How to build an Uber JAR (Fat JAR) using SBT within IntelliJ IDEA?

3 Step Process For Building Uber JAR/Fat JAR in IntelliJ Idea:

Uber JAR/Fat JAR : JAR file having all external libraray dependencies in it.

1. Adding SBT Assembly plugin in IntelliJ Idea
   
   

   Go to ProjectName/project/target/plugins.sbt file and add this line addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.12.0")

2. Adding Merge,Discard and Do Not Add strategy in build.sbt
   
   

   Go to ProjectName/build.sbt file and add the Strategy for Packaging of an Uber JAR
   

   Merge Strategy : If there is conflict in two packages about a version of library then which one to pack in Uber JAR.
   Discard Strategy : To remove some files from library which you do not want to package in Uber JAR.
   Do not Add Strategy : Do not add some package to Uber JAR.
   For ex: spark-core will be already present at your Spark Cluster.So we should not package this in Uber JAR
   

   Merge Strategy and Discard Strategy Basic Code :

   assemblyMergeStrategy in assembly := { case PathList("META-INF", xs @ _*) => MergeStrategy.discard case x => MergeStrategy.first }

   So you are asking to discard META-INF files using this command MergeStrategy.discard and for rest of the files you are taking the first occurrence of library file if there is any conflict by using this command MergeStrategy.first.

   Do not Add Strategy Basic Code :

   libraryDependencies += "org.apache.spark" %% "spark-core" % "1.4.1" %"provided"

   If we do not want to add the spark-core to our Uber JAR file as it will be already on our clutser, so we are adding the % "provided" at end of it library dependency.
   

3. Building Uber JAR with all its dependencies
   
   

   In terminal type sbt assembly for building up the package

Voila!!! Uber JAR is built. JAR will be in ProjectName/target/scala-XX

Finally I totally skip using IntelliJ IDEA to avoid generating noise in my global understanding :)

I started reading the official SBT tutorial.

I created my project with the following file structure :

my-project/project/assembly.sbt
my-project/src/main/scala/myPackage/MyMainObject.scala
my-project/build.sbt

Added the sbt-assembly plugin in my assembly.sbt file. Allowing me to build a fat JAR :

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.12.0")

My minimal build.sbt looks like :

lazy val root = (project in file(".")).
  settings(
    name := "my-project",
    version := "1.0",
    scalaVersion := "2.11.4",
    mainClass in Compile := Some("myPackage.MyMainObject")        
  )

val sparkVersion = "1.2.0"

libraryDependencies ++= Seq(
  "org.apache.spark" %% "spark-core" % sparkVersion % "provided",
  "org.apache.spark" %% "spark-streaming" % sparkVersion % "provided",
  "org.apache.spark" %% "spark-streaming-twitter" % sparkVersion
)

// META-INF discarding
mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
   {
    case PathList("META-INF", xs @ _*) => MergeStrategy.discard
    case x => MergeStrategy.first
   }
}

Note: The % "provided" means not to include the dependency in the final fat JAR (those libraries are already included in my workers)

Note: META-INF discarding inspired by this answser.

Note: Meaning of % and %%

Now I can build my fat JAR using SBT (how to install it) by running the following command in my /my-project root folder:

sbt assembly

My fat JAR is now located in the new generated /target folder :

/my-project/target/scala-2.11/my-project-assembly-1.0.jar

Hope that helps someone else.

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For those who wants to embeed SBT within IntelliJ IDE: How to run sbt-assembly tasks from within IntelliJ IDEA?