Why this custom java method in servlet class having childeventlistener of firebase gets inturrepted from completion?

Question

I have this processRequest(..) method in a Servlet class, where I am checking user's email and password in isLoginSuccessful(..) method using ChildEventListener of Firebase and want to redirect the user to home.jsp if true or back to index.jsp if false. But the boolean loginSuccess in this isLoginSuccessful(..) method returns false, hence redirect back to index.jsp then returns true. Means isLoginSuccessful(..) gets interrupted. How to fix this?

processRequest() method :

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    response.setContentType("text/html;charset=UTF-8");
    out = response.getWriter();

    // initialize firebaseapp
    FirebaseAppProvider.getFirebaseApp(getServletContext());

    // email password from textbox
    String user_email = request.getParameter("loginEmail");
    String user_password = request.getParameter("loginPassword");

    boolean loginSuccessful = isLoginSuccessful(user_email, user_password, response);

    if (loginSuccessful) {
        System.out.println("login if");
        response.sendRedirect("home.jsp");
    } else {
        System.out.println("login else");
        out.println("Login Failed");
        response.sendRedirect("index.jsp#download");
    }      

}

isLoginSuccessful() method :

 boolean loginSuccess = false;
private  boolean isLoginSuccessful(String userEmail, String userPassword, HttpServletResponse response) {

    FirebaseDatabase dbFire = FirebaseDatabase.getInstance();
    DatabaseReference dbRefUsers = dbFire.getReference().child("Users");     

    System.out.println("isLoginSuccessful method called");        

    dbRefUsers.orderByChild("user_email").equalTo(userEmail).addChildEventListener(new ChildEventListener() {

        @Override
        public void onChildAdded(DataSnapshot ds, String string) {

            System.out.println("on Child added called");

            User user = ds.getValue(User.class);
            boolean pwTrue = (userPassword.equals(user.getUser_password()));

                if (pwTrue) {
                    System.out.println("if onChildAdded called");
                    loginSuccess =true;                      
                    System.out.println("loginSuccess = " + loginSuccess);
                } else {
                    System.out.println("else onChildAdded called");
                  loginSuccess = false;
                }
        }

        @Override
        public void onChildChanged(DataSnapshot ds, String string) {
            throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
        }

        @Override
        public void onChildRemoved(DataSnapshot ds) {
            throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
        }

        @Override
        public void onChildMoved(DataSnapshot ds, String string) {
            throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
        }

        @Override
        public void onCancelled(DatabaseError de) {
            throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
        }
    });

    System.out.println("loginSuccess = " + loginSuccess);

    return loginSuccess;
}

Console logs

Info:   isLoginSuccessful method called
Info:   loginSuccess = false
Info:   login else
Info:   on Child added called
Info:   if onChildAdded called
Info:   loginSuccess = true 

What is getting happened :

1. processRequest() gets called

2. isLoginSuccessful() gets called where loginSuccess is false

3.if statement from processRequest() gets called

4. isLoginSuccessful() gets called again where loginSuccess is true now.

What I want is :

1. processRequest() gets called

2. isLoginSuccessful() gets called one time where loginSuccess is true

3.if statement from processRequest() gets called