@Override
    public long nextDelayMillis() {
        final long delayMillis = retryBackoff.nextDelayMillis(1);
        if (delayMillis < 0) {
            throw new IllegalStateException(
                    "retryBackoff.nextDelayMillis(1) returned a negative value for " + endpoint +
                    ": " + delayMillis);
        }

        return delayMillis;
    }
Keeps the {@link Future}s which were scheduled via this {@link ScheduledExecutorService}.
Note that this field is also used as a lock.
core/src/main/java/com/linecorp/armeria/client/endpoint/healthcheck/DefaultHealthCheckerContext.java