How to Resolve Google Authentication Errors When Accessing Token Endpoint from JVM?

What are the common reasons for Google authentication errors when accessing the token endpoint from a JVM application?

Experiencing Google authentication errors when accessing the token endpoint from a Java Virtual Machine (JVM) application can be frustrating. Understanding the root causes of these errors and knowing how to resolve them is crucial for smooth authentication using Google's API services.

import java.net.HttpURLConnection;
import java.net.URL;

public class GoogleAuthTokenFetcher {
    public static void main(String[] args) {
        try {
            String url = "https://oauth2.googleapis.com/token";
            URL obj = new URL(url);
            HttpURLConnection con = (HttpURLConnection) obj.openConnection();
            con.setRequestMethod("POST");
            // Add additional request properties as needed
            // con.setDoOutput(true);
            // Write your POST data here if necessary

            int responseCode = con.getResponseCode();
            System.out.println("Response Code : " + responseCode);
            // Handle the response
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Causes

• Incorrect client ID or client secret
• Invalid redirect URI registered in Google Cloud Console
• Expired or incorrectly formatted OAuth tokens
• Insufficient permissions for the requested scopes
• Networking issues preventing access to Google's servers

Solutions

• Verify that the client ID and client secret match those in your Google Cloud Console project.
• Ensure the redirect URI exactly matches the one registered in the API console, including the scheme (http vs https).
• Handle token expiration appropriately and refresh tokens before making API calls.
• Double-check that the requested scopes are included and valid for your application.
• Test network connectivity to ensure that your application can reach Google's token endpoint.

Mistake: Using different redirect URIs in the application and Google Console.

Solution: Always ensure that the redirect URI in your application matches one of the URIs registered in the Google Cloud project.

Mistake: Not handling token expiration properly, leading to invalid token errors.

Solution: Implement a mechanism to refresh tokens when they expire before making API requests.

Mistake: Incorrectly formatted requests sent to the token endpoint.

Solution: Refer to Google’s API documentation to ensure that request parameters are correctly formatted and all required fields are included.