Firebase Login Authentication in Java

Introduction: The Firebase Authentication provides methods to create and manage users that use their email addresses and passwords or federated identity provider or phone number to sign in.

Federated Identity Provider: Federated identity providers like Google, Facebook, Apple, GitHub, and Twitter, etc.

Working: To sign a user into an app  we can use the user's email address and password or select any of the federated identity providers then we will get an authentication token from firebase SDK on client-side now we can send this token to our own backend server and authenticate it

Also Read: Creating Asynchronous Methods in Spring Boot

Backend side Implementation :

1. Add the firebase admin SDK

Before this, we need the following

A Firebase project
A service account to communicate with Firebase
A configuration file with the service account's credentials

If you do not have a Firebase project, you need to create one by visiting the Firebase Console.

Add the SDK

 dependencies {
  implementation 'com.google.firebase:firebase-admin:7.1.0'
}

or

<dependency>
  <groupId>com.google.firebase</groupId>
  <artifactId>firebase-admin</artifactId>
  <version>7.1.0</version>
</dependency>

Also Read: 10 points for writing and maintaining APIs

2. Initialize the SDK

To generate a private key file :
1. Go to Firebase Console and open Setting>Service Accounts
2. Click Generate New Private Key
3. Store the JSON file containing private key

application.properties file 

     app.firebase-config="path of service-account-file.json"

FirebaseConfiguration.java file

public class FirebaseConfiguration {

    @Value("${app.firebase-config}")
    private String firebaseConfig;

    private void initialize() throws IOException {

          FirebaseOptions options = new FirebaseOptions.Builder()
                                   .setCredentials( GoogleCredentials.fromStream( new ClassPathResource( firebaseConfig ).getInputStream() ) )
                                   .build();

          if ( FirebaseApp.getApps().isEmpty() )
            {
               this.firebaseApp = FirebaseApp.initializeApp( options );
            }
            else
            {
                this.firebaseApp = FirebaseApp.getInstance();
            }

   }

}

Also Read: Rest Template in Spring Boot

3. Verify Id tokens 

// idToken comes from the client app
FirebaseToken decodedToken = FirebaseAuth.getInstance().verifyIdToken(idToken);
String uid = decodedToken.getUid();
 

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