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

}

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.

I had the same error NoClassDefFoundError FirebaseThreadManagers regarding this line of code: private ThreadManager threadManager = FirebaseThreadManagers.DEFAULT_THREAD_MANAGER;

Solution

Rebuild the gradle file. After the gradle file rebuilt the Kotlin app ran as expected.