Thanks!
obj.getMesh().setVertexController(
new GenericVertexController() {
@Override
public void apply() {
for (int c = 0; c < this.getSourceNormals().length; c++) {
final SimpleVector temp = this
.getSourceNormals()[c];
temp.scalarMul(-1f);
this.getDestinationNormals()[c] = temp;
}
}
}, false);
obj.getMesh().applyVertexController();
obj.getMesh().removeVertexController();
worked.