急！求高手解决 servlet异常:Cannot forward after response has been committed

Question

servlet代码：
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
HttpSession session = request.getSession();
try {
if(username != null && password != null){
if("admin".equals(username) && "123".equals(password)){
session.setAttribute("username", username);
session.setAttribute("password", password);
request.getRequestDispatcher("/main.jsp").forward(request, response);//报错语句
return;
}else{
request.getRequestDispatcher("/index.jsp").forward(request, response);
return;
}
}
} catch (Exception e) {
logger.error("login err in servlet",e);
}
}

Answer 1

request.getRequestDispatcher("/main.jsp").forward(request, response);//报错语句
return;这个return是干什么的？？