# Build File Basics

version 9.0.0

Generally, a build script (`build.gradle(.kts)`) details build configuration, tasks, and plugins.

Every Gradle build comprises at least one build script.

## Build scripts

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

The Groovy DSL and the Kotlin DSL are the only accepted languages for Gradle scripts.

In multi-project builds, each subproject typically has its own build file located in its root directory.

Inside a build script, you’ll typically specify:

- Plugins: Tools that extend Gradle’s functionality for tasks like compiling code, running tests, or packaging artifacts.

- Dependencies: External libraries and tools your project uses.

Specifically, build scripts contain two main types of dependencies:

- Gradle and Build Script Dependencies: These include plugins and libraries required by Gradle itself or the build script logic.

- Project Dependencies: Libraries required directly by your project’s source code to compile and run correctly.

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

app/build.gradle.kts

    plugins {   (1)
        // Apply the application plugin to add support for building a CLI application in Java.
        application
    }
    dependencies {  (2)
        // Use JUnit Jupiter for testing.
        testImplementation(libs.junit.jupiter)

        testRuntimeOnly("org.junit.platform:junit-platform-launcher")

        // This dependency is used by the application.
        implementation(libs.guava)
    }
    application {   (3)
        // Define the main class for the application.
        mainClass = "org.example.App"
    }

app/build.gradle

    plugins {   (1)
        // Apply the application plugin to add support for building a CLI application in Java.
        id 'application'
    }
    dependencies {  (2)
        // Use JUnit Jupiter for testing.
        testImplementation libs.junit.jupiter

        testRuntimeOnly 'org.junit.platform:junit-platform-launcher'

        // This dependency is used by the application.
        implementation libs.guava
    }
    application {   (3)
        // Define the main class for the application.
        mainClass = 'org.example.App'
    }

1 | Add plugins.
2 | Add dependencies.
3 | Use convention properties.

## 1. Add plugins

Plugins extend Gradle’s functionality and can contribute tasks to a project.

Adding a plugin to a build is called applying a plugin and makes additional functionality available.

app/build.gradle.kts

    plugins {   (1)
        // Apply the application plugin to add support for building a CLI application in Java.
        application
    }

app/build.gradle

    plugins {   (1)
        // Apply the application plugin to add support for building a CLI application in Java.
        id 'application'
    }

The `application` plugin facilitates creating an executable JVM application.

Applying the Application plugin also implicitly applies the Java plugin. The `java` plugin adds Java compilation along with testing and bundling capabilities to a project.

## 2. Add dependencies

Your project needs external libraries to compile, run, and test.

In this example, the project uses `JUnit Jupiter` for testing and Google’s `Guava` library in the main application code:

app/build.gradle.kts

    dependencies {  (2)
        // Use JUnit Jupiter for testing.
        testImplementation(libs.junit.jupiter)

        testRuntimeOnly("org.junit.platform:junit-platform-launcher")

        // This dependency is used by the application.
        implementation(libs.guava)
    }

app/build.gradle

    dependencies {  (2)
        // Use JUnit Jupiter for testing.
        testImplementation libs.junit.jupiter

        testRuntimeOnly 'org.junit.platform:junit-platform-launcher'

        // This dependency is used by the application.
        implementation libs.guava
    }

## 3. Use convention properties

A plugin adds tasks to a project. It also adds properties and methods to a project.

The `application` plugin defines tasks that package and distribute an application, such as the `run` task.

The Application plugin provides a way to declare the main class of a Java application, which is required to execute the code.

app/build.gradle.kts

    application {   (3)
        // Define the main class for the application.
        mainClass = "org.example.App"
    }

app/build.gradle

    application {   (3)
        // Define the main class for the application.
        mainClass = 'org.example.App'
    }

In this example, the main class (i.e., the point where the program’s execution begins) is `org.example.App`.

Build scripts are evaluated during the configuration phase of a build, and they serve as the main entry point for defining a (sub)project’s build logic. In addition to applying plugins and setting convention properties, build scripts can:

- Declare dependencies

- Configure tasks

- Reference shared settings (from version catalogs or convention plugins)

To learn more about scripting the build file, see Writing Build Files.

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