ZohoBooks Rest API and Self Client Option

Hi,

I'm developing stand-alone java application that needs to use ZohoBooks API. But since this is a single java application that runs every night, there is no user and authorization, authentication should be made automatically between my app and zohobooks.

My questions are:

for stand alone option in https://accounts.zoho.eu/developerconsole it says the code will expire in 3 7 or 10 minutes. So how can I get this code automatically? Every time when I start my app should i get this key manually?

There is no information in zohobooks api docs, but for zoho creator we can get auth token with making HTTP post: https://www.zoho.com/creator/help/api/prerequisites/generate-auth-token.html. I've tried this option for zohobooks but it didn't worked, is there any way getting auth code directly using email/username and password?

Is there any validity time for refresh tokens? or are they valid until I revoke?

Do ZohoBooks provide another OAuth2 Grand Type except "authorization_code", for example can I use "Passwords" grand type (https://oauth.net/2/grant-types/password/) or any other grand type for getting access_token?

Thanks