Very limited reflection is possible, but is not recommended. Use it in cases, when mod doesn't have integration.

// side: client
// This is a startup script. This script changes a java variable in ProjectE mod by accessing code directly

// Listen to post-init event, after all mods have loaded
events.listen('postinit', function (event) {
  // Loads Java class field
  var devEnvField = utils.field('moze_intel.projecte.PECore', 'DEV_ENVIRONMENT')
  // Changes public static boolean DEV_ENVIRONMENT of PECore class to false