注册第一个能登录，再次注册一个，就不行了

package com.my.servlet;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.my.domain.User;

/**
 * 登录的servlet
 */
@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		List<User> list=(List<User>)this.getServletContext().getAttribute("list");
		System.out.println(list);
		String username=req.getParameter("username");
		String password=req.getParameter("password");
		boolean flag=false;
		for (User user : list) {
			if(user.getUsername().equals(username)&&user.getPassword().equals(password)) {
				flag=true;
				req.setAttribute("username", username);
				//req.getRequestDispatcher("/success.jsp").forward(req, resp);
			}
			if(flag) {
				req.getRequestDispatcher("/success.jsp").forward(req, resp);
				
			}else {
				req.getRequestDispatcher("/fail.jsp").forward(req, resp);
				
			}
		}
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(req, resp);
	}
	

}