google api , the problem of null refresh token

http://stackoverflow.com/questions/10827920/google-oauth-refresh-token-is-not-being-received

The refresh_token is only provided on the first authorization from the user. Subsequent authorizations, such as the kind you make while testing an OAuth2 integration, will not return the refresh_token again. :)

1. Go to your account security settings: https://www.google.com/settings/u/1/security.
2. Click the edit button next to "Authorizing applications and sites".
3. Then click "Revoke Access" next to your app.
4. The next OAuth2 request you make will return a refresh_token.

Alternatively, you can add the query parameter approval_prompt=force to the OAuth redirect (see Google's OAuth 2.0 for Web Server Applications page).

This will prompt the user to authorize the application again and will always return a refresh_token.