Plugins 

There’s a getting started page focused on using existing plugins, which you may want to read first.

A plugin is a way to use external code in a build definition. A plugin can be a library used to implement a task (you might use Knockoff to write a markdown processing task). A plugin can define a sequence of sbt settings that are automatically added to all projects or that are explicitly declared for selected projects. For example, a plugin might add a proguard task and associated (overridable) settings. Finally, a plugin can define new commands (via the commands setting).

sbt 0.13.5 introduces auto plugins, with improved dependency management among the plugins and explicitly scoped auto importing. Going forward, our recommendation is to migrate to the auto plugins. The Plugins Best Practices page describes the currently evolving guidelines to writing sbt plugins. See also the general best practices.

Using an auto plugin 

A common situation is when using a binary plugin published to a repository. If you’re adding sbt-assembly, create project/assembly.sbt with the following:

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.11.2")

Alternatively, you can create project/plugins.sbt with all of the desired sbt plugins, any general dependencies, and any necessary repositories:

addSbtPlugin("org.example" % "plugin" % "1.0")

addSbtPlugin("org.example" % "another-plugin" % "2.0")

// plain library (not an sbt plugin) for use in the build definition
libraryDependencies += "org.example" % "utilities" % "1.3"

resolvers += "Example Plugin Repository" at "https://example.org/repo/"

Many of the auto plugins automatically add settings into projects, however, some may require explicit enablement. Here’s an example:

lazy val util = (project in file("util")).
  enablePlugins(FooPlugin, BarPlugin).
  disablePlugins(plugins.IvyPlugin).
  settings(
    name := "hello-util"
  )

See using plugins in the Getting Started guide for more details on using plugins.

By Description 

A plugin definition is a project under project/ folder. This project’s classpath is the classpath used for build definitions in project/ and any .sbt files in the project’s base directory. It is also used for the eval and set commands.

Specifically,

  1. Managed dependencies declared by the project/ project are retrieved and are available on the build definition classpath, just like for a normal project.
  2. Unmanaged dependencies in project/lib/ are available to the build definition, just like for a normal project.
  3. Sources in the project/ project are the build definition files and are compiled using the classpath built from the managed and unmanaged dependencies.
  4. Project dependencies can be declared in project/plugins.sbt (similarly to build.sbt file in a normal project) or project/project/Build.scala (similarly to project/Build.scala in a normal project) and will be available to the build definition sources. Think of project/project/ as the build definition for the build definition (worth to repeat it here again: “sbt is recursive”, remember?).

The build definition classpath is searched for sbt/sbt.plugins descriptor files containing the names of sbt.AutoPlugin or sbt.Plugin implementations.

The reload plugins command changes the current build to the (root) project’s project/ build definition. This allows manipulating the build definition project like a normal project. reload return changes back to the original build. Any session settings for the plugin definition project that have not been saved are dropped.

An auto plugin is a module that defines settings to automatically inject into projects. In addition an auto plugin provides the following feature:

Plugin dependencies 

When a traditional plugin wanted to reuse some functionality from an existing plugin, it would pull in the plugin as a library dependency, and then it would either:

  1. add the setting sequence from the dependency as part of its own setting sequence, or
  2. tell the build users to include them in the right order.

This becomes complicated as the number of plugins increase within an application, and becomes more error prone. The main goal of auto plugin is to alleviate this setting dependency problem. An auto plugin can depend on other auto plugins and ensure these dependency settings are loaded first.

Suppose we have the SbtLessPlugin and the SbtCoffeeScriptPlugin, which in turn depends on the SbtJsTaskPlugin, SbtWebPlugin, and JvmPlugin. Instead of manually activating all of these plugins, a project can just activate the SbtLessPlugin and SbtCoffeeScriptPlugin like this:

lazy val root = (project in file(".")).
  enablePlugins(SbtLessPlugin, SbtCoffeeScriptPlugin)

This will pull in the right setting sequence from the plugins in the right order. The key notion here is you declare the plugins you want, and sbt can fill in the gap.

A plugin implementation is not required to produce an auto plugin, however. It is a convenience for plugin consumers and because of the automatic nature, it is not always appropriate.

Global plugins 

The ~/.sbt/0.13/plugins/ directory is treated as a global plugin definition project. It is a normal sbt project whose classpath is available to all sbt project definitions for that user as described above for per-project plugins.

Creating an auto plugin 

A minimal sbt plugin is a Scala library that is built against the version of Scala that sbt runs (currently, 2.10.6) or a Java library. Nothing special needs to be done for this type of library. A more typical plugin will provide sbt tasks, commands, or settings. This kind of plugin may provide these settings automatically or make them available for the user to explicitly integrate.

To make an auto plugin, create a project and configure sbtPlugin to true.

sbtPlugin := true

Then, write the plugin code and publish your project to a repository. The plugin can be used as described in the previous section.

First, in an appropriate namespace, define your auto plugin object by extending sbt.AutoPlugin.

projectSettings and buildSettings 

With auto plugins, all provided settings (e.g. assemblySettings) are provided by the plugin directly via the projectSettings method. Here’s an example plugin that adds a command named hello to sbt projects:

package sbthello

import sbt._
import Keys._

object HelloPlugin extends AutoPlugin {
  override lazy val projectSettings = Seq(commands += helloCommand)
  lazy val helloCommand =
    Command.command("hello") { (state: State) =>
      println("Hi!")
      state
    }
}

This example demonstrates how to take a Command (here, helloCommand) and distribute it in a plugin. Note that multiple commands can be included in one plugin (for example, use commands ++= Seq(a,b)). See Commands for defining more useful commands, including ones that accept arguments and affect the execution state.

If the plugin needs to append settings at the build-level (that is, in ThisBuild) there’s a buildSettings method. The settings returned here are guaranteed to be added to a given build scope only once regardless of how many projects for that build activate this AutoPlugin.

override def buildSettings: Seq[Setting[_]] = Nil

The globalSettings is appended once to the global settings (in Global). These allow a plugin to automatically provide new functionality or new defaults. One main use of this feature is to globally add commands, such as for IDE plugins.

override def globalSettings: Seq[Setting[_]] = Nil

Use globalSettings to define the default value of a setting.

Implementing plugin dependencies 

Next step is to define the plugin dependencies.

package sbtless

import sbt._
import Keys._
object SbtLessPlugin extends AutoPlugin {
  override def requires = SbtJsTaskPlugin
  override lazy val projectSettings = ...
}

The requires method returns a value of type Plugins, which is a DSL for constructing the dependency list. The requires method typically contains one of the following values:

Root plugins and triggered plugins 

Some plugins should always be explicitly enabled on projects. we call these root plugins, i.e. plugins that are “root” nodes in the plugin dependency graph. An auto plugin is by default a root plugin.

Auto plugins also provide a way for plugins to automatically attach themselves to projects if their dependencies are met. We call these triggered plugins, and they are created by overriding the trigger method.

For example, we might want to create a triggered plugin that can append commands automatically to the build. To do this, set the requires method to return empty (this is the default), and override the trigger method with allRequirements.

package sbthello

import sbt._
import Keys._

object HelloPlugin2 extends AutoPlugin {
  override def trigger = allRequirements
  override lazy val buildSettings = Seq(commands += helloCommand)
  lazy val helloCommand =
    Command.command("hello") { (state: State) =>
      println("Hi!")
      state
    }
}

The build user still needs to include this plugin in project/plugins.sbt, but it is no longer needed to be included in build.sbt. This becomes more interesting when you do specify a plugin with requirements. Let’s modify the SbtLessPlugin so that it depends on another plugin:

package sbtless
import sbt._
import Keys._
object SbtLessPlugin extends AutoPlugin {
  override def trigger = allRequirements
  override def requires = SbtJsTaskPlugin
  override lazy val projectSettings = ...
}

As it turns out, PlayScala plugin (in case you didn’t know, the Play framework is an sbt plugin) lists SbtJsTaskPlugin as one of it required plugins. So, if we define a build.sbt with:

lazy val root = (project in file(".")).
  enablePlugins(PlayScala)

then the setting sequence from SbtLessPlugin will be automatically appended somewhere after the settings from PlayScala.

This allows plugins to silently, and correctly, extend existing plugins with more features. It also can help remove the burden of ordering from the user, allowing the plugin authors greater freedom and power when providing feature for their users.

Controlling the import with autoImport 

When an auto plugin provides a stable field such as val or object named autoImport, the contents of the field are wildcard imported in set, eval, and .sbt files. In the next example, we’ll replace our hello command with a task to get the value of greeting easily. In practice, it’s recommended to prefer settings or tasks to commands.

package sbthello

import sbt._
import Keys._

object HelloPlugin3 extends AutoPlugin {
  object autoImport {
    val greeting = settingKey[String]("greeting")
    val hello = taskKey[Unit]("say hello")
  }
  import autoImport._
  override def trigger = allRequirements
  override lazy val buildSettings = Seq(
    greeting := "Hi!",
    hello := helloTask.value)
  lazy val helloTask =
    Def.task {
      println(greeting.value)
    }
}

Typically, autoImport is used to provide new keys - SettingKeys, TaskKeys, or InputKeys - or core methods without requiring an import or qualification.

Example Plugin 

An example of a typical plugin:

build.sbt:

sbtPlugin := true

name := "sbt-obfuscate"

organization := "org.example"

ObfuscatePlugin.scala:

package sbtobfuscate

import sbt._

object ObfuscatePlugin extends AutoPlugin {
  // by defining autoImport, the settings are automatically imported into user's `*.sbt`
  object autoImport {
    // configuration points, like the built-in `version`, `libraryDependencies`, or `compile`
    val obfuscate = taskKey[Seq[File]]("Obfuscates files.")
    val obfuscateLiterals = settingKey[Boolean]("Obfuscate literals.")

    // default values for the tasks and settings
    lazy val baseObfuscateSettings: Seq[Def.Setting[_]] = Seq(
      obfuscate := {
        Obfuscate(sources.value, (obfuscateLiterals in obfuscate).value)
      },
      obfuscateLiterals in obfuscate := false
    )
  }

  import autoImport._
  override def requires = sbt.plugins.JvmPlugin

  // This plugin is automatically enabled for projects which are JvmPlugin.
  override def trigger = allRequirements

  // a group of settings that are automatically added to projects.
  override val projectSettings =
    inConfig(Compile)(baseObfuscateSettings) ++
    inConfig(Test)(baseObfuscateSettings)
}

object Obfuscate {
    def apply(sources: Seq[File]): Seq[File] := sources
}

Usage example 

A build definition that uses the plugin might look like. obfuscate.sbt:

obfuscateLiterals in obfuscate := true

Global plugins example 

The simplest global plugin definition is declaring a library or plugin in ~/.sbt/0.13/plugins/build.sbt:

libraryDependencies += "org.example" %% "example-plugin" % "0.1"

This plugin will be available for every sbt project for the current user.

In addition:

  1. Edit the global plugin code
  2. reload the project you want to use the modified plugin in
  3. sbt will rebuild the plugin and use it for the project.

    Additionally, the plugin will be available in other projects on the machine without recompiling again. This approach skips the overhead of publishLocal and cleaning the plugins directory of the project using the plugin.

These are all consequences of ~/.sbt/0.13/plugins/ being a standard project whose classpath is added to every sbt project’s build definition.

Using a library in a build definition example 

As an example, we’ll add the Grizzled Scala library as a plugin. Although this does not provide sbt-specific functionality, it demonstrates how to declare plugins.

1a) Manually managed 

  1. Download the jar manually from https://oss.sonatype.org/content/repositories/releases/org/clapper/grizzled-scala2.8.1/1.0.4/grizzled-scala2.8.1-1.0.4.jar
  2. Put it in project/lib/

1b) Automatically managed: direct editing approach 

Edit project/plugins.sbt to contain:

libraryDependencies += "org.clapper" %% "grizzled-scala" % "1.0.4"

If sbt is running, do reload.

1c) Automatically managed: command-line approach 

We can change to the plugins project in project/ using reload plugins.

$ sbt
> reload plugins
[info] Set current project to default (in build file:/Users/sbt/demo2/project/)
>

Then, we can add dependencies like usual and save them to project/plugins.sbt. It is useful, but not required, to run update to verify that the dependencies are correct.

> set libraryDependencies += "org.clapper" %% "grizzled-scala" % "1.0.4"
...
> update
...
> session save
...

To switch back to the main project use reload return:

> reload return
[info] Set current project to root (in build file:/Users/sbt/demo2/)

1d) Project dependency 

This variant shows how to use sbt’s external project support to declare a source dependency on a plugin. This means that the plugin will be built from source and used on the classpath.

Edit project/plugins.sbt

lazy val root = (project in file(".")).dependsOn(assemblyPlugin)

lazy val assemblyPlugin = uri("git://github.com/sbt/sbt-assembly")

If sbt is running, run reload.

Note that this approach can be useful used when developing a plugin. A project that uses the plugin will rebuild the plugin on reload. This saves the intermediate steps of publishLocal and update. It can also be used to work with the development version of a plugin from its repository.

It is however recommended to explicitly specify the commit or tag by appending it to the repository as a fragment:

lazy val assemblyPlugin = uri("git://github.com/sbt/sbt-assembly#0.9.1")

One caveat to using this method is that the local sbt will try to run the remote plugin’s build. It is quite possible that the plugin’s own build uses a different sbt version, as many plugins cross-publish for several sbt versions. As such, it is recommended to stick with binary artifacts when possible.

2) Use the library 

Grizzled Scala is ready to be used in build definitions. This includes the eval and set commands and .sbt and project/*.scala files.

> eval grizzled.sys.os

In a build.sbt file:

import grizzled.sys._
import OperatingSystem._

libraryDependencies ++=
    if(os == Windows)
        Seq("org.example" % "windows-only" % "1.0")
    else
        Seq.empty

Best Practices 

If you’re a plugin writer, please consult the Plugins Best Practices page; it contains a set of guidelines to help you ensure that your plugin is consistent and plays well with other plugins.

Contents

sbt Reference Manual
      1. Plugins