@Nullable
    @Override
    public TlsEngineType tlsEngineType() {
        final String strTlsEngineType = getNormalized("tlsEngineType");
        if (strTlsEngineType == null) {
            return null;
        }
        switch (strTlsEngineType) {
            case "jdk":
                return TlsEngineType.JDK;
            case "openssl":
                return TlsEngineType.OPENSSL;
            default:
                throw new IllegalArgumentException(
                        String.format("%s isn't one of 'jdk' or 'openssl'", strTlsEngineType));
        }
    }
Implementation of {@link FlagsProvider} which provides values from JVM options to {@link Flags}.
core/src/main/java/com/linecorp/armeria/common/SystemPropertyFlagsProvider.java