FirebaseOptions options = new FirebaseOptions.Builder()
    .setServiceAccount(getServletContext().getResourceAsStream("/WEB-INF/xxx.json"))
    .setDatabaseUrl("https://xxx.firebaseio.com")
    .build();

FirebaseApp.initializeApp(options);


DatabaseReference ref = FirebaseDatabase
                .getInstance()
                .getReference();

ref.child("users").setValue("aaa");

var config = {
    apiKey: "xxx",
    authDomain: "xxx.firebaseapp.com",
    databaseURL: "https://xxx.firebaseio.com",
    storageBucket: "xxx",
    messagingSenderId: "xxx"
};
firebase.initializeApp(config);

var database = firebase.database();

var myRef = firebase.database().ref("users");

myRef.on('value', function(snapshot) {
    alert(5);
});