Scala Cookbook

18.9. Specifying a Main Class to Run

Problem

You have multiple main methods in objects in your project, and you want to specify which main method should be run when you type sbt run, or specify the main method that should be invoked when your project is packaged as a JAR file.

Solution

If you have multiple main methods in your project and want to specify which main method to run when typing sbt run, add a line like this to your build.sbt file:

// set the main class for 'sbt run'
mainClass in (Compile, run) := Some("com.alvinalexander.Foo")

This class can either contain a main method, or extend the App trait.

To specify the class that will be added to the manifest when your application is packaged as a JAR file, add this line to your build.sbt file:

// set the main class for packaging the main jar
mainClass in (Compile, packageBin) := Some("com.alvinalexander.Foo")

That setting tells SBT to add the following line to the META-INF/MANIFEST.MF file in your JAR when you run sbt package:

Main-Class: com.alvinalexander.Foo

Using run-main

When running your application with SBT, you can also use SBT’s run-main command to specify the class to run. Invoke it like this from your operating system command line:

$ sbt "run-main com.alvinalexander.Foo"
[info] Loading global plugins from /Users/Al/.sbt/plugins
[info] Running com.alvinalexander.Foo
hello
[success] Total time: 1 s

Invoke it like this from inside the SBT shell:

$ sbt

> run-main com.alvinalexander.Foo [info] Running com.alvinalexander.Foo hello [success] ...