should add null argument to ScriptEngineManager: ScriptEngineManager manager = new ScriptEngineManager(null);
Javadoc says:
This constructor loads the implementations of ScriptEngineFactory visible to the given ClassLoader using the service provider mechanism.
If loader is null, the script engine factories that are bundled with the platform and that are in the usual extension directories (installed extensions) are loaded.
https://github.com/yannrichet/rsession/blob/2ad82f336fa00a0a8362460947b087150d7d8522/src/main/java/org/math/R/R2jsSession.java#L123
should add null argument to ScriptEngineManager:
ScriptEngineManager manager = new ScriptEngineManager(null);
Javadoc says: This constructor loads the implementations of ScriptEngineFactory visible to the given ClassLoader using the service provider mechanism. If loader is null, the script engine factories that are bundled with the platform and that are in the usual extension directories (installed extensions) are loaded.