static int validateNonNegative(int value, String fieldName) {
        if (value < 0) {
            throw new IllegalArgumentException(fieldName + ": " + value + " (expected: >= 0)");
        }
        return value;
    }
Initialized later by {@link Server} via {@link #setServer(Server)}.
core/src/main/java/com/linecorp/armeria/server/DefaultServerConfig.java