(Java) Get GMail SMTP OAuth2 Access Token for Desktop App

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Note: This example requires Chilkat v10.1.2 or greater.

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import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // To further clarify, see OAuth 2.0 Authorization Flow

    CkOAuth2 oauth2 = new CkOAuth2();

    // For Google OAuth2, set the listen port equal to the port used
    // in the Authorized Redirect URL for the Client ID.
    // For example, in this case the Authorized Redirect URL would be http://localhost:55568/
    // Your app should choose a port not likely not used by any other application.
    oauth2.put_ListenPort(55568);

    oauth2.put_AuthorizationEndpoint("https://accounts.google.com/o/oauth2/v2/auth");
    oauth2.put_TokenEndpoint("https://www.googleapis.com/oauth2/v4/token");

    // Replace these with actual values.
    oauth2.put_ClientId("GOOGLE-CLIENT-ID");
    oauth2.put_ClientSecret("GOOGLE-CLIENT-SECRET");

    // To use Gmail's OAuth2 for sending emails via the SMTP protocol, you need to request the https://mail.google.com/ scope. 
    // See About GMail SMTP OAuth2 Scopes
    oauth2.put_Scope("https://mail.google.com/");

    // Begin the OAuth2 Authorization code flow.  This returns a URL that should be loaded in a browser.
    String url = oauth2.startAuth();
    if (oauth2.get_LastMethodSuccess() != true) {
        System.out.println(oauth2.lastErrorText());
        return;
        }

    // Launch the default browser on the system and navigate to the url.
    // The LaunchBrowser method was added in Chilkat v10.1.2.
    boolean success = oauth2.LaunchBrowser(url);
    if (success == false) {
        System.out.println(oauth2.lastErrorText());
        return;
        }

    // Wait for the user to approve or deny authorization in the browser.
    int numMsWaited = 0;
    while ((numMsWaited < 90000) && (oauth2.get_AuthFlowState() < 3)) {
        oauth2.SleepMs(100);
        numMsWaited = numMsWaited+100;
        }

    // If the browser does not respond within the specified time, AuthFlowState will be:
    // 
    // 1: Waiting for Redirect – The OAuth2 background thread is waiting for the browser's redirect request.
    // 2: Waiting for Final Response – The thread is awaiting the final access token response.
    // In either case, cancel the background task initiated by StartAuth.

    if (oauth2.get_AuthFlowState() < 3) {
        oauth2.Cancel();
        System.out.println("No response from the browser!");
        return;
        }

    // Check AuthFlowState to determine if authorization was granted, denied, or failed:
    // 
    // 3: Success – OAuth2 flow completed, the background thread exited, and the successful response is in AccessTokenResponse.
    // 4: Access Denied – OAuth2 flow completed, the background thread exited, and the error response is in AccessTokenResponse.
    // 5: Failure – OAuth2 flow failed before completion, the background thread exited, and error details are in FailureInfo.

    if (oauth2.get_AuthFlowState() == 5) {
        System.out.println("OAuth2 failed to complete.");
        System.out.println(oauth2.failureInfo());
        return;
        }

    if (oauth2.get_AuthFlowState() == 4) {
        System.out.println("OAuth2 authorization was denied.");
        System.out.println(oauth2.accessTokenResponse());
        return;
        }

    if (oauth2.get_AuthFlowState() != 3) {
        System.out.println("Unexpected AuthFlowState:" + oauth2.get_AuthFlowState());
        return;
        }

    // Save the full JSON access token response to a file.
    CkStringBuilder sbJson = new CkStringBuilder();
    sbJson.Append(oauth2.accessTokenResponse());
    sbJson.WriteFile("qa_data/tokens/_gmailSmtp.json","utf-8",false);

    // The saved JSON response looks like this:

    // 	{
    // 	 "access_token": "ya39.Ci-XA_C5bGgRDC3UaD-h0_NeL-DVIQnI2gHtBBBHkZzrwlARkwX6R3O0PCDEzRlfaQ",
    // 	 "token_type": "Bearer",
    // 	 "expires_in": 3600,
    // 	 "refresh_token": "1/r_2c_7jddspcdfesrrfKqfXtqo08D6Q-gUU0DsdfVMsx0c"
    // 	}
    // 
    System.out.println("OAuth2 authorization granted!");
    System.out.println("Access Token = " + oauth2.accessToken());
  }
}