Question

我创建了两个servlet：

UserReceiverServlet从表单接收用户名。然后，它将用户名设置为属性，并将请求转发给UserDisplayServlet。

UserDisplayServlet会将用户名添加到cookie中，然后显示已设置的属性和存储的cookie的当前大陆。

然而，虽然我已经设法确定cookie肯定是在创建并且值正在存储，但是当这个Servlet去寻找它时，它找不到它

这是UserDisplayServlet。它通过request.getAttribute（“username”）正确接收值，所以attributeUsername不是问题......

/**
 * 
 */
package hu.flux.user;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.http.*;

/**
 * @author Brian Kessler
 *
 */
public class UserDisplayServlet extends HttpServlet {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    public void service (HttpServletRequest request, HttpServletResponse response)
    {
        response.setContentType("text/html");

        String attributeUsername = (String) request.getAttribute("username"); 

        String cookieUsername = null;
    for (Cookie cookie: request.getCookies())
        { if (cookie.getName() == "username") 
            { 
                String value = cookie.getValue();
                if (value.isEmpty()) { value = "--- EMPTY ---"; }
                cookieUsername = cookie.getValue(); 
                break; 
            } 
        }
    if (cookieUsername == null) {cookieUsername = "--- NOT FOUND ---"; }

    String newCookieValue;
    if (!(cookieUsername.isEmpty())) { newCookieValue = cookieUsername;}
    else if (!(attributeUsername.isEmpty())) { newCookieValue = cookieUsername;}
    else { newCookieValue = "UNKOWN";}

        Cookie usernameCookie = new Cookie ("username", attributeUsername);
        usernameCookie.setMaxAge(24*60*60);
        response.addCookie(usernameCookie);

        PrintWriter out = null;
        try { out = response.getWriter(); } 
        catch (IOException e) 
        { 
            System.err.print ("Cannot getWriter():" + e);
            e.printStackTrace();
        }


        out.println("<html>");
        out.println("<head>");
            out.println("<title>User Display Servlet</title>");
        out.println("</head>");
        out.println("<body");

        if ((attributeUsername != null) && (!(attributeUsername.isEmpty())))
        {
            out.println ("<p>I have an attribute which says your username is " 
                    + request.getAttribute("username") + "</p>");
        }
        else { out.println ("<p>The attribute hasn't been set.</p>"); }


        if ((cookieUsername != null) && (!(cookieUsername.isEmpty())))
        {
            out.println ("<p>You have a cookie which says your username is " + cookieUsername + "</p>");
            out.println ("</body></html>");
        }
        else { out.println ("<p>The cookie hasn't been set.</p>"); }


    }


}

如果它有用，这里是接收输入并将其转发到上述servlet的servlet：

/**
 * 
 */
package hu.flux.user;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.*;

/**
 * This simple servlet receives a username from a form,
 * stores the name in a context attribute,
 * and forwards the context attribute to another servlet
 * which displays the name.
 *  
 * @author Brian Kessler
 *
 */
public class UserReceiverServlet extends HttpServlet 
{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    public void service (HttpServletRequest request, HttpServletResponse response)
    {
        String username = request.getParameter("username");
        request.setAttribute("username", username);

        try {

            getServletConfig()
                .getServletContext()
                .getRequestDispatcher("/userdisplay")
                .forward(request, response);
        } catch (ServletException e) {
            System.err.println ("Can't forward: " + e);
            e.printStackTrace();
        } catch (IOException e) {
            System.err.println ("Can't forward: " + e);
            e.printStackTrace();
        }


    }
}

我缺少什么来检索存储在cookie中的值？

Answer 1

发现问题：

if (cookie.getName() == "username")

应该是：

if (name.equals("username"))

总是绊倒我！

Answer 2

你怎么知道还没有设置cookie？ HTML正文肯定不会告诉你，因为内容是在服务器端而不是客户端评估的。如果您真的希望页面告诉您是否已成功设置cookie，您需要在HTML中嵌入一些javascript来访问浏览器的cookie存储区。

如果我试图解决这个问题，我首先要查看服务器和客户端的HTTP响应。在服务器端启用webapp容器支持的响应转储/日志记录;例如对于Tomcat，打开RequestDumperValve。在客户端，使用浏览器的首选项对话框（或其他）查看您正在与之交互的服务器的cookie存储的内容。然后使用浏览器的Javascript调试器（或其他）查看响应头是否包含用于设置cookie的标头。（如果您使用的是firefox，请安装firebug插件。）

为什么我的Java HttpServlet不能找到我的cookie值？

2 个答案: