Could not initialize class FirebaseThreadManagers error on Firebase

Question

I'm attempting to manipulate my Firebase database from Google App Engine. Having followed tutorials, I'm getting the following error -

java.lang.NoClassDefFoundError: Could not initialize class com.google.firebase.internal.FirebaseThreadManagers

Sometimes I see the following:

org.slf4j.LoggerFactory is a restricted class. Please see the Google  App Engine developer's guide for more details.

The error occurs at FirebaseOptions, as seen in my code below.

        FileInputStream serviceAccount = new FileInputStream("WEB-INF/MyApp.json");

    FirebaseOptions options = new FirebaseOptions.Builder()
            .setCredentials(GoogleCredentials.fromStream(serviceAccount))
            .setDatabaseUrl("https://MyApp.firebaseio.com")
            .build();

    FirebaseApp.initializeApp(options);

Here's the error in full

java.lang.NoClassDefFoundError: Could not initialize class com.google.firebase.internal.FirebaseThreadManagers at com.google.firebase.FirebaseOptions$Builder.(FirebaseOptions.java:147) at com.example.name.myapplication.backend.MyServlet.doGet(MyServlet.java:55) at javax.servlet.http.HttpServlet.service(HttpServlet.java:617) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166) at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:63) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at com.google.appengine.tools.development.DevAppServerModulesFilter.doRedirectedModuleRequest(DevAppServerModulesFilter.java:415) at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:128) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418) at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98) at org.mortbay.jetty.servlet.Dispatcher.forward(Dispatcher.java:327) at org.mortbay.jetty.servlet.Dispatcher.forward(Dispatcher.java:126) at com.google.appengine.tools.development.JettyContainerService.forwardToServer(JettyContainerService.java:458) at com.google.appengine.tools.development.Modules.forwardToInstance(Modules.java:372) at com.google.appengine.tools.development.DelegatingModulesFilterHelper.forwardToInstance(DelegatingModulesFilterHelper.java:95) at com.google.appengine.tools.development.DevAppServerModulesFilter.doRedirect(DevAppServerModulesFilter.java:326) at com.google.appengine.tools.development.DevAppServerModulesFilter.doFilter(DevAppServerModulesFilter.java:119) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418) at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:98) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:511) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle(Server.java:326) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542) at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409) at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

Here's my build.gradle for the backend

    buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.google.appengine:gradle-appengine-plugin:1.9.42'
    }
}

repositories {
    jcenter();
}

apply plugin: 'java'
apply plugin: 'war'
apply plugin: 'appengine'

sourceCompatibility = JavaVersion.VERSION_1_7
targetCompatibility = JavaVersion.VERSION_1_7

dependencies {
    appengineSdk 'com.google.appengine:appengine-java-sdk:1.9.42'
    compile 'javax.servlet:servlet-api:2.5'
    compile 'com.google.firebase:firebase-admin:5.5.0'
    compile 'com.squareup.retrofit2:retrofit:2.1.0'
    compile 'com.squareup.retrofit2:converter-gson:2.1.0'
    compile 'com.google.code.gson:gson:2.6.1'


    appengine {
        downloadSdk = true
        appcfg {
            oauth2 = true
        }
    }
}

Top level build file:

    buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'


        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
        classpath 'com.google.gms:google-services:3.1.0'
    }
}

allprojects {
    repositories {
        jcenter()
        maven { url "https://maven.google.com" }
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

You tagged this with both "android" and "google-app-engine". Seems like it should be one or the other, but not both. Are you using the firebase-admin SDK on GAE? — Doug Stevenson
Sorry, yes. I'm using Google App Engine as a backend for my Android app. This is solely to do with GAE. I am using Firebase-admin SDK. It's one of my dependencies alongside firebase-database and firebase-core. — PSLDev
I just tested your exact build.gradle, and it worked fine for me. My gut feeling is that having the android plugins in your project has left it in an inconsistent state. I would recommend cleaning the project directory (gradle clean), and try to re-execute the app. — Hiranya Jayathilaka
I've tried that and no difference. Gradle builds fine but when the Servlet is called I get that error. Please see my edit for another error I sometimes see, occurs at the same line - Firebase options. — PSLDev

Doug Stevenson Doug Stevenson · Accepted Answer · 2017-11-30T20:11:19

Don't use the firebase-admin dependency alongside the other Android client libraries in a server app. firebase-admin alone has everything you need to access Realtime Database in a JVM runtime. The Android client libraries will be of no use in a server app because they require Android infrastructure.

Could not initialize class FirebaseThreadManagers error on Firebase

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