OAuth 2 is an open standard for authorization that enables third-party applications to obtain limited access to a user account on a web service. It is commonly used for sign-in purposes, allowing users to log into a website or application using their existing credentials from a different service, such as Google.


  • Basic knowledge of Java and web development
  • An active Google Developer Account to obtain API keys
  • A web application with a server-side component

Step 1: Obtaining API Keys from Google

  1. Log in to the Google Developers Console.
  2. Create a new project or select an existing one.
  3. Go to the “Credentials” page and create a new OAuth client ID.
  4. Select “Web application” as the application type.
  5. Enter a name for the client ID and specify the authorized JavaScript origins and redirect URIs.
  6. Click “Create.”
  7. Obtain the Client ID from the “Credentials” page and keep it in a secure place.

Step 2: Implementing OAuth 2 on the Client Side

  1. In the HTML file, include the Google API JavaScript client library.
<script src="https://apis.google.com/js/platform.js" async defer></script>
  1. Add a Google Sign-In button to your HTML file.
<div class="g-signin2" data-onsuccess="onSignIn"></div>
  1. In the JavaScript file, implement the onSignIn function to send the user’s ID token to your server for verification.
function onSignIn(googleUser) {
  // Get the user's ID token
  var id_token = googleUser.getAuthResponse().id_token;
  // Send the token to your server for verification
  // ...

Step 3: Implementing OAuth 2 on the Server Side

  1. Verify the ID token on your server using the Google API client library for your preferred language, such as Java.
// Import the Google API client library
import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdTokenVerifier;

// Verify the ID token
GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(transport, jsonFactory)

GoogleIdToken idToken = verifier.verify(idTokenString);
if (idToken != null) {
  // ID token is valid
  // ...
} else {
  // ID token is invalid
  // ...

Step 4: Storing and Using User Information

  1. After verifying the ID token, you can extract the user information, such as the user’s email address and name, from the token.
Payload payload = idToken.getPayload();
String userId = payload.getSubject();  // User's unique ID
String email = payload.getEmail();     // User's email address
String name = (String) payload.get("name");  // User's name
  1. Store the user information in your database, or use it to personalize the user’s experience in your application.

Step 5: Handling Errors and Invalid Tokens

  1. Handle errors that may occur during the verification process, such as network errors or invalid tokens.
try {
  GoogleIdToken idToken = verifier.verify(idTokenString);
  // ID token is valid
  // ...
} catch (GeneralSecurityException | IOException e) {
  // Handle the error
  // ...
  1. Ensure that your application is secure by verifying the token on each request from the client and handling invalid tokens appropriately.

Step 6: Using OAuth 2 with Other Providers

  1. Follow the provider’s documentation to obtain API keys and implement OAuth 2 in a similar manner to the example with Google.
  2. Modify the verification process to match the provider’s specifications, such as using a different client library or different verification endpoint.

Additional Considerations:

  • OAuth 2 provides access to a user’s resources, so it is important to understand the security implications of using OAuth 2 in your application.
  • OAuth 2 provides multiple authorization flows to accommodate different use cases. Choose the appropriate flow for your application based on your security requirements and user experience goals.


By implementing OAuth 2 on both the client and server side, you can enable users to log into your web application using their Google credentials or credentials from other providers. This can greatly improve the user experience and reduce the friction of creating and remembering separate login credentials for your application.

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