[Java][Servlet] Cannot call sendRedirect() after the response has been committed

做一個Login Demo的時候，寫了如下代碼：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    super.doPost(request, response);
    
    String userName = request.getParameter("userName");
    String password = request.getParameter("password");

    if (userName.equals("test")) {
        response.sendRedirect(request.getContextPath() + "/success/success.jsp");
    } else {
        response.sendRedirect(request.getContextPath() + "/error/login-failed.jsp");
    }
}

運行程序時報錯了Cannot call sendRedirect() after the response has been committed，意思已經很明確了，就是說不能在一個response做了提交之后再重定向，也就是說再重定向之前不要去改變response，包括如下部分：

• 修改response中參數的值，cookie,session 等
• 重定向

請注意在做doPost(..)的時候我這里調用了super.doPost(..)，而super.doPost(..)做了什么呢？

protected void doPost(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {

    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_post_not_supported");
    if (protocol.endsWith("1.1")) {
        resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
    } else {
        resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
    }
}

response被修改了，我們注釋掉super.doPost(..)，程序正常運行。