@ExtensionPoint
public interface OAuthServiceProvider {
  /**
   * Returns the URL where you should redirect your users to
   * authenticate your application.
   */
  String getAuthorizationUrl();

  /**
   * Retrieve the access token
   */
  OAuthToken getAccessToken(OAuthVerifier verifier);

   /**
   * After establishing of secure communication channel,
   * this method supossed to access the protected resoure
   * and retrieve the user infos.
   */
  OAuthUserInfo getUserInfo(OAuthToken token) throws IOException;
}

public class GitHubApi {

  private static final String AUTHORIZE_URL =
      "https://github.com/login/oauth/authorize?client_id=%s&redirect_uri=%s";

  @Override
  public String getAccessTokenEndpoint() {
    return "https://github.com/login/oauth/access_token";
  }
  [...]

public class GoogleApi {

  private static final String AUTHORIZE_URL =
      "https://accounts.google.com/o/oauth2/auth?response_type=code&client_id=%s&redirect_uri=%s&scope=%s";

  @Override
  public String getAccessTokenEndpoint() {
    return "https://accounts.google.com/o/oauth2/token?grant_type=authorization_code";
  }
  [...]