showMessageDialog() method cause the program get stuck in Java Web Project

I'm using eclipse to develop a Java Web Dynamic Project. While calling the function JOptionPane.showMessageDialog(null, "Invalid Username or Password"); both in running or debugging, the program will get stuck, and no alert window appears. And the program could never forward. PS: I'm using Tomcat 8.0 Server in eclipse Java EE to test the project. And the following is a piece of my code:

protected void doPost(HttpServletRequest req, HttpServletResponse resp)    throws ServletException, IOException {               
JOptionPane.showMessageDialog(null, "Invalid Username or Password");

And the function of doPost is proved to be right. So what's the reason why the program get stuck and how to fix it?