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[[writing_build_scripts]]
= Writing Build Scripts

The initialization phase in the Gradle Build lifecycle finds the root project and subprojects included in your <<directory_layout#dir:project_root,project root directory>> using the settings file.

image::author-gradle-6.png[]

Then, for each project included in the settings file, Gradle creates a link:{javadocPath}org/gradle/api/Project.html[`Project`] instance.

Gradle then looks for a corresponding build script file, which is used in the configuration phase.

== Build Scripts

Every Gradle build comprises one or more *projects*; a _root_ project and _subprojects_.

A project typically corresponds to a software component that needs to be built, like a library or an application.
It might represent a library JAR, a web application, or a distribution ZIP assembled from the JARs produced by other projects.

On the other hand, it might represent a thing to be done, such as deploying your application to staging or production environments.

Gradle scripts are written in either Groovy DSL or Kotlin DSL (domain-specific language).

A *build script* configures a *project* and is associated with an object of type link:{javadocPath}/org/gradle/api/Project.html[`Project`].

image::author-gradle-4.png[Build,500,63,align="left"]

As the build script executes, it configures `Project`.

The build script is either a `\*.gradle` file in Groovy or a `*.gradle.kts` file in Kotlin.

IMPORTANT: _Build scripts_ configure `Project` objects and their children.

== The `Project` object

The `Project` object is part of the link:{javadocPath}/org/gradle/api/Project.html[Gradle API]:

- In the Groovy DSL, the `Project` object documentation is found link:{groovyDslPath}/org.gradle.api.Project.html[here].
- In the Kotlin DSL, the `Project` object documentation is found link:{kotlinDslPath}/gradle/org.gradle.api/-project/index.html[here].

Many top-level properties and blocks in a build script are part of the Project API.

For example, the following build script uses the link:{groovyDslPath}/org.gradle.api.Project.html#org.gradle.api.Project:name[Project.name] property to print the name of the project:

====
include::sample[dir="snippets/tutorial/projectApi/kotlin",files="build.gradle.kts[tags=project-name]"]
include::sample[dir="snippets/tutorial/projectApi/groovy",files="build.gradle[tags=project-name]"]
----
$ gradle -q check
include::{snippetsPath}/tutorial/projectApi/tests/projectApi.out[]
----
====

Both `println` statements print out the same property.

The first uses the top-level reference to the `name` property of the `Project` object.
The second statement uses the `project` property available to any build script, which returns the associated `Project` object.

[[sec:standard_project_properties]]
=== Standard project properties

The `Project` object exposes a standard set of properties in your build script.

The following table lists a few commonly used properties:

[%header%autowidth,compact]
|===
| Name | Type | Description

| `name`
| `String`
| The name of the project directory.

| `path`
| `String`
| The fully qualified name of the project.

| `description`
| `String`
| A description for the project.

| `dependencies`
| `DependencyHandler`
| Returns the dependency handler of the project.

| `repositories`
| `RepositoryHandler`
| Returns the repository handler of the project.

| `layout`
| `ProjectLayout`
| Provides access to several important locations for a project.

| `group`
| `Object`
| The group of this project.

| `version`
| `Object`
| The version of this project.
|===

The following table lists a few commonly used methods:

[%autowidth.stretch]
|===
| Name | Description

| `uri()`
| Resolves a file path to a URI, relative to the project directory of this project.

| `task()`
| Creates a Task with the given name and adds it to this project.
|===

== Build Script structure

The Build script is composed of `{ ... }`, a special object in both Groovy and Kotlin.
This object is called a _lambda_ in Kotlin or a _closure_ in Groovy.

Simply put, the `plugins{ }` block is a method invocation in which a Kotlin _lambda_ object or Groovy _closure_ object is passed as the argument.
It is the short form for:

[source,text]
----
plugins(function() {
    id("plugin")
})
----

Blocks are mapped to Gradle API methods.

The code inside the function is executed against a `this` object called a _receiver_ in Kotlin lambda and a _delegate_ in Groovy closure.
Gradle determines the correct `this` object and invokes the correct corresponding method.
The `this` of the method invocation `id("plugin")` object is of type link:{javadocPath}/org/gradle/plugin/use/PluginDependenciesSpec.html[`PluginDependenciesSpec`].

The build script is essentially composed of Gradle API calls built on top of the DSLs.
Gradle executes the script line by line, top to bottom.

Let's take a look at an example and break it down:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[]"]
====
<1> Apply plugins to the build.
<2> Define the locations where dependencies can be found.
<3> Add dependencies.
<4> Set properties.
<5> Register and configure tasks.

=== 1. Apply plugins to the build
Plugins are used to extend Gradle. They are also used to modularize and reuse project configurations.

Plugins can be applied using the `PluginDependenciesSpec` plugins script block.

The plugins block is preferred:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=plugins]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=plugins]"]
====

In the example, the `application` plugin, which is included with Gradle, has been applied, describing our project as a Java application.

The Kotlin gradle plugin, version 1.9.0, has also been applied.
This plugin is not included with Gradle and, therefore, has to be described using a `plugin id` and a `plugin version` so that Gradle can find and apply it.

=== 2. Define the locations where dependencies can be found

A project generally has a number of dependencies it needs to do its work.
Dependencies include plugins, libraries, or components that Gradle must download for the build to succeed.

The build script lets Gradle know where to look for the binaries of the dependencies.
More than one location can be provided:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=repo]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=repo]"]
====

In the example, the `guava` library and the JetBrains Kotlin plugin (`org.jetbrains.kotlin.jvm`) will be downloaded from the link:https://repo.maven.apache.org/maven2/[Maven Central Repository].

=== 3. Add dependencies

A project generally has a number of dependencies it needs to do its work.
These dependencies are often libraries of precompiled classes that are imported in the project's source code.

Dependencies are managed via <<glossary.adoc#sub:terminology_configuration,configurations>> and are retrieved from repositories.

Use the `DependencyHandler` returned by `Project.getDependencies()` method to manage the dependencies.
Use the `RepositoryHandler` returned by `Project.getRepositories()` method to manage the repositories.

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=dep]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=dep]"]
====

In the example, the application code uses Google's `guava` libraries.
Guava provides utility methods for collections, caching, primitives support, concurrency, common annotations, string processing, I/O, and validations.

=== 4. Set properties

A plugin can add properties and methods to a project using extensions.

The link:{groovyDslPath}/org.gradle.api.Project.html[`Project`] object has an associated link:{javadocPath}/org/gradle/api/plugins/ExtensionContainer.html[`ExtensionContainer`] object that contains all the settings and properties for the plugins that have been applied to the project.

In the example, the `application` plugin added an `application` property, which is used to detail the main class of our Java application:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=app]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=app]"]
====

=== 5. Register and configure tasks

Tasks perform some basic piece of work, such as compiling classes, or running unit tests, or zipping up a WAR file.

While tasks are typically defined in plugins, you may need to register or configure tasks in build scripts.

*Registering* a task adds the task to your project.

You can register tasks in a project using the link:{javadocPath}/org/gradle/api/tasks/TaskContainer.html[`TaskContainer.register(java.lang.String)`] method:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=task]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=task]"]
====

You may have seen usage of the link:{javadocPath}/org/gradle/api/tasks/TaskContainer.html[`TaskContainer.create(java.lang.String)`] method *which should be avoided*.

[source,kotlin]
----
tasks.create<Zip>("zip-reports") { }
----

TIP: `register()`, which enables <<task_configuration_avoidance.adoc#task_configuration_avoidance,task configuration avoidance>>, is preferred over `create()`.

You can locate a task to configure it using the `TaskCollection.named(java.lang.String)` method:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=test]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=test]"]
====

The example below configures the link:{groovyDslPath}/org.gradle.api.tasks.javadoc.Javadoc.html[`Javadoc`] task to automatically generate HTML documentation from Java code:

====
include::sample[dir="snippets/plugins/simple/kotlin/sub-project-a", files="build.gradle.kts[tags=doc]"]
include::sample[dir="snippets/plugins/simple/groovy/sub-project-a", files="build.gradle[tags=doc]"]
====

== Build Scripting

A build script is made up of zero or more statements and script blocks:

[source,text]
----
println(project.layout.projectDirectory);
----

Statements can include method calls, property assignments, and local variable definitions:

[source,text]
----
version = '1.0.0.GA'
----

A script block is a method call which takes a closure/lambda as a parameter:

[source,text]
----
configurations {
}
----

The closure/lambda configures some delegate object as it executes:

[source,text]
----
repositories {
    google()
}
----

A build script is also a Groovy or a Kotlin script:

====
include::sample[dir="snippets/tutorial/upper/kotlin", files="build.gradle.kts[]"]
include::sample[dir="snippets/tutorial/upper/groovy", files="build.gradle[]"]
----
$ gradle -q upper
include::{snippetsPath}/tutorial/upper/tests/upper.out[]
----
====

It can contain elements allowed in a Groovy or Kotlin script, such as method definitions and class definitions:

====
include::sample[dir="snippets/tutorial/count/kotlin", files="build.gradle.kts[]"]
include::sample[dir="snippets/tutorial/count/groovy", files="build.gradle[]"]

----
$ gradle -q count
include::{snippetsPath}/tutorial/count/tests/count.out[]
----
====

[[sec:dynamic_tasks]]
=== Flexible task registration

Using the capabilities of the Groovy or Kotlin language, you can register multiple tasks in a loop:

====
include::sample[dir="snippets/tutorial/dynamic/kotlin", files="build.gradle.kts[]"]
include::sample[dir="snippets/tutorial/dynamic/groovy", files="build.gradle[]"]

----
$ gradle -q task1
include::{snippetsPath}/tutorial/dynamic/tests/dynamic.out[]
----
====

[[sec:declaring_variables]]
=== Declare Variables

Build scripts can declare two variables: *local variables* and *extra properties*.

[[sec:local_variables]]
==== Local Variables

[.multi-language-text.lang-kotlin]
Declare local variables with the `val` keyword. Local variables are only visible in the scope where they have been declared. They are a feature of the underlying Kotlin language.

[.multi-language-text.lang-groovy]
Declare local variables with the `def` keyword. Local variables are only visible in the scope where they have been declared. They are a feature of the underlying Groovy language.

====
include::sample[dir="snippets/tutorial/localVariables/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/tutorial/localVariables/groovy",files="build.gradle[]"]
====

[[sec:extra_properties]]
==== Extra Properties

Gradle's enhanced objects, including projects, tasks, and source sets, can hold user-defined properties.

[.multi-language-text.lang-kotlin]
Add, read, and set extra properties via the owning object's `extra` property. Alternatively, you can access extra properties via Kotlin delegated properties using `by extra`.

[.multi-language-text.lang-groovy]
Add, read, and set extra properties via the owning object's `ext` property. Alternatively, you can use an `ext` block to add multiple properties simultaneously.

====
include::sample[dir="snippets/tutorial/extraProperties/kotlin",files="build.gradle.kts[tags=extraProperties]"]
include::sample[dir="snippets/tutorial/extraProperties/groovy",files="build.gradle[tags=extraProperties]"]

----
$ gradle -q printProperties
include::{snippetsPath}/tutorial/extraProperties/tests/extraProperties.out[]
----
====

[.multi-language-text.lang-kotlin]
This example adds two extra properties to the `project` object via `by extra`. Additionally, this example adds a property named `purpose` to each source set by setting `extra["purpose"]` to `null`. Once added, you can read and set these properties via `extra`.

[.multi-language-text.lang-groovy]
This example adds two extra properties to the `project` object via an `ext` block. Additionally, this example adds a property named `purpose` to each source set by setting `ext.purpose` to `null`. Once added, you can read and set all these properties just like predefined ones.

Gradle requires special syntax for adding a property so that it can fail fast. For example, this allows Gradle to recognize
when a script attempts to set a property that does not exist. You can access extra properties anywhere where you can
access their owning object. This gives extra properties a wider scope than local variables.
Subprojects can access extra properties on their parent projects.

For more information about extra properties, see
link:{groovyDslPath}/org.gradle.api.plugins.ExtraPropertiesExtension.html[ExtraPropertiesExtension] in the API documentation.

[[sec:configuring_arbitrary_objects]]
=== Configure Arbitrary Objects

The example `greet()` task shows an example of arbitrary object configuration:

====
include::sample[dir="snippets/tutorial/configureObject/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/tutorial/configureObject/groovy",files="build.gradle[]"]

----
$ gradle -q greet
include::{snippetsPath}/tutorial/configureObject/tests/configureObject.out[]
----
====

[[sec:closure_delegate]]
==== Closure Delegates

Each closure has a `delegate` object. Groovy uses this delegate to look up variable and method references to nonlocal variables and closure parameters.
Gradle uses this for _configuration closures_,
where the `delegate` object refers to the object being configured.

====
include::sample[dir="snippets/tutorial/groovy/groovy",files="build.gradle[tags=closureDelegates]"]
====

[[script-default-imports]]
== Default imports

To make build scripts more concise, Gradle automatically adds a set of import statements to scripts.

As a result, instead of writing `throw new org.gradle.api.tasks.StopExecutionException()`, you can write `throw new StopExecutionException()` instead.

Gradle implicitly adds the following imports to each script:

[source,groovy]
----
include::default-imports.txt[]
----

[[sec:build_script_external_dependencies]]
[[sec:configuring_arbitrary_objects_using_an_external_script]]
[[sec:closures_as_the_last_parameter_in_a_method]]

[.text-right]
**Next Step:** <<tutorial_using_tasks.adoc#tutorial_using_tasks,Learn how to use Tasks>> >>
