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[[custom_plugins]]
= Understanding Plugins

Gradle comes with a set of powerful core systems such as dependency management, task execution, and project configuration.
But everything else it can do is supplied by plugins.

Plugins encapsulate logic for specific tasks or integrations, such as compiling code, running tests, or deploying artifacts.
By applying plugins, users can easily add new features to their build process without having to write complex code from scratch.

This plugin-based approach allows Gradle to be lightweight and modular.
It also promotes code reuse and maintainability, as plugins can be shared across projects or within an organization.

Before reading this chapter, it's recommended that you first read the <<gradle_directories.adoc#gradle_directories,Learning The Basics>> and complete the <<partr1_gradle_init.adoc#partr1_gradle_init,Tutorial>>.

== Plugins Introduction

Plugins can be sourced from Gradle or the Gradle community.
But when users want to organize their build logic or need specific build capabilities not provided by existing plugins, they can develop their own.

As such, we distinguish between three different kinds of plugins:

1. *Core Plugins* - plugins that come from Gradle.
2. *Community Plugins* - plugins that come from link:https://plugins.gradle.org/[Gradle Plugin Portal] or a public repository.
3. *Local or Custom Plugins* - plugins that you develop yourself.

=== Core Plugins
The term *core plugin* refers to a plugin that is part of the Gradle distribution such as the <<java_library_plugin.adoc#java_library_plugin,Java Library Plugin>>.
They are always available.

=== Community Plugins
The term *community plugin* refers to a plugin published to the Gradle Plugin Portal (or another public repository) such as the link:https://plugins.gradle.org/plugin/com.diffplug.gradle.spotless[Spotless Plugin].
In order for Gradle to find the plugin, you must declare the repository that provides the plugin.

=== Local or Custom Plugins
The term *local or custom plugin* refers to a plugin you write yourself for your own build.

== Custom plugins

There are three types of *custom plugins*:

[cols="~,~,~,~,~"]
|===
|# |Type |Location: |Most likely: |Benefit:

|<<sec:build_script_plugins,1>>
|<<sec:build_script_plugins,Build script plugins>>
|Build script
|A local plugin
|Plugin is automatically compiled and included in the classpath of the build script.

|<<sec:precompile_script_plugin,2>>
|<<sec:precompile_script_plugin,Precompiled script plugins>>
|<<sharing_build_logic_between_subprojects.adoc#sec:using_buildsrc,`buildSrc`>> folder
|A convention plugin
|Plugin is automatically compiled, tested, and available on the classpath of the build script. The plugin is visible to every build script used by the build.

|<<sec:custom_plugins_standalone_project,3>>
|<<sec:custom_plugins_standalone_project,Binary plugins>>
|Standalone project
|A shared plugin
|Plugin JAR is produced and published. The plugin can be used in multiple builds and shared with others.
|===

[[sec:build_script_plugins]]
== Build script plugins

*Build script plugins* are typically small, local plugins written in build files for tasks specific to a single build or project and do not need to be reused across multiple projects.

To create a Gradle plugin, you need to write a class that implements the link:{javadocPath}/org/gradle/api/Plugin.html[Plugin] interface in one of your build files.

[[sec:writing_a_simple_plugin]]
The following sample creates a `GreetingPlugin`, which adds a `hello` task to a project:

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

----
$ gradle -q hello
include::{snippetsPath}/customPlugins/customPlugin/tests/customPlugin.out[]
----

The `Project` object is passed as a parameter in `apply()`, which the plugin can use to configure the project however it needs to (such as adding tasks, configuring dependencies, etc.)

[[sec:precompile_script_plugin]]
== Precompiled script plugins

*Precompiled script plugins* are compiled into class files and packaged into a JAR before they are executed.
They are best used as *convention plugins* that share build logic across projects or as a way to neatly organize build logic.

To create a precompiled script plugin, you can:

1. Use Gradle's Kotlin DSL - The plugin is a `.gradle.kts` file, and apply `id("kotlin-dsl")`.
2. Use Gradle's Groovy DSL - The plugin is a `.gradle` file, and apply `id("groovy-gradle-plugin")`.
3. Write a Java, Scala, Kotlin, or Groovy class - The plugin is an abstract class that implements the `Plugin<Project>` interface, and apply `id("java-gradle-plugin")`.

To apply a precompiled script plugin, you need to know its ID.
The ID is derived from the plugin script's filename and its (optional) package declaration.

For example, the script `src/main/\*/java-library-convention.gradle(.kts)` has a plugin ID of `java-library-convention` (assuming it has no package declaration).
Likewise, `src/main/\*/my/java-library-convention.gradle(.kts)` has a plugin ID of `my.java-library-convention` as long as it has a package declaration of `my`.

Precompiled script plugin names have two important limitations:

* They cannot start with `org.gradle`.
* They cannot have the same name as a <<plugin_reference#plugin_reference,core plugin>>.

When the plugin is applied to a project, Gradle creates an instance of the plugin class and calls the instance's link:{javadocPath}/org/gradle/api/Plugin.html#apply-T-[Plugin.apply()] method.

NOTE: A new instance of a `Plugin` is created within each project applying that plugin.

Let's rewrite the `GreetingPlugin` script plugin as a precompiled script plugin:

====
include::sample[dir="snippets/plugins/greetingPlugin/kotlin",files="buildSrc/src/main/kotlin/com/example/ConventionGreetingPlugin.gradle.kts[]"]
include::sample[dir="snippets/plugins/greetingPlugin/groovy",files="buildSrc/src/main/groovy/com/example/ConventionGreetingPlugin.gradle[]"]
====

The `GreetingPlugin` can now be applied in other subprojects' builds by using its ID:

====
include::sample[dir="snippets/plugins/greetingPlugin/kotlin",files="app/build.gradle.kts[tag=use-plugin]"]
include::sample[dir="snippets/plugins/greetingPlugin/groovy",files="app/build.gradle[tag=use-plugin]"]
====

[[sec:convention_plugins]]
== Convention Plugins

A *convention plugin* is typically a plugin that configures existing core and community plugins with your own conventions (i.e. default values) such as setting the Java version by using `java.toolchain.languageVersion.set(JavaLanguageVersion.of(17))`.
Convention plugins are also used to enforce project standards and help streamline the build process.
They can apply and configure plugins, create new tasks and extensions, set dependencies, and much more.

Let's take an example build with three subprojects: one for `data-model`, one for `database-logic` and one for `app` code.
The project has the following structure:

[source, text]
----
.
├── buildSrc
│   ├── src
│   │   └──...
│   └── build.gradle.kts
├── data-model
│   ├── src
│   │   └──...
│   └── build.gradle.kts
├── database-logic
│   ├── src
│   │   └──...
│   └── build.gradle.kts
├── app
│   ├── src
│   │   └──...
│   └── build.gradle.kts
└── settings.gradle.kts
----

The build file of the `database-logic` subproject is as follows:

====
include::sample[dir="snippets/plugins/pluginProject/kotlin",files="database-logic/build.gradle.kts[]"]
include::sample[dir="snippets/plugins/pluginProject/groovy",files="database-logic/build.gradle[]"]
====

We apply the `java-library` plugin and add the `org.jetbrains.kotlin.jvm` plugin for Kotlin support.
We also configure Kotlin, Java, tests and more.

Our build file is beginning to grow...

The more plugins we apply and the more plugins we configure, the larger it gets.
There's also repetition in the build files of the `app` and `data-model` subprojects, especially when configuring common extensions like setting the Java version and Kotlin support.

To address this, we use convention plugins.
This allows us to avoid repeating configuration in each build file and keeps our build scripts more concise and maintainable.
In convention plugins, we can encapsulate arbitrary build configuration or custom build logic.

To develop a convention plugin, we recommend using <<sharing_build_logic_between_subprojects.adoc#sec:using_buildsrc,`buildSrc`>> – which represents a completely separate Gradle build.
`buildSrc` has its own settings file to define where dependencies of this build are located.

We add a Kotlin script called `my-java-library.gradle.kts` inside the `buildSrc/src/main/kotlin` directory.
Or conversely, a Groovy script called `my-java-library.gradle` inside the `buildSrc/src/main/groovy` directory.
We put all the plugin application and configuration from the `database-logic` build file into it:

====
include::sample[dir="snippets/plugins/pluginProject/kotlin",files="buildSrc/src/main/kotlin/my-java-library.gradle.kts[]"]
include::sample[dir="snippets/plugins/pluginProject/groovy",files="buildSrc/src/main/groovy/my-java-library.gradle[]"]
====

The name of the file `my-java-library` is the ID of our brand-new plugin, which we can now use in all of our subprojects.

The `database-logic` build file becomes much simpler by removing all the redundant build logic and applying our convention `my-java-library` plugin instead:

====
[.multi-language-sample]
=====
.database-logic/build.gradle.kts
[source,kotlin]
----
plugins {
    id("my-java-library")
}
----
=====
[.multi-language-sample]
=====
..database-logic/build.gradle
[source,groovy]
----
plugins {
    id('my-java-library')
}
----
=====
====

This convention plugin enables us to easily share common configurations across all our build files.
Any modifications can be made in one place, simplifying maintenance.

[[sec:custom_plugins_standalone_project]]
== Binary plugins

*Binary plugins* in Gradle are plugins that are built as standalone JAR files and applied to a project using the `plugins{}` block in the build script.

Let's move our `GreetingPlugin` to a standalone project so that we can publish it and share it with others.
The plugin is essentially moved from the `buildSrc` folder to its own build called `greeting-plugin`.

NOTE: You can publish the plugin from `buildSrc`, but this is not recommended practice. Plugins that are ready for publication should be in their own build.

`greeting-plugin` is simply a Java project that produces a JAR containing the plugin classes.

The easiest way to package and publish a plugin to a repository is to use the <<java_gradle_plugin.adoc#java_gradle_plugin,Gradle Plugin Development Plugin>>.
This plugin provides the necessary tasks and configurations (including the plugin metadata) to compile your script into a plugin that can be applied in other builds.

Here is a simple build script for the `greeting-plugin` project using the Gradle Plugin Development Plugin:

====
include::sample[dir="snippets/plugins/customPlugin/kotlin/java-gradle-plugin",files="build.gradle.kts[tags=use-and-configure-plugin]"]
include::sample[dir="snippets/plugins/customPlugin/groovy/java-gradle-plugin",files="build.gradle[tags=use-and-configure-plugin]"]
====

For more on publishing plugins, see <<publishing_gradle_plugins.adoc#plugin-publishing-plugin,Publishing Plugins>>.

==  Project vs Settings vs Init plugins

In the example used through this section, the plugin accepts the link:{groovyDslPath}/org.gradle.api.Project.html[Project] type as a type parameter.
Alternatively, the plugin can accept a parameter of type link:{groovyDslPath}/org.gradle.api.initialization.Settings.html[Settings] to be applied in a settings script, or a parameter of type link:{groovyDslPath}/org.gradle.api.invocation.Gradle.html[Gradle] to be applied in an initialization script.

The difference between these types of plugins lies in the scope of their application:

Project Plugin :: A project plugin is a plugin that is applied to a specific project in a build.
It can customize the build logic, add tasks, and configure the project-specific settings.

Settings Plugin :: A settings plugin is a plugin that is applied in the `settings.gradle` or `settings.gradle.kts` file.
It can configure settings that apply to the entire build, such as defining which projects are included in the build, configuring build script repositories, and applying common configurations to all projects.

Init Plugin :: An init plugin is a plugin that is applied in the `init.gradle` or `init.gradle.kts` file.
It can configure settings that apply globally to all Gradle builds on a machine, such as configuring the Gradle version, setting up default repositories, or applying common plugins to all builds.
