private SelfSignedCertificate selfSignedCertificate() throws CertificateException {
        if (selfSignedCertificate == null) {
            assert defaultHostname != null;
            return selfSignedCertificate = new SelfSignedCertificate(defaultHostname);
        }
        return selfSignedCertificate;
    }
Returns a newly-created {@link VirtualHost} based on the properties of this builder and the services
added to this builder.
core/src/main/java/com/linecorp/armeria/server/VirtualHostBuilder.java