You can pass directly into Spark the parameters of the keystore, like this:
Spark.secure(keyStorePath, keyStorePassword, trustStorePath, trustStorePassword);
Those are all strings, and for example, the keystore values could be:
String keyStorePath = "/home/user/keys/private-key.jks";
String keyStorePassword = "password";
This way, Spark will have the parameters to create the SslContextFactory inside, as you can see here: