static <T> Sampler<T> of(String specification) {
        requireNonNull(specification, "specification");
        switch (specification.trim()) {
            case "always":
            case "true":
                return Sampler.always();
            case "never":
            case "false":
                return Sampler.never();
        }

        final List<String> components = KEY_VALUE_SPLITTER.splitToList(specification);
        checkArgument(components.size() == 2, IAE_MSG_TEMPLATE, specification);

        final String key = components.get(0);
        final String value = components.get(1);
        try {
            switch (key) {
                case "random":
                    return Sampler.random(Float.parseFloat(value));
                case "rate-limit":
                case "rate-limiting":
                case "rate-limited":
                    return Sampler.rateLimiting(Integer.parseInt(value));
                default:
            }
        } catch (Exception e) {
            throw new IllegalArgumentException(
                    String.format(Locale.ROOT, IAE_MSG_TEMPLATE, specification), e);
        }

        throw new IllegalArgumentException(
                String.format(Locale.ROOT, IAE_MSG_TEMPLATE, specification));
    }
A sampler that will never be sampled.
core/src/main/java/com/linecorp/armeria/common/util/Samplers.java