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[[writing_plugins]]
= Writing Plugins

If Gradle or the Gradle community does not offer the specific capabilities your project needs, creating your own custom plugin could be a solution.

Additionally, if you find yourself duplicating build logic across subprojects and need a better way to organize it, convention plugins can help.

== Script plugin

A plugin is any class that implements the link:{javadocPath}/org/gradle/api/Plugin.html[`Plugin`] interface.
For example, this is a "hello world" *plugin*:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="build.gradle.kts[tags=plugin]"]
include::sample[dir="snippets/unused/plugins/simple/groovy", files="build.gradle[tags=plugin]"]
====
<1> Extend the `org.gradle.api.Plugin` interface.
<2> Override the `apply` method.
<3> `apply` the plugin to the project.

=== 1. Extend the `org.gradle.api.Plugin` interface

Create a class that extends the `Plugin` interface:

====
[.multi-language-sample]
=====
.build.gradle.kts
[source, kotlin]
----
abstract class SamplePlugin : Plugin<Project> {
}
----
=====
[.multi-language-sample]
=====
.build.gradle
[source,groovy]
----
class SamplePlugin implements Plugin<Project> {
}
----
=====
====

=== 2. Override the `apply` method

Add tasks and other logic in the `apply()` method:

====
[.multi-language-sample]
=====
.build.gradle.kts
[source, kotlin]
----
override fun apply() {

}
----
=====
[.multi-language-sample]
=====
.build.gradle
[source,groovy]
----
void apply(Project project) {

}
----
=====
====

=== 3. `apply` the plugin to your project

When `SamplePlugin` is applied in your project, Gradle calls the `fun apply() {}` method defined.
This adds the `ScriptPlugin` task to your project:

====
[.multi-language-sample]
=====
.build.gradle.kts
[source, kotlin]
----
apply<SamplePlugin>()
----
=====
[.multi-language-sample]
=====
.build.gradle
[source,groovy]
----
apply plugin: SamplePlugin
----
=====
====

Note that this is a simple `hello-world` example and does *not* reflect best practices.

IMPORTANT: Script plugins are *not* recommended.

The best practice for developing plugins is to create _convention plugins_ or _binary plugins_.

== Pre-compiled script plugin

Pre-compiled script plugins offer an easy way to rapidly prototype and experiment.
They let you package build logic as `*.gradle(.kts)` script files using the Groovy or Kotlin DSL.
These scripts reside in specific directories, such as `src/main/groovy` or `src/main/kotlin`.

To apply one, simply use its `ID` derived from the script filename (without `.gradle`).
You can think of the file itself as the plugin, so you do not need to subclass the `Plugin` interface in a precompiled script.

Let's take a look at an example with the following structure:

[source,text]
----
.
└── buildSrc
    ├── build.gradle.kts
    └── src
       └── main
          └── kotlin
             └── my-create-file-plugin.gradle.kts
----

Our `my-create-file-plugin.gradle.kts` file contains the following code:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="buildSrc/src/main/kotlin/my-create-file-plugin.gradle.kts[]"]
include::sample[dir="snippets/unused/plugins/simple/groovy", files="buildSrc/src/main/groovy/my-create-file-plugin.gradle[]"]
====

The pre-compiled script can now be applied in the `build.gradle(.kts`) file of any subproject:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="build.gradle.kts[tags=apply-conv]"]
include::sample[dir="snippets/unused/plugins/simple/groovy", files="build.gradle[tags=apply-conv]"]
====

The `createMyFileTaskInConventionPlugin` task from the plugin is now available in your subproject.

== Binary Plugins

A binary plugin is a plugin that is implemented in a compiled language and is packaged as a JAR file.
It is resolved as a dependency rather than compiled from source.

For most use cases, convention plugins must be updated infrequently.
Having each developer execute the plugin build as part of their development process is wasteful, and we can instead distribute them as binary dependencies.

There are two ways to update the convention plugin in the example above into a binary plugin.

1. Use <<composite_builds.adoc#composite_builds,composite builds>>:
+
.settings.gradle.kts
[source,kotlin]
----
includeBuild("my-plugin")
----

2. <<publishing_gradle_plugins.adoc#sec:publishing_your_plugin,Publish the plugin>> to a repository:
+
.build.gradle.kts
[source,kotlin]
----
plugins {
    id("com.gradle.plugin.my-plugin") version "1.0.0"
}
----

Let's go with the second solution.
This plugin has been re-written in Kotlin and is called `MyCreateFileBinaryPlugin.kt`.
It is still stored in `buildSrc`:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="buildSrc/src/main/kotlin/MyCreateFileBinaryPlugin.kt[]"]
====

The plugin can be published and given an `id` using a `gradlePlugin{}` block so that it can be referenced in the root:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="buildSrc/build.gradle.kts[tags=plugin-3]"]
include::sample[dir="snippets/unused/plugins/simple/groovy", files="buildSrc/build.gradle[tags=plugin-3]"]
====

Then, the plugin can be applied in the build file:

====
include::sample[dir="snippets/unused/plugins/simple/kotlin", files="build.gradle.kts[tags=apply]"]
include::sample[dir="snippets/unused/plugins/simple/groovy", files="build.gradle[tags=apply]"]
====

Consult the <<plugins.adoc#custom_plugins,Developing Plugins chapter>> to learn more.
