How to use a FireBase as a Auth provider (untested)

Angular

Install firebase to angular

npm install firebase @angular/fire

Initialize firebase in Angular app

import { initializeApp } from "firebase/app";

const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "YOUR_AUTH_DOMAIN",
  projectId: "YOUR_PROJECT_ID",
  storageBucket: "YOUR_STORAGE_BUCKET",
  messagingSenderId: "YOUR_MESSAGING_SENDER_ID",
  appId: "YOUR_APP_ID",
};

const app = initializeApp(firebaseConfig);

Log user in

import { getAuth, signInWithPopup, GoogleAuthProvider } from "firebase/auth";

const auth = getAuth();
const provider = new GoogleAuthProvider();

signInWithPopup(auth, provider)
  .then((result) => {
    const user = result.user;
    console.log("User signed in:", user);
  })
  .catch((error) => {
    console.error("Error during sign-in:", error);
  });

Send the id to SpringBoot

const user = auth.currentUser;
if (user) {
  user.getIdToken().then((idToken) => {
    // Send idToken to backend for verification
    this.http.post('/api/authenticate', { token: idToken }).subscribe();
  });
}

SpringBoot

Configure Firebase Admin SDK dependency

<dependency>
  <groupId>com.google.firebase</groupId>
  <artifactId>firebase-admin</artifactId>
  <version>8.1.0</version>
</dependency>

Initialize Firebase in SpringBoot

import com.google.firebase.FirebaseApp;
import com.google.firebase.FirebaseOptions;
import com.google.auth.oauth2.GoogleCredentials;

@PostConstruct
public void initFirebase() {
    FirebaseOptions options = FirebaseOptions.builder()
        .setCredentials(GoogleCredentials.fromStream(new FileInputStream("path/to/serviceAccountKey.json")))
        .build();
    FirebaseApp.initializeApp(options);
}

Verify the token in a controller

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseAuthException;
import com.google.firebase.auth.FirebaseToken;

@PostMapping("/api/authenticate")
public ResponseEntity<?> authenticate(@RequestBody Map<String, String> body) {
    try {
        String idToken = body.get("token");
        FirebaseToken decodedToken = FirebaseAuth.getInstance().verifyIdToken(idToken);
        String uid = decodedToken.getUid();

        // Perform additional logic (e.g., store/retrieve user data from MySQL)
        return ResponseEntity.ok("User authenticated: " + uid);
    } catch (FirebaseAuthException e) {
        return ResponseEntity.status(HttpStatus.UNAUTHORIZED).body("Invalid token");
    }
}

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