Connect an Android app

Each Firebase Android SDK has a useEmulator(host, port) method. Call it once for each product, before the app uses that product. The Application class is a good place, since it runs before any activity.

Connect

Kotlin

app/src/main/java/com/example/shop/ShopApplication.kt

package com.example.shop

import android.app.Application
import com.google.firebase.Firebase
import com.google.firebase.auth.auth
import com.google.firebase.firestore.firestore
import com.google.firebase.functions.functions
import com.google.firebase.storage.storage

class ShopApplication : Application() {
    override fun onCreate() {
        super.onCreate()
        if (BuildConfig.DEBUG) {
            // 10.0.2.2 is the Android emulator's name for your machine.
            Firebase.auth.useEmulator("10.0.2.2", 9099)
            Firebase.firestore.useEmulator("10.0.2.2", 8080)
            Firebase.storage.useEmulator("10.0.2.2", 9199)
            Firebase.functions.useEmulator("10.0.2.2", 5001)
        }
    }
}

Java

app/src/main/java/com/example/shop/ShopApplication.java

package com.example.shop;

import android.app.Application;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.functions.FirebaseFunctions;
import com.google.firebase.storage.FirebaseStorage;

public class ShopApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        if (BuildConfig.DEBUG) {
            // 10.0.2.2 is the Android emulator's name for your machine.
            FirebaseAuth.getInstance().useEmulator("10.0.2.2", 9099);
            FirebaseFirestore.getInstance().useEmulator("10.0.2.2", 8080);
            FirebaseStorage.getInstance().useEmulator("10.0.2.2", 9199);
            FirebaseFunctions.getInstance().useEmulator("10.0.2.2", 5001);
        }
    }
}

Name the class in the manifest’s <application android:name=".ShopApplication">. BuildConfig.DEBUG keeps the calls out of release builds; since version 8 of the Android Gradle plugin, it needs buildFeatures { buildConfig = true } in the app’s build.gradle.kts.

Use the ports from your firebase.json if they are not the defaults. The console’s Connect dialog prints the Kotlin lines with your ports: pick Android emulator in its Host menu to get 10.0.2.2. If your functions run in another region, connect that region’s instance: Firebase.functions("europe-west1") in Kotlin, FirebaseFunctions.getInstance("europe-west1") in Java.

Firestore’s useEmulator also turns SSL off for that connection, so it needs no other setting.

Which host

Where the app runs	Host
The Android emulator	10.0.2.2
A phone or tablet on your network	Your machine’s network address, with Firenook started with --host 0.0.0.0

Inside the Android emulator, 127.0.0.1 is the emulator itself, not your machine. For a real device, start Firenook so it listens on your network:

npx firenook emulators:start --host 0.0.0.0

Firenook has no authentication: with --host 0.0.0.0, anyone on the network can reach the emulators and the data, so use it only on a network you trust.

Allow cleartext traffic

The emulators speak plain HTTP. Android blocks cleartext traffic by default for apps that target Android 9 (API level 28) or later, so Authentication, Storage and Functions requests fail until you allow it for the emulator’s host. Do it in a network security config that only debug builds use, so a release build keeps blocking cleartext:

app/src/debug/res/xml/network_security_config.xml

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartextTrafficPermitted="true">
        <domain includeSubdomains="false">10.0.2.2</domain>
        <!-- Your machine's address, for a real device: -->
        <domain includeSubdomains="false">192.168.1.20</domain>
    </domain-config>
</network-security-config>

app/src/debug/AndroidManifest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
    <application android:networkSecurityConfig="@xml/network_security_config" />
</manifest>

Files under app/src/debug/ go into debug builds only, and Gradle merges this manifest into the main one.

The offline cache

Firestore keeps an offline cache on the device, and the SDK does not clear it when the emulator’s data changes. Without --keep-data, every Firenook start is empty, so the cache can still hold documents from the last run. Google suggests a memory cache while you use the emulator:

Kotlin

import com.google.firebase.firestore.firestoreSettings
import com.google.firebase.firestore.memoryCacheSettings

Firebase.firestore.firestoreSettings = firestoreSettings {
    setLocalCacheSettings(memoryCacheSettings {})
}

Java

import com.google.firebase.firestore.FirebaseFirestoreSettings;
import com.google.firebase.firestore.MemoryCacheSettings;

FirebaseFirestoreSettings settings = new FirebaseFirestoreSettings.Builder()
        .setLocalCacheSettings(MemoryCacheSettings.newBuilder().build())
        .build();
FirebaseFirestore.getInstance().setFirestoreSettings(settings);

Set it in the same place as useEmulator, before the app reads or writes. Settings and useEmulator combine, in either order.

Next

Building for iOS too? See Apple platforms. For cross-platform apps, see Flutter or React Native.