SkillConstraintViolation how to return true

As far as I know the default constraint type of skill is hard constraint define in com.graphhopper.jsprit.core.problem.constraint. But, when I check out the SolutionAnalyser with this following method.

public Boolean hasSkillConstraintViolation(VehicleRoute route) {
    if (route == null) throw new IllegalArgumentException("route is missing.");
    return stateManager.getRouteState(route, skill_id, Boolean.class);

It return true if the skill violated on the route. So, I just want to know how it can be done.