HttpProxyGlobal.java

import com.google.auth.oauth2.GoogleCredentials;
import com.google.cloud.firestore.DocumentSnapshot;
import com.google.cloud.firestore.Firestore;
import com.google.firebase.FirebaseApp;
import com.google.firebase.FirebaseOptions;
import com.google.firebase.messaging.FirebaseMessaging;
import com.google.firebase.messaging.Message;
import com.google.firebase.messaging.Notification;

public class Main {

  public static void main(String[] args) throws Exception {
    System.setProperty("https.proxyHost", "localhost");
    System.setProperty("https.proxyPort", "3128");
    System.setProperty("com.google.api.client.should_use_proxy", "true");

    FirebaseOptions options =
        FirebaseOptions.builder()
            .setCredentials(GoogleCredentials.getApplicationDefault())
            .build();
    FirebaseApp.initializeApp(options);
    Firestore firestore = FirestoreClient.getFirestore();
    DocumentSnapshot snapshot = firestore.collection("users").document("alice").get().get();

    FirebaseMessaging messaging = FirebaseMessaging.getInstance();
    String response = messaging.send(Message.builder()
        .setTopic(snapshot.getString("topicName"))
        .setNotification(new Notification(
            "Hello from the other side",
            "I must have called this proxy a thousand times"))
        .build());
    System.out.println(response);
  }
}