Dockerizing Your Scala App

Dockerizing a Scala application is pretty easy.

The first concern is creating a fat JAR. Now, we all come from different backgrounds, including Maven/Gradle and different plugins that handle this issue. If you use sbt, the way to go is to use the sbt-assembly plugin.

To use it, we should add it to our project/plugins.sbt file. If the file does not exist, create it.

logLevel := Level.Warn

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.14.6")

So by executing...

sbt clean assembly

...we will end up with a fat JAR located at the target/scala-**/**.jar path.

Now the easy part is putting our application inside Docker. Thus, a Dockerfile is needed.

We will use OpenJDK Alpine as a base image.

FROM openjdk:8-jre-alpine

ADD target/scala-**/your-fat-jar app.jar

ENTRYPOINT ["java","-jar","/app.jar"]

The above approach works all right and gives you the control needed to customize your build process. For a more bootstrapped experience, you can use the sbt native packager.

All you need to do is to add the plugin to the project/plugins.sbt file.

logLevel := Level.Warn

addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.3.4")

Then we specify the main class of our application and enable the Java and Docker plugins from the native packager at the build.sbt file.

mainClass in Compile := Some("your.package.MainClass")

enablePlugins(JavaAppPackaging)
enablePlugins(DockerPlugin)

The next step is to issue the sbt command.

sbt docker:publishLocal

This command will build your application, include the binaries needed to the JAR, containerize your application, and publish it to your local Maven repo.