HttpProxySpecialized.java

import com.google.api.client.http.HttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.auth.http.HttpTransportFactory;
import com.google.auth.oauth2.GoogleCredentials;
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;

import java.net.InetSocketAddress;
import java.net.Proxy;

public class Main {

  public static void main(String[] args) throws Exception {
    InetSocketAddress address = new InetSocketAddress("localhost", 3128);
    final HttpTransport transport = new NetHttpTransport.Builder()
        .setProxy(new Proxy(Proxy.Type.HTTP, address))
        .build();
    HttpTransportFactory transportFactory = new HttpTransportFactory() {
      @Override
      public HttpTransport create() {
        return transport;
      }
    };

    FirebaseOptions options =
        FirebaseOptions.builder()
            .setCredentials(GoogleCredentials.getApplicationDefault(transportFactory))
            .setHttpTransport(transport)
            .build();
    FirebaseApp.initializeApp(options);

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