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");
    }

}

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;
}

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