# Chapter 27. Gradle Plugins

Gradle at its core intentionally provides very little for real world automation. All of the useful features, like the ability to compile Java code, are added by plugins. Plugins add new tasks (e.g. `JavaCompile`), domain objects (e.g. `SourceSet`), conventions (e.g. Java source is located at `src/main/java`) as well as extending core objects and objects from other plugins.

In this chapter we discuss how to use plugins and the terminology and concepts surrounding plugins.

## 27.1. What plugins do

Applying a plugin to a project allows the plugin to extend the project's capabilities. It can do things such as:

- Extend the Gradle model (e.g. add new DSL elements that can be configured)
- Configure the project according to conventions (e.g. add new tasks or configure sensible defaults)
- Apply specific configuration (e.g. add organizational repositories or enforce standards)

By applying plugins, rather than adding logic to the project build script, we can reap a number of benefits. Applying plugins:

- Promotes reuse and reduces the overhead of maintaining similar logic across multiple projects
- Allows a higher degree of modularization, enhancing comprehensibility and organization
- Encapsulates imperative logic and allows build scripts to be as declarative as possible

## 27.2. Types of plugins

There are two general types of plugins in Gradle, script plugins and binary plugins. Script plugins are additional build scripts that further configure the build and usually implement a declarative approach to manipulating the build. They are typically used within a build although they can be externalized and accessed from a remote location. Binary plugins are classes that implement the `Plugin` interface and adopt a programmatic approach to manipulating the build. Binary plugins can reside within a build script, within the project hierarchy or externally in a plugin jar.

A plugin often starts out as a script plugin (because they are easy to write) and then, as the code becomes more valuable, it's migrated to a binary plugin that can be easily tested and shared between multiple projects or organizations.

## 27.3. Using plugins

To use the build logic encapsulated in a plugin, Gradle needs to perform two steps. First, it needs to resolve the plugin, and then it needs to apply the plugin to the target, usually a `Project`.

Resolving a plugin means finding the correct version of the jar which contains a given plugin and adding it the script classpath. Once a plugin is resolved, its API can be used in a build script. Script plugins are self-resolving in that they are resolved from the specific file path or URL provided when applying them. Core binary plugins provided as part of the Gradle distribution are automatically resolved.

Applying a plugin means actually executing the plugin's `Plugin.apply(T)` on the Project you want to enhance with the plugin. Applying plugins is idempotent. That is, you can safely apply any plugin multiple times without side effects.

The most common use case for using a plugin is to both resolve the plugin and apply it to the current project. Since this is such a common use case, it's recommended that build authors use the plugins DSL to both resolve and apply plugins in one step. The feature is technically still incubating, but it works well, and should be used by most users.

## 27.4. Script plugins

Example 27.1. Applying a script plugin

`build.gradle`

    apply from: 'other.gradle'

Script plugins are automatically resolved and can be applied from a script on the local filesystem or at a remote location. Filesystem locations are relative to the project directory, while remote script locations are specified with an HTTP URL. Multiple script plugins (of either form) can be applied to a given target.

## 27.5. Binary plugins

You apply plugins by their plugin id, which is a globally unique identifier, or name, for plugins. Core Gradle plugins are special in that they provide short names, such as `'java'` for the core `JavaPlugin`. All other binary plugins must use the fully qualified form of the plugin id (e.g. `com.github.foo.bar`), although some legacy plugins may still utilize a short, unqualified form. Where you put the plugin id depends on whether you are using the plugins DSL or the buildscript block.

### 27.5.1. Locations of binary plugins

A plugin is simply any class that implements the `Plugin` interface. Gradle provides the core plugins (e.g. `JavaPlugin`) as part of its distribution which means they are automatically resolved. However, non-core binary plugins need to be resolved before they can be applied. This can be achieved in a number of ways:

- Including the plugin from the plugin portal or a custom repository using the plugins DSL (see Section 27.5.2, “Applying plugins with the plugins DSL”).
- Including the plugin from an external jar defined as a buildscript dependency (see the section called “Applying plugins with the buildscript block”).
- Defining the plugin as a source file under the buildSrc directory in the project (see Section 43.4, “Build sources in the `buildSrc` project”).
- Defining the plugin as an inline class declaration inside a build script.

For more on defining your own plugins, see Chapter 41, Writing Custom Plugins.

### 27.5.2. Applying plugins with the plugins DSL

The plugins DSL is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The new plugins DSL provides a succinct and convenient way to declare plugin dependencies. It works with the Gradle plugin portal to provide easy access to both core and community plugins. The plugins DSL block configures an instance of `PluginDependenciesSpec`.

To apply a core plugin, the short name can be used:

Example 27.2. Applying a core plugin

`build.gradle`

    plugins {
        id 'java'
    }

To apply a community plugin from the portal, the fully qualified plugin id must be used:

Example 27.3. Applying a community plugin

`build.gradle`

    plugins {
        id "com.jfrog.bintray" version "0.4.1"
    }

See `PluginDependenciesSpec` for more information on using the Plugin DSL.

#### Limitations of the plugins DSL

This way of adding plugins to a project is much more than a more convenient syntax. The plugins DSL is processed in a way which allows Gradle to determine the plugins in use very early and very quickly. This allows Gradle to do smart things such as:

- Optimize the loading and reuse of plugin classes.
- Allow different plugins to use different versions of dependencies.
- Provide editors detailed information about the potential properties and values in the buildscript for editing assistance.

This requires that plugins be specified in a way that Gradle can easily and quickly extract, before executing the rest of the build script. It also requires that the definition of plugins to use be somewhat static.

There are some key differences between the new plugin mechanism and the “traditional” `apply()` method mechanism. There are also some constraints, some of which are temporary limitations while the mechanism is still being developed and some are inherent to the new approach.

##### Constrained Syntax

The new `plugins {}` block does not support arbitrary Groovy code. It is constrained, in order to be idempotent (produce the same result every time) and side effect free (safe for Gradle to execute at any time).

The form is:

    plugins {
        id «plugin id» version «plugin version» [apply «false»]
    }

Where `«plugin version»` and `«plugin id»` must be constant, literal, strings and the `apply` statement with a `boolean` can be used to disable the default behavior of applying the plugin immediately (e.g. you want to apply it only in `subprojects`). No other statements are allowed; their presence will cause a compilation error.

The `plugins {}` block must also be a top level statement in the buildscript. It cannot be nested inside another construct (e.g. an if-statement or for-loop).

##### Can only be used in build scripts

The `plugins {}` block can currently only be used in a project's build script. It cannot be used in script plugins, the settings.gradle file or init scripts.

Future versions of Gradle will remove this restriction.

If the restrictions of the new syntax are prohibitive, the recommended approach is to apply plugins using the buildscript {} block.

#### Applying plugins to subprojects

If you have a multi-project build, you probably want to apply plugins to some or all of the subprojects in your build, but not to the `root` or `master` project. The default behavior of the `plugins {}` block is to immediately `resolve` and `apply` the plugins. But, you can use the `apply false` syntax to tell Gradle not to apply the plugin to the current project and then use `apply plugin: «plugin version»` in the `subprojects` block:

Example 27.4. Applying plugins only on certain subprojects.

`settings.gradle`

    include 'helloA'
    include 'helloB'
    include 'goodbyeC'

`build.gradle`

    plugins {
      id "org.gradle.sample.hello" version "1.0.0" apply false
      id "org.gradle.sample.goodbye" version "1.0.0" apply false
    }

    subprojects { subproject ->
        if (subproject.name.startsWith("hello")) {
            apply plugin: 'org.gradle.sample.hello'
        }
        if (subproject.name.startsWith("goodbye")) {
            apply plugin: 'org.gradle.sample.goodbye'
        }
    }

If you then run `gradle hello` you'll see that only the helloA and helloB subprojects had the hello plugin applied.

    gradle/subprojects/docs/src/samples/plugins/multiproject $> gradle hello
    Parallel execution is an incubating feature.
    :helloA:hello
    :helloB:hello
    Hello!
    Hello!

    BUILD SUCCESSFUL

#### Custom Plugin Repositories

The `pluginRepositories {}` DSL is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

By default, the `plugins {}` DSL resolves plugins from the public Gradle Plugin Portal. Many build authors would also like to resolve plugins from private Maven or Ivy repositories because the plugins contain proprietary implementation details, or just to have more control over what plugins are available to their builds.

To specify custom plugin repositories, add a `pluginRepositories {}` block to the `settings.gradle` file:

Example 27.5. Using plugins from custom plugin repositories.

`settings.gradle`

    pluginRepositories {
      maven {
        url 'maven-repo'
      }
      gradlePluginPortal()
      ivy {
        url 'ivy-repo'
      }
    }

This tells Gradle to first look in the Maven repository at `maven-repo` when resolving plugins and then to check the Gradle Plugin Portal if the plugins are not found in the Maven repository. If you don't want the Gradle Plugin Portal to be searched, omit the `gradlePluginPortal()` line. Finally, the Ivy repository at `ivy-repo` will be checked.

The `pluginRepositories {}` block may only appear in the `settings.gradle` file, and must be the first block in the file. Custom Maven and Ivy plugin repositories must contain plugin marker artifacts in addition to the artifacts which actually implement the plugin. For more information on publishing plugins to custom repositories read Chapter 42, The Java Gradle Plugin Development Plugin.

See `PluginRepositoriesSpec` for complete documentation for using the `pluginRepositories {}` block.

#### Plugin Marker Artifacts

Since the `plugins {}` DSL block only allows for declaring plugins by their globally unique plugin `id` and `version` properties, Gradle needs a way to look up the coordinates of the plugin implementation artifact. To do so, Gradle will look for a Plugin Marker Artifact with the coordinates `plugin.id:plugin.id.gradle.plugin:plugin.version`. This marker needs to have a dependency on the actual plugin implementation. Publishing these markers is automated by the java-gradle-plugin.

For example, the following complete sample from the `sample-plugins` project shows how to publish a `org.gradle.sample.hello` plugin and a `org.gradle.sample.goodbye` plugin to both an Ivy and Maven repository using the combination of the java-gradle-plugin, the maven-publish plugin, and the ivy-publish plugin.

Example 27.6. Complete Plugin Publishing Sample

`build.gradle`

    plugins {
      id 'java-gradle-plugin'
      id 'maven-publish'
      id 'ivy-publish'
    }

    group 'org.gradle.sample'
    version '1.0.0'

    gradlePlugin {
      plugins {
        hello {
          id = "org.gradle.sample.hello"
          implementationClass = "org.gradle.sample.hello.HelloPlugin"
        }
        goodbye {
          id = "org.gradle.sample.goodbye"
          implementationClass = "org.gradle.sample.goodbye.GoodbyePlugin"
        }
      }
    }

    publishing {
      repositories {
        maven {
          url "../consuming/maven-repo"
        }
        ivy {
          url "../consuming/ivy-repo"
        }
      }
    }

Running `gradle publish` in the sample directory causes the following repo layouts to exist:

### 27.5.3. Legacy Plugin Application

With the introduction of the plugins DSL, users should have little reason to use the legacy method of applying plugins. It is documented here in case a build author cannot use the plugins DSL due to restrictions in how it currently works.

#### Applying Binary Plugins

Example 27.7. Applying a binary plugin

`build.gradle`

    apply plugin: 'java'

Plugins can be applied using a plugin id. In the above case, we are using the short name ‘`java`’ to apply the `JavaPlugin`.

Rather than using a plugin id, plugins can also be applied by simply specifying the class of the plugin:

Example 27.8. Applying a binary plugin by type

`build.gradle`

    apply plugin: JavaPlugin

The `JavaPlugin` symbol in the above sample refers to the the `JavaPlugin`. This class does not strictly need to be imported as the `org.gradle.api.plugins` package is automatically imported in all build scripts (see Section 18.8, “Default imports”). Furthermore, it is not necessary to append `.class` to identify a class literal in Groovy as it is in Java.

#### Applying plugins with the buildscript block

Binary plugins that have been published as external jar files can be added to a project by adding the plugin to the build script classpath and then applying the plugin. External jars can be added to the build script classpath using the `buildscript {}` block as described in Section 43.6, “External dependencies for the build script”.

Example 27.9. Applying a plugin with the buildscript block

`build.gradle`

    buildscript {
        repositories {
            jcenter()
        }
        dependencies {
            classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:0.4.1"
        }
    }

    apply plugin: "com.jfrog.bintray"

## 27.6. Finding community plugins

Gradle has a vibrant community of plugin developers who contribute plugins for a wide variety of capabilities. The Gradle plugin portal provides an interface for searching and exploring community plugins.

## 27.7. More on plugins

This chapter aims to serve as an introduction to plugins and Gradle and the role they play. For more information on the inner workings of plugins, see Chapter 41, Writing Custom Plugins.