private static String validateHostname(String hostname) {
        requireNonNull(hostname, "hostname");
        if (hostname.isEmpty()) {
            throw new IllegalArgumentException("hostname is empty.");
        }
        return hostname;
    }
Sets the hostname of an embedded Tomcat.
tomcat10/src/main/java/com/linecorp/armeria/server/tomcat/TomcatServiceBuilder.java