如何从servlet发出消息并在jsp中显示 [英] how to message from servlet and display in jsp

问题描述

我正在尝试做一些看起来很小但失败的事情.登录失败但无法登录时，我正在尝试将响应消息发送回jsp.到目前为止，我只能重定向回jsp，但不能在其上显示来自servlet的消息.这是重定向的servlet部分:

I'm trying to do something that looks small but it's failing. I'm trying to send a response message back to a jsp when login fails but not being able. As of now I can only redirect back to the jsp but cannot display a message from the servlet on it. This is the servlet part of the redirection:

                if (count > 0) {
                res.sendRedirect("adminHome.jsp");
            } else {

                res.sendRedirect("index.jsp");
            }

我尝试使用PrintWriter和重定向来打印消息，但是失败了，因为我无法在JSP中获得如何接收消息的方法.我还读到我不应该重定向，而应该从servlet转发.我怎样才能做到这一点?请帮助从Servlet转发的代码补丁以及在JSP中接收的代码.谢谢

I tried to print a message using PrintWriter and the redirect but failed because I couldn't get how to receive the message in the JSP. I also read that I shouldn't redirect but rather I should just forward from the servlet. How can I do this? Please help with the code patch to forward from servlet as well as that one to receive in JSP. Thanks

推荐答案

如果您坚持使用重定向而不是转发，那么您有2个选择:

If you insist to use redirect instead of forward, then you have 2 options:

1. 将消息作为请求参数传递

1. Pass the message as request parameter

String message = "hello";
res.sendRedirect("adminHome.jsp?message=" + URLEncoder.encode(message, "UTF-8"));

以便您可以在JSP中将其显示如下

so that you can display it in JSP as follows

<p>Message: ${param.message}</p>

它也仅在浏览器地址栏中可见，并且您不能以这种方式传递非标准Java对象.

It's only visible in the browser address bar as well and you aren't able to pass non-standard Java objects this way.

将其存储在会话中

String message = "hello";
req.getSession().setAttribute("message", message);
res.sendRedirect("adminHome.jsp");

，以便您可以在JSP中显示(并删除)它，如下所示:

so that you can display (and remove) it in JSP as follows:

<p>Message: ${message}</p>
<c:remove var="message" scope="session" /> 

删除非常重要，否则整个会话都将粘贴在那里.

Removing is important, otherwise it sticks there for the entire session.

---

但是，如果您愿意使用转发而不是重定向，那么它会更优雅:

---

However, if you're open to using forward instead of redirect, it's more elegant:

String message = "hello";
req.setAttribute("message", message);
req.getRequestDispatcher("/adminHome.jsp").forward(req, res);

并在JSP中如下显示

    <p>Message: ${message}</p>

另请参见:

• 我们的Servlets Wiki页面-包含一个Hello World，它也可以处理消息传递
• EL Wiki页面-解释所有有关${}的事情.

See also:

• Our Servlets wiki page - contains a Hello World which also treats messaging
• EL wiki page - explains all about those ${} things.

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