Gradle provides a powerful mechanism to allow customizing the build based on the current environment. This mechanism also supports tools that wish to integrate with Gradle.

Note that this is completely different from the “init” task provided by the “build-init” plugin (see Build Init Plugin).

Basic usage

Initialization scripts (a.k.a. init scripts) are similar to other scripts in Gradle. These scripts, however, are run before the build starts. Here are several possible uses:

  • Set up enterprise-wide configuration, such as where to find custom plugins.

  • Set up properties based on the current environment, such as a developer’s machine vs. a continuous integration server.

  • Supply personal information about the user that is required by the build, such as repository or database authentication credentials.

  • Define machine specific details, such as where JDKs are installed.

  • Register build listeners. External tools that wish to listen to Gradle events might find this useful.

  • Register build loggers. You might wish to 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 (see Using buildSrc to extract imperative logic for details of this feature).

Using an init script

There are several ways to use an init script:

  • Specify a file on the command line. The command line option is -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 of the files specified on the command line does not exist.

  • Put a file called init.gradle (or init.gradle.kts for Kotlin) in the USER_HOME/.gradle/ directory.

  • Put a file that ends with .gradle (or .init.gradle.kts for Kotlin) in the USER_HOME/.gradle/init.d/ directory.

  • Put a file that ends with .gradle (or .init.gradle.kts for Kotlin) in the GRADLE_HOME/init.d/ directory, in the Gradle distribution. This allows you to package up a custom Gradle distribution containing some custom build logic and plugins. You can combine this with the Gradle wrapper as a way 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. This allows, for example, a tool to specify an init script on the command line and the user to put one in their home directory for defining the environment and both scripts will run when Gradle is executed.

Writing an init script

Similar to 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 delegate to this Gradle instance.

Each init script also implements the Script interface.

Configuring projects from an init script

You can use an init script to configure the projects in the build. This works in a similar way 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. This sample uses this feature to configure an extra repository to be used only for certain environments.

Example 1. Using init script to perform extra configuration before projects are evaluated
build.gradle
repositories {
    mavenCentral()
}

task showRepos {
    doLast {
        println "All repos:"
        println repositories.collect { it.name }
    }
}
init.gradle
allprojects {
    repositories {
        mavenLocal()
    }
}
build.gradle.kts
repositories {
    mavenCentral()
}

task("showRepos") {
    doLast {
        println("All repos:")
        //TODO:kotlin-dsl remove filter once we're no longer on a kotlin eap
        println(repositories.map { it.name }.filter { it != "maven" })
    }
}
init.gradle.kts
allprojects {
    repositories {
        mavenLocal()
    }
}

Output when applying the init script

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

External dependencies for the init script

In External dependencies for the build script it was explained how to add external dependencies to a build script. Init scripts can also declare dependencies. You do this with the initscript() method, passing in a closure which declares the init script classpath.

Example 2. Declaring external dependencies for an init script
init.gradle
initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.apache.commons:commons-math:2.0'
    }
}
init.gradle.kts
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.

Example 3. An init script with external dependencies
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)
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))

Output when applying the init script

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

Init script plugins

Similar to a Gradle build script or a Gradle settings file, plugins can be applied on init scripts.

Example 4. Using plugins in init scripts
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
/*
 * Copyright 2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// tag::show-repos-task[]
repositories{
    mavenCentral()
}

 task showRepositories {
     doLast {
         repositories.each {
             println "repository: ${it.name} ('${it.url}')"
         }
     }
}
// end::show-repos-task[]
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
// tag::show-repos-task[]
repositories{
    mavenCentral()
}

task("showRepositories") {
    doLast {
        repositories.map { it as MavenArtifactRepository }.forEach {
            println("repository: ${it.name} ('${it.url}')")
        }
    }
}
// end::show-repos-task[]

Output when applying the init script

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