code: private static Constraint loadConstraint(Annotation context) {
          Constraint constraint = null;
          final ServiceLoader<Constraint> constraints = ServiceLoader.load(Constraint.class);
  
          for (Constraint aConstraint : constraints) {
              try {
                  aConstraint.getClass().getDeclaredMethod("check", context.annotationType());
                  constraint = aConstraint;
                  break;
              } catch (NoSuchMethodException e) {
                  // Look for next implementation if method not found with required signature.
              }
          }
  
          if (constraint == null) {
              throw new IllegalStateException("Couldn't found any implementation of " + Constraint.class.getName());
          }
          return constraint;
      }
docs: we have only one implementation on classpath.