Cannot forward after response has been committe

Question

[code="java"]
private void login(HttpServletRequest req, HttpServletResponse resp) throws SQLException, IOException, ServletException {
// TODO Auto-generated method stub
String username=req.getParameter("username");
String password=req.getParameter("password");
String captcha=req.getParameter("captcha");
String captcha1=(String)req.getSession().getAttribute("captcha");
if(!captcha.equals(captcha1)){
req.setAttribute("error","验证码错误");
req.getRequestDispatcher("/security/login.jsp").forward(req, resp);
}
User user=userDao.login(username,password);
if(user!=null){
user.setLast_login(new Date());
userDao.update(user);
req.getSession().setAttribute("user", user);
req.getRequestDispatcher("index.jsp").forward(req, resp);
}else{
req.setAttribute("error", "用户名或密码错误");
req.getRequestDispatcher("/security/login.jsp").forward(req,resp);
}
}
[/code]
第一次登录 输入错误的用户名密码 能成功跳转回去 提示错误。
登录成功之后，再一次返回login.jsp界面 输入一个错误的用户名密码 这时报错：[code="java"]
java.lang.IllegalStateException: Cannot forward after response has been committed
[/code]
但是输入正确的用户名密码却一切正常。起初我以为是成功后跳转使用了response 但是我改成了request之后也一样出错？

Answer 1

[quote]
private void login(HttpServletRequest req, HttpServletResponse resp) throws SQLException, IOException, ServletException {
// TODO Auto-generated method stub
String username=req.getParameter("username");
String password=req.getParameter("password");
String captcha=req.getParameter("captcha");
String captcha1=(String)req.getSession().getAttribute("captcha");
if(!captcha.equals(captcha1)){
req.setAttribute("error","验证码错误");
req.getRequestDispatcher("/security/login.jsp").forward(req, resp);
//////////////////////////////
return; //登入出错就返回,不能让其继续运行
///////////////////////////////
}
User user=userDao.login(username,password);
if(user!=null){
user.setLast_login(new Date());
userDao.update(user);
req.getSession().setAttribute("user", user);
req.getRequestDispatcher("index.jsp").forward(req, resp);
}else{
req.setAttribute("error", "用户名或密码错误");
req.getRequestDispatcher("/security/login.jsp").forward(req,resp);
}
}

[/quote]