java.lang.RuntimeException: No main class detected - scala

Question

I am trying to run Scala application using docker. I've created a sample project with the following structure.
build.sbt
name := "test"
version := "1.0"
scalaVersion := "2.12.2"
project/build.properties
sbt.version = 0.13.15
src/main/scala/HelloWorld.scala
object HelloWorld {
def main(args: Array[String]): Unit = {
println("Hello, world!")
}
}
Now if I do sbt run, everything works fine and I have the lovely hello-world greeting.
$ sbt run
...
[info] Set current project to test (in build file:/Users/yuchen/Documents/test/)
[info] Running HelloWorld
Hello, world!
[success] Total time: 1 s, completed 22-May-2017 4:30:28 PM
I aded a Dockerfile:
FROM openjdk:8
ENV SCALA_VERSION 2.12.2
ENV SBT_VERSION 0.13.15
RUN touch /usr/lib/jvm/java-8-openjdk-amd64/release
RUN \
curl -fsL http://downloads.typesafe.com/scala/$SCALA_VERSION/scala-$SCALA_VERSION.tgz | tar xfz - -C /root/ && \
echo >> /root/.bashrc && \
echo 'export PATH=~/scala-$SCALA_VERSION/bin:$PATH' >> /root/.bashrc
RUN \
curl -L -o sbt-$SBT_VERSION.deb http://dl.bintray.com/sbt/debian/sbt-$SBT_VERSION.deb && \
dpkg -i sbt-$SBT_VERSION.deb && \
rm sbt-$SBT_VERSION.deb && \
apt-get update && \
apt-get install sbt && \
sbt sbtVersion
WORKDIR /root
CMD sbt run
They are all copied from https://hub.docker.com/r/hseeberger/scala-sbt/~/dockerfile/ and with the additional last line CMD sbt run.
And I tried running docker commands:
docker build -t test .
docker tag test test/test:1.0
docker run test/test:1.0
However, I keep getting this error:
[info] Set current project to root (in build file:/root/)
[info] Updating {file:/root/}root...
[info] Resolving org.fusesource.jansi#jansi;1.4 ...
[info] Done updating.
java.lang.RuntimeException: No main class detected.
at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last compile:run for the full output.
[error] (compile:run) No main class detected.
[error] Total time: 0 s, completed May 23, 2017 4:00:19 AM
What does it mean and how do I resolve this?
FYI: I am aware of http://www.scala-sbt.org/sbt-native-packager/formats/docker.html, but just want to try writing Dockerfile as I am learning this.

Answer 1

I don't really know sbt. But this appears that you are not copying the project into the image (there is no COPY or ADD in the Dockerfile).
Try doing something like:
FROM openjdk:8
ENV SCALA_VERSION 2.12.2
ENV SBT_VERSION 0.13.15
RUN touch /usr/lib/jvm/java-8-openjdk-amd64/release
RUN \
curl -fsL http://downloads.typesafe.com/scala/$SCALA_VERSION/scala-$SCALA_VERSION.tgz | tar xfz - -C /root/ && \
echo >> /root/.bashrc && \
echo 'export PATH=~/scala-$SCALA_VERSION/bin:$PATH' >> /root/.bashrc
RUN \
curl -L -o sbt-$SBT_VERSION.deb http://dl.bintray.com/sbt/debian/sbt-$SBT_VERSION.deb && \
dpkg -i sbt-$SBT_VERSION.deb && \
rm sbt-$SBT_VERSION.deb && \
apt-get update && \
apt-get install sbt && \
sbt sbtVersion
WORKDIR /myapp
CMD sbt run
COPY . /myapp
The addition here is just a COPY to copy all the code to /myapp and to changing the working directory to this folder as well. This would copy the build context to /myapp and then the WORKDIR should run sbt run from the same folder.