Webots scala-controller example

Webots is an open source and multi-platform desktop application used to simulate robots. Robot controllers may be programmed in C, C++, Python, Java, MATLAB or ROS with a simple API covering all the basic robotics needs. This project is an example showing how to setup an sbt project for Scala to write an example robot controller. The project uses Scala 3, but the code will compile on Scala 2 as well.

Before you start

Before trying this, first make sure that you followed Tutorial-1 for Java. A common hurdle is to get the environment variable JAVA_HOME set correctly. Although it is explained in the chaper Using Java of the Cyberbotics User Guide, I used the following approach to launch the simulator on macOS:

1. Open a Terminal
2. Ensure that the environment variable JAVA_HOME is set correctly.
3. Launch Webots from the Terminal: open -a "Webots"

Configure dependency

At the end of Tutorial 1, have a look on the Console to see how the Java controller is launched. It should look something like this.

INFO: EPuckGoForward: Starting controller: java -XstartOnFirstThread -classpath /Applications/Webots.app/Contents/lib/controller/java/Controller.jar:YOUR_WEBOTS_PROJECT_FOLDER/controllers/EPuckGoForward/ -Djava.library.path=/Applications/Webots.app/Contents/lib/controller/java EPuckGoForward

So, we'll need to point to /Applications/Webots.app/Contents/lib/controller/java/Controller.jar (on Linux/Window this will be another location) as an unmanaged library in your scala sbt project. The easiest way to do this is to create a soft link in the lib folder of this project. On Mac/Linux:

$ cd lib
$ ln -s /Applications/Webots.app/Contents/lib/controller/java/Controller.jar

Fat jar

We need to get all the dependencies that Webots is not aware of, including Scala's default dependencies, in a single JAR-file. This can be done with sbt-assembly, so build this project using

$ sbt assembly

Configure the Webots project

Last step is to make sure that the fat JAR-file can be found by the Webots project. For the example in Tutorial 1, go and look where the EPuckGoForward.java is stored, e.g.: YOUR_WEBOTS_PROJECT_FOLDER/controllers/EPuckGoForward/

First remove the EPuckGoForward.class file in that folder (if present) to make sure this is not used i.s.o. the JAR-file that you built with sbt. Webots expects that the jar file is located here and is named exactly as the name you gave it in the webots project. So, let's create here another soft link to our JAR file. On Mac/Linux:

$ cd YOUR_WEBOTS_PROJECT_FOLDER/controllers/EPuckGoForward/
$ ln -s YOUR_SCALA_PROJECT/target/scala-3.2.1/scala-controller-assembly-0.0.1.jar EPuckGoForward.jar

That's it! Reset and start the simulation and it should run your Scala code.