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Plugins — sbt Documentation

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Original source (www.scala-sbt.org)
Tags: scala sbt build-definition library-dependencies
Clipped on: 2014-07-31

Plugins

Introduction

A plugin is essentially a way to use external code in a build definition. A plugin can be a library used to implement a task. For example, 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. Also, Commands can be added with the commands setting

The Plugins Best Practices page describes the currently evolving guidelines to writing sbt plugins. See also the general General Best Practices.

Using a binary sbt plugin

A common situation is using a binary plugin published to a repository. Create project/plugins.sbt with 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 "http://example.org/repo/"

See the rest of the page for more information on creating and using plugins.

By Description

A plugin definition is a project in <main-project>/project/. This project's classpath is the classpath used for build definitions in <main-project>/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 or project/project/Build.scala and will be available to the build definition sources. Think of project/project/ as the build definition for the build definition.

The build definition classpath is searched for sbt/sbt.plugins descriptor files containing the names of Plugin implementations. A Plugin is a module that defines settings to automatically inject to projects. Additionally, all Plugin modules are wildcard imported for the eval and set commands and .sbt files. A Plugin implementation is not required to produce a plugin, however. It is a convenience for plugin consumers and because of the automatic nature, it is not always appropriate.

The reload plugins command changes the current build to <current-build>/project/. 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.

Note: At runtime, all plugins for all builds are loaded in a separate, parent class loader of the class loaders for builds. This means that plugins will not see classes or resources from build definitions.

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.

By Example

Using a library in a build definition

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.

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/harrah/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:

> reload return
[info] Set current project to root (in build file:/Users/harrah/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 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)
        ("org.example" % "windows-only" % "1.0") :: Nil
    else
        Nil

Creating a plugin

Introduction

A minimal plugin is a Scala library that is built against the version of Scala that sbt runs (currently, 2.10.4) or a Java library. Nothing special needs to be done for this type of library, as shown in the previous section. 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.

Description

To make a plugin, create a project and configure sbtPlugin to true. Then, write the plugin code and publish your project to a repository. The plugin can be used as described in the previous section.

  • Automatically importing selective names to .sbt files.
  • Specifying plugin dependencies.
  • Automatically activating itself when all dependencies are present.
  • Specifying projectSettings, buildSettings, and globalSettings as appropriate.

When an AutoPlugin provides a stable field such as val or object named autoImport, the contents of the field are wildcard imported in in set, eval, and .sbt files. Typically, this is used to provide new keys (SettingKey, TaskKey, or InputKey) or core methods without requiring an import or qualification.

The AutoPlugin's projectSettings is automatically appended to each project's settings, when its dependencies also exist on that project. The requires method defines the dependencies to other plugins. The trigger method defines the conditions by which this plugin's settings are automatically activated. The buildSettings is appended to each build's settings (that is, in ThisBuild). 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. Use globalSettings to define the default value of a setting.

Example Plugin

An example of a typical plugin:

build.sbt:

sbtPlugin := true

name := "sbt-obfuscate"

organization := "org.example"

Plugin.scala:

package sbtobfuscate

import sbt._

object Plugin extends AutoPlugin
{
    // by definging autoImport, these 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[sbt.Def.Setting[_]] = Seq(
            obfuscate := {
                Obfuscate(sources.value, (obfuscateLiterals in obfuscate).value)
            },
            obfuscateLiterals in obfuscate := false
        )
    }

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

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

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

Root Plugins

Some plugins should always be explicitly enabled on projects. Sbt calls these root plugins, i.e. plugins that are "root" nodes in the plugin depdendency graph. AutoPlugin by default defines a root plugin.

Example command root plugin

A basic plugin that adds commands looks like:

build.sbt

sbtPlugin := true

name := "sbt-sample"

organization := "org.example"

Plugin.scala

package sbtsample

import sbt._
import Keys._
object Plugin extends AutoPlugin
{
  override lazy val projectSettings = Seq(commands += myCommand)

  lazy val myCommand =
    Command.command("hello") { (state: State) =>
      println("Hi!")
      state
    }
}

This example demonstrates how to take a Command (here, myCommand) 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.

For a user to consume this plugin, it requires an explicit include via the Project instance. Here's what their local sbt will look like.

build.sbt

val root = Project("example-plugin-usage", file(".")).setPlugins(MyPlugin)

The setPlugins method allows projects to explicitly define the `RootPlugin`s they wish to consume. `AutoPlugin`s are automatically added to the project as appropriate.

Projects can also exclude any type of plugin using the disablePlugins method. For example, if we wish to remove the JvmModule settings (compile,`test`,`run`), we modify our build.sbt as follows:

val root = Project("example-plugin-usage", file(".")).setPlugins(MyPlugin).disablePlugins(plugins.JvmModule)

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. Jars may be placed directly in ~/.sbt/0.13/plugins/lib/ and will be available to every build definition for the current user.
  2. Dependencies on plugins built from source may be declared in ~/.sbt/0.13/plugins/project/Build.scala as described at .scala Build Definition.
  3. A Plugin may be directly defined in Scala source files in ~/.sbt/0.13/plugins/, such as ~/.sbt/0.13/plugins/MyPlugin.scala. ~/.sbt/0.13/plugins//build.sbt should contain sbtPlugin := true. This can be used for quicker turnaround when developing a plugin initially:
    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.

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 with and plays well with other plugins.

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