Google app engine standard + Firebase database

I have a Google app engine + firebase database backend architecture. I am writing a servlet, which should get some values from database, make calculations and build a response with this values. The problem is that onDataChange() method is called asynchronously. At first I'd introduce my code and then continue:

//Here I initialize a listener that would be called when the onDataChange() 
//method is finished to make the code synchronous.
    responseReadyListener = new ResponseReadyListener() {
            @Override
            public void onResponseReady() {
                responseReady[0] = true;
                synchronized (MyServlet.this) {
                    MyServlet.this.notify();
                }
            }
        };
        ref.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                    //notify the main thread that the response can be sent
                    responseReadyListener.onResponseReady();
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
        //the loop that stops the main thread until onDataChange() finishes
        while (!responseReady[0]) {
            try {
                synchronized (this) {
                    if (!responseReady[0]) {
                       this.wait();
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

Now the question, recently I have read that only one servlet instance is created to respond to http requests. That is why I cant use synchronized(this) as it will stop synchronized threads of all client responses the server gets (i need to stop only 1 main thread, the main thread of only one request). How to get rid of the method asynchronicity properly?