Is it necessary to call FirebaseApp.initializeApp() to initialize firebase?

Question

I added firebase to my android project to use firebase cloud messaging. I followed the documentation and I didn't find any instruction to call FirebaseApp.initializeApp().

My app works fine, except for once it crashed with following error.

Caused by: java.lang.IllegalStateException: Default FirebaseApp is not initialized in this process com.my.app. Make sure to call FirebaseApp.initializeApp(Context) first.
at com.google.firebase.FirebaseApp.getInstance(Unknown Source)
at com.google.firebase.iid.FirebaseInstanceId.getInstance(Unknown Source)
at com.my.app.core.ApplicationEx.onCreate(ApplicationEx.java:79)

When I searched for the error, the resolution given is to call FirebaseApp.initializeApp() at the startup.

I am wondering whether this is really necessary, since documentation didn't mention it and my app worked (mostly) fine without it.

Does anyone know whether calling FirebaseApp.initializeApp() is really necessary, and what else could have caused the error I mentioned above?

Following is my build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.my.app"
        minSdkVersion 17
        targetSdkVersion 26
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }
    flavorDimensions "appType"
    productFlavors {
        passenger {
            dimension "appType"
            applicationId "com.my.app.passenger"
            versionCode 1
            versionName "1"
        }
        driver {
            dimension "appType"
            applicationId "com.my.app.driver"
            versionCode 1
            versionName "1"
        }
        admin {
            dimension "appType"
            applicationId "com.my.app.admin"
            versionCode 1
            versionName "1"
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
        debug {
            testCoverageEnabled true
        }
        packagingOptions {
            exclude 'META-INF/ASL2.0'
            exclude 'META-INF/LICENSE'
            exclude 'META-INF/NOTICE'
            exclude 'META-INF/NOTICE.txt'
            exclude 'META-INF/LICENSE.txt'
            exclude 'META-INF/MANIFEST.MF'
        }
    }
}

repositories {
    maven { url 'https://oss.sonatype.org/content/repositories/snapshots' }
}

dependencies {
    implementation project(path: ':cards')
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation "com.android.support:design:${supportVersion}"
    implementation "com.android.support:support-v4:${supportVersion}"
    implementation "com.android.support:appcompat-v7:${supportVersion}"
    implementation "com.android.support:cardview-v7:${supportVersion}"
    implementation "com.android.support:gridlayout-v7:${supportVersion}"
    implementation "com.google.android.gms:play-services-maps:${googlePlayServicesVersion}"
    implementation "com.google.android.gms:play-services-location:${googlePlayServicesVersion}"
    implementation "com.google.android.gms:play-services-places:${googlePlayServicesVersion}"
    implementation "com.google.android.gms:play-services-gcm:${googlePlayServicesVersion}"
    implementation "com.google.android.gms:play-services-ads:${googlePlayServicesVersion}"
    implementation "com.google.android.gms:play-services-auth:${googlePlayServicesVersion}"
    implementation 'com.google.maps:google-maps-services:0.2.5'
    implementation "com.google.firebase:firebase-messaging:${googlePlayServicesVersion}"
    implementation "com.loopj.android:android-async-http:${asyncHttpVersion}"
    implementation "com.android.support.test.espresso:espresso-idling-resource:${espressoVersion}"
    implementation 'com.android.support:multidex:1.0.2'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    implementation 'org.slf4j:slf4j-api:1.7.25'
    implementation 'com.github.tony19:logback-android-core:1.1.1-6'
    implementation 'ch.acra:acra:4.9.2'
    implementation('com.github.tony19:logback-android-classic:1.1.1-6') {
        exclude group: 'com.google.android', module: 'android'      // workaround issue #73
    }
    testImplementation 'org.testng:testng:6.9.6'
    testImplementation 'org.mockito:mockito-core:1.10.19'
    testImplementation 'org.powermock:powermock-api-mockito:1.6.5'
    testImplementation 'org.powermock:powermock-module-junit4-rule-agent:1.6.5'
    testImplementation 'org.powermock:powermock-module-junit4-rule:1.6.5'
    testImplementation 'org.powermock:powermock-module-junit4:1.6.5'
    androidTestImplementation "com.android.support:support-annotations:${supportVersion}"
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test:rules:1.0.1'
    androidTestImplementation 'org.testng:testng:6.9.6'
    androidTestImplementation 'org.mockito:mockito-core:1.10.19'
    androidTestImplementation 'com.google.dexmaker:dexmaker:1.2'
    androidTestImplementation 'com.google.dexmaker:dexmaker-mockito:1.2'
    androidTestImplementation 'com.android.support.test.uiautomator:uiautomator-v18:2.1.3'
    androidTestImplementation("com.android.support.test.espresso:espresso-core:${espressoVersion}", {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
}

apply plugin: 'com.google.gms.google-services'

Answer 1

The Firebase SDKs generally don't support the use of processes other than the main process. If and when ACRA kicks in and starts another process, its own process will create a new Application subclass for that process. This is because every app process must have at exactly one Application object instantiated.

What this means for your app is that this other process should never use Firebase APIs. This means you'll need to find another place to get that IID token.

(Note that the Firebase SDKs are automatically initialized by a ContentProvider that's merged into your app by default - you should never have to call FirebaseApp.initializeApp() unless you've removed this ContentProvider or you aren't using the google-services plugin.)

Typically, when apps need to get the IID token, they create a subclass of FirebaseInstanceIdService, as described in the documentation. This service is notified every time a new token is known. That's the place where you should be retrieving it and sending it to your server.