property reference and method call in the init script will be delegated to this Gradle instance.

Each init script implements the Script interface.

When writing init scripts, pay attention to the scope of the reference you are trying to access. For example, properties loaded from gradle.properties are available on Settings or Project instances but not on the Gradle one.

Configuring projects from an init script

You can use an init script to configure the projects in the build. This works similarly to configuring projects in a multi-project build.

The following sample shows how to perform extra configuration from an init script before the projects are evaluated:

build.gradle.kts

repositories {
    mavenCentral()
}

tasks.register("showRepos") {
    val repositoryNames = repositories.map { it.name }
    doLast {
        println("All repos:")
        println(repositoryNames)
    }
}

init.gradle.kts

allprojects {
    repositories {
        mavenLocal()
    }
}

build.gradle

repositories {
    mavenCentral()
}

tasks.register('showRepos') {
    def repositoryNames = repositories.collect { it.name }
    doLast {
        println "All repos:"
        println repositoryNames
    }
}

init.gradle

allprojects {
    repositories {
        mavenLocal()
    }
}

This sample uses this feature to configure an additional repository to be used only for specific environments.

> gradle --init-script init.gradle.kts -q showRepos
All repos:
[MavenLocal, MavenRepo]

> gradle --init-script init.gradle -q showRepos
All repos:
[MavenLocal, MavenRepo]

Adding external dependencies

Init scripts can also declare dependencies with the initscript() method, passing in a closure that declares the init script classpath.

Declaring external dependencies for an init script:

init.gradle.kts

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.apache.commons:commons-math:2.0")
    }
}

init.gradle

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.apache.commons:commons-math:2.0'
    }
}

The closure passed to the initscript() method configures a ScriptHandler instance. You declare the init script classpath by adding dependencies to the classpath configuration.

This is the same way you declare, for example, the Java compilation classpath. You can use any of the dependency types described in Declaring Dependencies, except project dependencies.

Having declared the init script classpath, you can use the classes in your init script as you would any other classes on the classpath. The following example adds to the previous example and uses classes from the init script classpath.

An init script with external dependencies:

init.gradle.kts

import org.apache.commons.math.fraction.Fraction

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.apache.commons:commons-math:2.0")
    }
}

println(Fraction.ONE_FIFTH.multiply(2))

build.gradle.kts

tasks.register("doNothing")

init.gradle

import org.apache.commons.math.fraction.Fraction

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.apache.commons:commons-math:2.0'
    }
}

println Fraction.ONE_FIFTH.multiply(2)

build.gradle

tasks.register('doNothing')

> gradle --init-script init.gradle.kts -q doNothing
2 / 5

> gradle --init-script init.gradle -q doNothing
2 / 5

Applying plugins

Plugins can be applied to init scripts like a Gradle build script or a Gradle settings file.

Using plugins in init scripts:

init.gradle.kts

apply<EnterpriseRepositoryPlugin>()

class EnterpriseRepositoryPlugin : Plugin<Gradle> {
    companion object {
        const val ENTERPRISE_REPOSITORY_URL = "https://repo.gradle.org/gradle/repo"
    }

    override fun apply(gradle: Gradle) {
        // ONLY USE ENTERPRISE REPO FOR DEPENDENCIES
        gradle.allprojects {
            repositories {

                // Remove all repositories not pointing to the enterprise repository url
                all {
                    if (this !is MavenArtifactRepository || url.toString() != ENTERPRISE_REPOSITORY_URL) {
                        project.logger.lifecycle("Repository ${(this as? MavenArtifactRepository)?.url ?: name} removed. Only $ENTERPRISE_REPOSITORY_URL is allowed")
                        remove(this)
                    }
                }

                // add the enterprise repository
                add(maven {
                    name = "STANDARD_ENTERPRISE_REPO"
                    url = uri(ENTERPRISE_REPOSITORY_URL)
                })
            }
        }
    }
}

build.gradle.kts

repositories{
    mavenCentral()
}

data class RepositoryData(val name: String, val url: URI)

tasks.register("showRepositories") {
    val repositoryData = repositories.withType<MavenArtifactRepository>().map { RepositoryData(it.name, it.url) }
    doLast {
        repositoryData.forEach {
            println("repository: ${it.name} ('${it.url}')")
        }
    }
}

init.gradle

apply plugin: EnterpriseRepositoryPlugin

class EnterpriseRepositoryPlugin implements Plugin<Gradle> {

    private static String ENTERPRISE_REPOSITORY_URL = "https://repo.gradle.org/gradle/repo"

    void apply(Gradle gradle) {
        // ONLY USE ENTERPRISE REPO FOR DEPENDENCIES
        gradle.allprojects { project ->
            project.repositories {

                // Remove all repositories not pointing to the enterprise repository url
                all { ArtifactRepository repo ->
                    if (!(repo instanceof MavenArtifactRepository) ||
                          repo.url.toString() != ENTERPRISE_REPOSITORY_URL) {
                        project.logger.lifecycle "Repository ${repo.url} removed. Only $ENTERPRISE_REPOSITORY_URL is allowed"
                        remove repo
                    }
                }

                // add the enterprise repository
                maven {
                    name "STANDARD_ENTERPRISE_REPO"
                    url ENTERPRISE_REPOSITORY_URL
                }
            }
        }
    }
}

build.gradle

repositories{
    mavenCentral()
}

@Immutable
class RepositoryData {
    String name
    URI url
}

tasks.register('showRepositories') {
    def repositoryData = repositories.collect { new RepositoryData(it.name, it.url) }
    doLast {
        repositoryData.each {
            println "repository: ${it.name} ('${it.url}')"
        }
    }
}

> gradle --init-script init.gradle.kts -q showRepositories
repository: STANDARD_ENTERPRISE_REPO ('https://repo.gradle.org/gradle/repo')

> gradle --init-script init.gradle -q showRepositories
repository: STANDARD_ENTERPRISE_REPO ('https://repo.gradle.org/gradle/repo')

The plugin in the init script ensures that only a specified repository is used when running the build.

When applying plugins within the init script, Gradle instantiates the plugin and calls the plugin instance’s Plugin.apply(T) method.

The gradle object is passed as a parameter, which can be used to configure all aspects of a build. Of course, the applied plugin can be resolved as an external dependency as described in External dependencies for the init script