首先session是同一PC同一浏览器共享的.比如如下代码：
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
HttpSession hs = request.getSession(); //存入session
String user = request.getParameter("user");
hs.setAttribute("user", user);
response.sendRedirect("wel.jsp");
}
如果同时在同一PC同一浏览器登录系统，那么session["user"]的值会被第二次改写。那怎么解决呢？
解决办法：只需要在session中存的key不同就可以，然后通过request传递key即可。如下：
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
HttpSession hs = request.getSession();
String user = request.getParameter("user"); //存入session，以用户名作为key，避免重写
hs.setAttribute(user, user);
//以后request参数传递给下个页面，以便调用对应session
response.sendRedirect("wel.jsp?user="+user);
}