TreeItem selected = tree.getSelection()[0];
IResource resource = (IResource) selected.getData();
String resourcePath = resource.getFullPath().toString();
for (MMenuElement element : contextMenu.getChildren()) {
  if (!(element instanceof MHandledMenuItem))
    continue;
  
  MHandledMenuItem item = (MHandledMenuItem) element;
  for (MParameter param : item.getParameters()) {
    if (!param.getName().equals("com.example.commandparameter.resource_path"))
      continue;
    param.setValue(resourcePath);
  }
}