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Deploy fat JARs. Restart processes.

sbt-assembly is a sbt 0.10+ port of an awesome sbt plugin by codahale:

assembly-sbt is a simple-build-tool plugin for building a single JAR file of your project which includes all of its dependencies, allowing to deploy the damn thing as a single file without dicking around with shell scripts and lib directories or, worse, welding your configuration to your deployable in the form of a WAR file.


  • Simple Build Tool
  • The burning desire to have a simple deploy procedure.

Using Published Plugin

For sbt 0.11.2, 0.11.3, and 0.12.x add sbt-assembly as a dependency in project/plugins.sbt:

resolvers += Resolver.url("artifactory", url(""))(Resolver.ivyStylePatterns)

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.8.3")

Using Source Dependency

Alternately, you can have sbt checkout and build the plugin's source from version control.

Specify sbt-assembly.git as a dependency in project/project/build.scala:

import sbt._

object Plugins extends Build {
  lazy val root = Project("root", file(".")) dependsOn(
    uri("git://") // where XX is tag or sha1

(You may need to check this project's tags to see what the most recent release is. I'm notoriously crap about updating the version numbers in my READMEs.)

Applying the Plugin to a Project (Adding the assembly Task)

First, make sure that you've added the plugin to your build (either the published builds or source from Git).

Now, add the following in your build.sbt:

import AssemblyKeys._ // put this at the top of the file


or, for full configuration:

import sbtassembly.Plugin._
import AssemblyKeys._

lazy val sub = Project("sub", file("sub"),
  settings = buildSettings ++ assemblySettings ++
             Seq( // your settings here

Now you'll have an awesome new assembly task which will compile your project, run your tests, and then pack your class files and all your dependencies into a single JAR file: target/scala_X.X.X/projectname-assembly-X.X.X.jar.

> assembly

If you specify a mainClass in assembly in build.sbt (or just let it autodetect one) then you'll end up with a fully executable JAR, ready to rock.

Here is the list of the keys you can rewire for assembly task.

target                        assembly-jar-name             test
assembly-option               main-class                    full-classpath
dependency-classpath          assembly-excluded-files       assembly-excluded-jars

For example the name of the jar can be set as follows in build.sbt:

jarName in assembly := "something.jar"

To skip the test during assembly,

test in assembly := {}

To exclude Scala library,

assembleArtifact in packageScala := false

To exclude the class files from the main sources,

assembleArtifact in packageBin := false

To exclude some jar file, first consider using "provided" dependency. The dependency will be part of compilation, but excluded from the runtime. Next, try creating a custom configuration that describes your classpath. If all efforts fail, here's a way to exclude jars:

excludedJars in assembly <<= (fullClasspath in assembly) map { cp => 
  cp filter { == "compile-0.1.0.jar"}

To exclude specific files, customize merge strategy.

To make a jar containing only the dependencies, type

> assembly-package-dependency

To set an explicit main class,

mainClass in assembly := Some("com.example.Main")

Merge Strategy

If multiple files share the same relative path (e.g. a resource named application.conf in multiple dependency JARs), the default strategy is to verify that all candidates have the same contents and error out otherwise. This behavior can be configured on a per-path basis using either one of the following built-in strategies or writing a custom one:

  • MergeStrategy.deduplicate is the default described above
  • MergeStrategy.first picks the first of the matching files in classpath order
  • MergeStrategy.last picks the last one
  • MergeStrategy.singleOrError bails out with an error message on conflict
  • MergeStrategy.concat simply concatenates all matching files and includes the result
  • MergeStrategy.filterDistinctLines also concatenates, but leaves out duplicates along the way
  • MergeStrategy.rename renames the files originating from jar files
  • MergeStrategy.discard simply discards matching files

The mapping of path names to merge strategies is done via the setting assembly-merge-strategy which can be augmented like so:

mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
    case "application.conf" => MergeStrategy.concat
    case "unwanted.txt"     => MergeStrategy.discard
    case x => old(x)

where the default is to

mergeStrategy in assembly := { 
  case "reference.conf" =>
  case PathList(ps @ _*) if isReadme(ps.last) || isLicenseFile(ps.last) =>
  case PathList("META-INF", xs @ _*) =>
    (xs map {_.toLowerCase}) match {
      case ("" :: Nil) | ("index.list" :: Nil) | ("dependencies" :: Nil) =>
      case ps @ (x :: xs) if ps.last.endsWith(".sf") || ps.last.endsWith(".dsa") =>
      case "plexus" :: xs =>
      case "services" :: xs =>
      case ("spring.schemas" :: Nil) | ("spring.handlers" :: Nil) =>
      case _ => MergeStrategy.deduplicate
  case _ => MergeStrategy.deduplicate

Custom MergeStrategys can find out where a particular file comes from using the sourceOfFileForMerge method on sbtassembly.AssemblyUtils, which takes the temporary directory and one of the files passed into the strategy as parameters.

Development Notes

Please use global .gitignore instead of adding editor junk files to .gitignore.


Copyright (c) 2010-2011 e.e d3si9n, Coda Hale

Published under The MIT License, see LICENSE

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