For Medium Article

Firebase.java

package firebase.jwt;

import com.google.auth.oauth2.GoogleCredentials;
import com.google.firebase.FirebaseApp;
import com.google.firebase.FirebaseOptions;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseAuthException;
import com.google.firebase.auth.FirebaseToken;
import io.micronaut.core.io.ResourceResolver;
import io.micronaut.core.io.scan.ClassPathResourceLoader;
import java.io.IOException;
import org.slf4j.LoggerFactory;

import javax.annotation.PostConstruct;
import javax.inject.Singleton;

@Singleton
class Firebase {

    private boolean initialized = false;

    @PostConstruct
    public void init() {
        if (initialized) {
            return;
        }

        try {
            var loader = new ResourceResolver()
                    .getLoader(ClassPathResourceLoader.class)
                    .get();

            var credentials = GoogleCredentials.fromStream(
                    loader.getResourceAsStream("firebase-adminsdk.json").get());

            var options = FirebaseOptions.builder()
                    .setCredentials(credentials)
                    .build();

            FirebaseApp.initializeApp(options);
            initialized = true;

        }
        catch (IOException e) {
            LoggerFactory
                    .getLogger(Firebase.class)
                    .error("Failed to initialize Firebase authentication", e);
        }
    }

    public FirebaseToken verifyIdToken(String idToken) throws FirebaseAuthException {
        return FirebaseAuth.getInstance().verifyIdToken(idToken);
    }

}