sbt / sbt-assembly

Deploy fat JARs. Restart processes. (port of codahale/assembly-sbt)

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Deploy fat JARs. Restart processes.

sbt-assembly is a sbt plugin originally ported from codahale's assembly-sbt, which I'm guessing was inspired by Maven's assembly plugin. The goal is simple: Create a fat JAR of your project with all of its dependencies.


  • sbt
  • The burning desire to have a simple deploy procedure.

Reporting Issues & Contributing

Before you email me, please read Issue Reporting Guideline carefully. Twice. (Don't email me)


Using Published Plugin

Bintray version

Add sbt-assembly as a dependency in project/plugins.sbt:

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "x.y.z")

(You may need to check this project's tags to see what the most recent release is.)


Since sbt-assembly is now an auto plugin that's triggered for all projects with JvmPlugin, it shouldn't require extra setup to include assembly task into your project. See migration guide for details on how to upgrade from older sbt-assembly.

Applying the plugin to multi-project build.sbt

For example, here's a multi-project build.sbt:

lazy val commonSettings = Seq(
  version := "0.1-SNAPSHOT",
  organization := "com.example",
  scalaVersion := "2.10.1",
  test in assembly := {}

lazy val app = (project in file("app")).
  settings(commonSettings: _*).
    mainClass in assembly := Some("com.example.Main"),
    // more settings here ...

lazy val utils = (project in file("utils")).
  settings(commonSettings: _*).
    assemblyJarName in assembly := "utils.jar",
    // more settings here ...

In the above example, both the app project and the utils project do not run tests during assembly. The app project sets a main class whereas the utils project sets the name of its jar file.

assembly task

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.

assemblyJarName               test                          mainClass
assemblyOutputPath            assemblyMergeStrategy         assemblyOption
assemblyExcludedJars          assembledMappings

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

assemblyJarName in assembly := "something.jar"

To skip the test during assembly,

test in assembly := {}

To set an explicit main class,

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

Excluding an explicit main class from your assembly requires something a little bit different though

packageOptions in assembly ~= { pos =>
  pos.filterNot { po =>

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 assemblyMergeStrategy which can be augmented as follows:

assemblyMergeStrategy in assembly := {
  case PathList("javax", "servlet", xs @ _*)         => MergeStrategy.first
  case PathList(ps @ _*) if ps.last endsWith ".html" => MergeStrategy.first
  case "application.conf"                            => MergeStrategy.concat
  case "unwanted.txt"                                => MergeStrategy.discard
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value


  • assemblyMergeStrategy in assembly expects a function. You can't do assemblyMergeStrategy in assembly := MergeStrategy.first!
  • Some files must be discarded or renamed otherwise to avoid breaking the zip (due to duplicate file name) or the legal license. Delegate default handling to (assemblyMergeStrategy in assembly) as the above pattern matching example.

By the way, the first case pattern in the above using PathList(...) is how you can pick javax/servlet/* from the first jar. If the default MergeStrategy.deduplicate is not working for you, that likely means you have multiple versions of some library pulled by your dependency graph. The real solution is to fix that dependency graph. You can work around it by MergeStrategy.first but don't be surprised when you see ClassNotFoundException.

Here is the default:

  val defaultMergeStrategy: String => MergeStrategy = { 
    case x if Assembly.isConfigFile(x) =>
    case PathList(ps @ _*) if Assembly.isReadme(ps.last) || Assembly.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.

Third Party Merge Strategy Plugins

Support for special-case merge strategies beyond the generic scope can be provided by companion plugins, below is a non-exhaustive list:


sbt-assembly can shade classes from your projects or from the library dependencies. Backed by Jar Jar Links, bytecode transformation (via ASM) is used to change references to the renamed classes.

    assemblyShadeRules in assembly := Seq(
      ShadeRule.rename("**" -> "shadeio.@1").inAll

Here are the shade rules:

  • ShadeRule.rename("x.**" -> "y.@1", ...).inAll This is the main rule.
  • ShadeRule.zap("a.b.c").inAll
  • ShadeRule.keep("x.**").inAll

The main ShadeRule.rename rule is used to rename classes. All references to the renamed classes will also be updated. If a class name is matched by more than one rule, only the first one will apply. The rename rules takes a vararg of String pairs in <pattern> -> <result> format:

  • <pattern> is a class name with optional wildcards. ** will match against any valid class name substring. To match a single package component (by excluding . from the match), a single * may be used instead.
  • <result> is a class name which can optionally reference the substrings matched by the wildcards. A numbered reference is available for every * or ** in the <pattern>, starting from left to right: @1, @2, etc. A special @0 reference contains the entire matched class name.

Instead of .inAll, call .inProject to match your project source, or call .inLibrary("commons-io" % "commons-io" % "2.4", ...) to match specific library dependencies. inProject and inLibrary(...) can be chained.

    assemblyShadeRules in assembly := Seq(
      ShadeRule.rename("**" -> "shadeio.@1").inLibrary("commons-io" % "commons-io" % "2.4", ...).inProject

The ShadeRule.zap rule causes any matched class to be removed from the resulting jar file. All zap rules are processed before renaming rules.

The ShadeRule.keep rule marks all matched classes as "roots". If any keep rules are defined all classes which are not reachable from the roots via dependency analysis are discarded when writing the output jar. This is the last step in the process, after renaming and zapping.

To see the verbose output for shading:

    logLevel in assembly := Level.Debug

Scala libraries

Scala classes contain an annotation which, among other things, contain all symbols referenced in that class. As of sbt-assembly XXX the rename rules will be applied to these annotations as well which makes it possible to compile or reflect against a shaded library.

This is currently limited to renaming packages. Renaming class names will not work and cause compiler errors when compiling against the shaded library.

Excluding JARs and files

If you need to tell sbt-assembly to ignore JARs, you're probably doing it wrong. assembly task grabs deps JARs from your project's classpath. Try fixing the classpath first.

% "provided" configuration

If you're trying to exclude JAR files that are already part of the container (like Spark), consider scoping the dependent library to "provided" configuration:

libraryDependencies ++= Seq(
  "org.apache.spark" %% "spark-core" % "0.8.0-incubating" % "provided",
  "org.apache.hadoop" % "hadoop-client" % "2.0.0-cdh4.4.0" % "provided"

Maven defines "provided" as:

This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.

The dependency will be part of compilation and test, but excluded from the runtime. If you Spark people want to include "provided" dependencies back to run, @douglaz has come up with a one-liner solution on StackOverflow sbt: how can I add "provided" dependencies back to run/test tasks' classpath?:

run in Compile := Defaults.runTask(fullClasspath in Compile, mainClass in (Compile, run), runner in (Compile, run)).evaluated

Exclude specific transitive deps

You might be thinking about excluding JAR files because of the merge conflicts. Merge conflict of *.class files indicate pathological classpath, often due to non-modular bundle JAR files or SLF4J, not the problem with assembly. Here's what happens when you try to create a fat JAR with Spark included:

[error] (*:assembly) deduplicate: different file contents found in the following:
[error] /Users/foo/.ivy2/cache/org.eclipse.jetty.orbit/javax.servlet/orbits/javax.servlet-2.5.0.v201103041518.jar:javax/servlet/SingleThreadModel.class
[error] /Users/foo/.ivy2/cache/org.mortbay.jetty/servlet-api/jars/servlet-api-2.5-20081211.jar:javax/servlet/SingleThreadModel.class

In the above case two separate JAR files javax.servlet-2.5.0.v201103041518.jar and servlet-api-2.5-20081211.jar are defining javax/servlet/SingleThreadModel.class! Similarly also conflicts on common-beanutils and EsotericSoftware/minlog. Here's how to evict specific transitive deps:

libraryDependencies ++= Seq(
  ("org.apache.spark" %% "spark-core" % "0.8.0-incubating").
    exclude("org.mortbay.jetty", "servlet-api").
    exclude("commons-beanutils", "commons-beanutils-core").
    exclude("commons-collections", "commons-collections").
    exclude("commons-logging", "commons-logging").
    exclude("com.esotericsoftware.minlog", "minlog")

See sbt's Exclude Transitive Dependencies for more details.

Sometimes it takes a bit of detective work to figure out which transitive deps to exclude. Play! comes with dist task, so assembly is not needed, but suppose we wanted to run assembly. It brings in signpost-commonshttp4, which leads to commons-logging. This conflicts with jcl-over-slf4j, which re-implements the logging API. Since the deps are added via build.sbt and playScalaSettings, here's one way to work around it:

libraryDependencies ~= { _ map {
  case m if m.organization == "" =>
    m.exclude("commons-logging", "commons-logging").
      exclude("", "sbt-link")
  case m => m

Excluding specific files

To exclude specific files, customize merge strategy:

assemblyMergeStrategy in assembly := {
  case PathList("about.html") => MergeStrategy.rename
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value

Splitting your project and deps JARs

To make a JAR file containing only the external dependencies, type

> assemblyPackageDependency

This is intended to be used with a JAR that only contains your project

assemblyOption in assembly := (assemblyOption in assembly).value.copy(includeScala = false, includeDependency = false)

NOTE: If you use -jar option for java, it will ignore -cp, so if you have multiple JAR files you have to use -cp and pass the main class: java -cp "jar1.jar:jar2.jar" Main

Excluding Scala library JARs

To exclude Scala library (JARs that start with scala- and are included in the binary Scala distribution) to run with scala command,

assemblyOption in assembly := (assemblyOption in assembly).value.copy(includeScala = false)


If all efforts fail, here's a way to exclude JAR files:

assemblyExcludedJars in assembly := { 
  val cp = (fullClasspath in assembly).value
  cp filter { == "compile-0.1.0.jar"}

Other Things

Content hash

You can also append SHA-1 fingerprint to the assembly file name, this may help you to determine whether it has changed and, for example, if it's necessary to deploy the dependencies,

assemblyOption in assembly := (assemblyOption in assembly).value.copy(appendContentHash = true)


By default for performance reasons, the result of unzipping any dependency JAR files to disk is cached from run-to-run. This feature can be disabled by setting:

assemblyOption in assembly := (assemblyOption in assembly).value.copy(cacheUnzip = false)

In addition the fat JAR is cached so its timestamp changes only when the input changes. This feature requires checking the SHA-1 hash of all *.class files, and the hash of all dependency *.jar files. If there are a large number of class files, this could take a long time, although with hashing of jar files, rather than their contents, the speed has recently been improved. This feature can be disabled by setting:

assemblyOption in assembly := (assemblyOption in assembly).value.copy(cacheOutput = false)

Prepending a launch script

Your can prepend a launch script to the fat jar. This script will be a valid shell and batch script and will make the jar executable on Unix and Windows. If you enable the shebang the file will be detected as an executable under Linux but this will cause an error message to appear on Windows. On Windows just append a ".bat" to the files name to make it executable.

import sbtassembly.AssemblyPlugin.defaultUniversalScript

assemblyOption in assembly := (assemblyOption in assembly).value.copy(prependShellScript = Some(defaultUniversalScript(shebang = false)))

assemblyJarName in assembly := s"${name.value}-${version.value}"

This will prepend the following shell script to the jar.

(#!/usr/bin/env sh)
@ 2>/dev/null # 2>nul & echo off & goto BOF
exec java -jar $JAVA_OPTS "$0" "$@"

@echo off
java -jar %JAVA_OPTS% "%~dpnx0" %*
exit /B %errorlevel%

You can also choose to prepend just the shell script to the fat jar as follows:

import sbtassembly.AssemblyPlugin.defaultShellScript

assemblyOption in assembly := (assemblyOption in assembly).value.copy(prependShellScript = Some(defaultShellScript))

assemblyJarName in assembly := s"${name.value}-${version.value}"

Publishing (Not Recommended)

Publishing fat JARs out to the world is discouraged because non-modular JARs cause much sadness. One might think non-modularity is convenience but it quickly turns into a headache the moment your users step outside of Hello World example code. If you still wish to publish your assembled artifact along with the publish task and all of the other artifacts, add an assembly classifier (or other):

artifact in (Compile, assembly) := {
  val art = (artifact in (Compile, assembly)).value

addArtifact(artifact in (Compile, assembly), assembly)

Q: Despite the concerned friends, I still want publish fat JARs. What advice do you have?

You would likely need to set up a front business to lie about what dependencies you have in pom.xml and ivy.xml. To do so, make a subproject for fat JAR purpose only where you depend on the dependencies, and make a second cosmetic subproject that you use only for publishing purpose:

lazy val fatJar = project
    depend on the good stuff
    skip in publish := true

lazy val cosmetic = project
    name := "shaded-something",
    // I am sober. no dependencies.
    packageBin in Compile := (assembly in (fatJar, Compile)).value


Copyright (c) 2010-2014 e.e d3si9n, Coda Hale

Published under The MIT License, see LICENSE