The main question is "How can I test this Pull Request?". I need FrappeExample.java and app/user credentials to successfully run it.

Login with (Role - System Manager)
Testsite : https://scribejava.mntechnique.com
username : scribejava@example.com
password : scribejava

I'll delete above credentials after testing.

Client Details Added in Example,

Login and change client details here

https://scribejava.mntechnique.com/desk#Form/OAuth%20Client/37763ecdb0

In this case serverURL keeps changing as per the hosting domain name.

For others who wish to use their own frappe server url, I can think of extending FrappeApi to CustomFrappeApi and using it instead. I've added setter and getter for this.

Any thing better you can suggest?

e.g.

public class CustomFrappeAPI extends FrappeApi {
    protected CustomFrappeAPI(){
        setServerURL("http://custom.frappe.url.tld");
    }

    private static class InstanceHolder {
        private static final CustomFrappeAPI INSTANCE = new CustomFrappeAPI();
    }

    public static CustomFrappeAPI instance() {
        return CustomFrappeAPI.InstanceHolder.INSTANCE;
    }
}

References :

Frappe Framework is Open Source Web App Framework
Setup frappe server using frappe/bench - https://github.com/frappe/bench
Setup OAuth 2 and use REST API - http://frappe.github.io/frappe/user/en/guides/integration/
frappe framework https://github.com/frappe/frappe

great work, well done.
I have merged this Pull Request to the master with some additions
1.I created generic OpenId token and extractor for Google and Frappe.
2.As for custom serverUrl, it's a required param to Frappe. So, it's the required param for instance() method.
3.credentials are not needed anymore. I have tested Example, and removed credential from the sources.

I have fixed up all commits to the single one - 6dbf10e
my additions - f42812f and 48d17c2