How to generate access token using refresh token through google drive API?

It's an old question but seems to me it wasn't completely answered, and I needed this information too so I'll post my answer.

If you want to use the Google Api Client Library, then you just need to have an access token that includes the refresh token in it, and then - even though the access token will expire after an hour - the library will refresh the token for you automatically.

In order to get an access token with a refresh token, you just need to ask for the offline access type (for example in PHP: $client->setAccessType("offline");) and you will get it. Just keep in mind you will get the access token with the refresh token only in the first authorization, so make sure to save that access token in the first time, and you will be able to use it anytime.

Hope that helps anyone :-)

If you are using web api then you should make a http POST call to URL : https://www.googleapis.com/oauth2/v4/token with following request body

client_id: <YOUR_CLIENT_ID>
client_secret: <YOUR_CLIENT_SECRET>
refresh_token: <REFRESH_TOKEN_FOR_THE_USER>
grant_type: refresh_token

refresh token never expires so you can use it any number of times. The response will be a JSON like this:

{
  "access_token": "your refreshed access token",
  "expires_in": 3599,
  "scope": "Set of scope which you have given",
  "token_type": "Bearer"
}

If you want to implement that yourself, the OAuth 2.0 flow for Web Server Applications is documented at https://developers.google.com/accounts/docs/OAuth2WebServer, in particular you should check the section about using a refresh token:

https://developers.google.com/accounts/docs/OAuth2WebServer#refresh