《Servlet、JSP和Spring MVC初学指南》——2.3　Cookies-阿里云开发者社区

本节书摘来自异步社区《Servlet、JSP和Spring MVC初学指南》一书中的第2章，第2.3节，作者：【加】Budi Kurniawan（克尼亚万） , 【美】Paul Deck著，更多章节内容可以访问云栖社区“异步社区”公众号查看

2.3　Cookies

URL重写和隐藏域仅适合保存无须跨越太多页面的信息。如果需要在多个页面间传递信息，则以上两种技术实现成本高昂，因为你不得不在每个页面都进行相应处理。幸运的是，Cookies技术可以帮助我们。

Cookies是一个很少的信息片段，可自动地在浏览器和Web服务器间交互，因此cookies可存储在多个页面间传递的信息。Cookie作为HTTP header的一部分，其传输由HTTP协议控制。此外，你可以控制cookies的有效时间。浏览器通常支持每个网站高达20个cookies。

Cookies的问题在于用户可以通过改变其浏览器设置来拒绝接受cookies。

要使用cookies，需要熟悉javax.servlet.http.Cookie类以及HttpServletRequest和HttpServlet Response两个接口。

可以通过传递name和value两个参数给Cookie 类的构造函数来创建一个cookies：

Cookie cookie = new Cookie(name, value);
如下是一个创建语言选择的cookie示例：

Cookie languageSelectionCookie = new Cookie("language", "Italian");
创建完一个Cookie对象后，你可以设置domain、path和maxAge属性。其中，maxAge 属性决定cookie何时过期。

要将cookie发送到浏览器，需要调用HttpServletResponse的add方法：

httpServletResponse.addCookie(cookie);
浏览器在访问同一Web服务器时，会将之前收到的cookie一并发送。

此外，Cookies也可以通过客户端的javascript脚本创建和删除，不过这些不在本书范围内。

服务端若要读取浏览器提交的cookie，可以通过HttpServletRequest接口的getCookies方法，该方法返回一个Cookie数组，若没有cookies则返回null。你需要遍历整个数组来查询某个特定名称的cookie。如下为查询名为maxRecords的cookie的示例：

Cookie[] cookies = request.getCookies();
Cookie maxRecordsCookie = null;
if (cookies != null) {
    for (Cookie cookie : cookies) {
        if (cookie.getName().equals("maxRecords")) {
            maxRecordsCookie = cookie;
            break;
        }
    }
}

目前，还没有类似于getCookieByName这样的方法来帮助简化工作。此外，也没有一个直接的方法来删除一个cookie，你只能创建一个同名的cookie，并将maxAge属性设置为0，并添加到HttpServletResponse接口中。如下为删除一个名为userName的cookie代码：

Cookie cookie = new Cookie("userName", "");
cookie.setMaxAge(0);
response.addCookie(cookie);

清单2.4的PreferenceServlet 类展示了如何通过cookies来进行会话管理，该Servlet允许用户通过修改四个cookie值来设定显示配置。

清单2.4　PreferenceServlet类

package app02a.cookie;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name = "PreferenceServlet",  urlPatterns = { "/preference" })
public class PreferenceServlet extends HttpServlet {
    private static final long serialVersionUID = 888L;
    public static final String MENU =
            "<div style='background:#e8e8e8;"
            + "padding:15px'>"
            + "<a href='cookieClass'>Cookie Class</a>&nbsp;&nbsp;"
            + "<a href='cookieInfo'>Cookie Info</a>&nbsp;&nbsp;"
            + "<a href='preference'>Preference</a>" + "</div>";

    @Override
    public void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException,
            IOException {
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.print("<html><head>" + "<title>Preference</title>"
                + "<style>table {" + "font-size:small;"
                + "background:NavajoWhite }</style>"
                + "</head><body>"
                + MENU
                + "Please select the values below:"
                + "<form method='post'>"
                + "<table>"
                + "<tr><td>Title Font Size: </td>"
                + "<td><select name='titleFontSize'>"
                + "<option>large</option>"
                + "<option>x-large</option>"
                + "<option>xx-large</option>"
                + "</select></td>"
                + "</tr>"
                + "<tr><td>Title Style & Weight: </td>"
                +"<td><select name='titleStyleAndWeight' multiple>"
                + "<option>italic</option>"
                + "<option>bold</option>"
                + "</select></td>"
                + "</tr>"
                + "<tr><td>Max. Records in Table: </td>"
                + "<td><select name='maxRecords'>"
                + "<option>5</option>"
                + "<option>10</option>"
                + "</select></td>"
                + "</tr>"
                + "<tr><td rowspan='2'>"
                + "<input type='submit' value='Set'/></td>"
                + "</tr>"
                + "</table>" + "</form>" + "</body></html>");
    }

    @Override
    public void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException,
            IOException {

        String maxRecords = request.getParameter("maxRecords");
        String[] titleStyleAndWeight = request
                .getParameterValues("titleStyleAndWeight");
        String titleFontSize =
                request.getParameter("titleFontSize");
        response.addCookie(new Cookie("maxRecords", maxRecords));
        response.addCookie(new Cookie("titleFontSize",
                titleFontSize));

        // delete titleFontWeight and titleFontStyle cookies first
        // Delete cookie by adding a cookie with the maxAge = 0;
        Cookie cookie = new Cookie("titleFontWeight", "");
        cookie.setMaxAge(0);
        response.addCookie(cookie);

        cookie = new Cookie("titleFontStyle", "");
        cookie.setMaxAge(0);
        response.addCookie(cookie);

        if (titleStyleAndWeight != null) {
            for (String style : titleStyleAndWeight) {
                if (style.equals("bold")) {
                    response.addCookie(new
                            Cookie("titleFontWeight", "bold"));
                } else if (style.equals("italic")) {
                    response.addCookie(new Cookie("titleFontStyle",
                            "italic"));
                }
            }
        }

        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.println("<html><head>" + "<title>Preference</title>"
                + "</head><body>" + MENU
                + "Your preference has been set."
                + "<br/><br/>Max. Records in Table: " + maxRecords
                + "<br/>Title Font Size: " + titleFontSize
                + "<br/>Title Font Style & Weight: ");

        // titleStyleAndWeight will be null if none of the options
        // was selected
        if (titleStyleAndWeight != null) {
            writer.println("<ul>");
            for (String style : titleStyleAndWeight) {
                writer.print("<li>" + style + "</li>");
            }
            writer.println("</ul>");
        }
        writer.println("</body></html>");
    }
}

PreferenceServlet的doGet方法展示一个包含多个输入项的表单，如图2.6所示。

screenshot

图2.6　通过cookies来管理用户偏好

表单上部有3个链接：Cookie Class、Cookie Info和Preference。它们可以导航到本应用的其他Servlet上。关于Cookie Class和Cookie Info，我们稍后介绍。

当用户提交表单时，Web服务器会调用PreferenceServlet的doPost方法，该方法创建4个cookies，即maxRecords、titleFontSize、titleFontStyle和titleFontWeight，并覆盖该cookie之前的值，然后将用户输入的值返回给浏览器。

可以通过如下URL访问PreferenceServlet：

http://localhost:8080/app02a/preference
CookieClassServlet（见清单2.5）和CookieInfoServlet（见清单2.6）各自应用这些cookie来格式化其内容。CookieClassServlet将Cookie的属性展示为一个HTML列表。

Cookie中的max Records值决定显示多少个列表项，可通过Preference Servlet调整该值。

清单2.5　CookieClassServlet类

package app02a.cookie;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name = "CookieClassServlet",
        urlPatterns = { "/cookieClass" })
public class CookieClassServlet extends HttpServlet {
    private static final long serialVersionUID = 837369L;

    private String[] methods = {
            "clone", "getComment", "getDomain",
            "getMaxAge", "getName", "getPath",
            "getSecure", "getValue", "getVersion",
            "isHttpOnly", "setComment", "setDomain",
            "setHttpOnly", "setMaxAge", "setPath",
            "setSecure", "setValue", "setVersion"
    };

    @Override
    public void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException,
            IOException {

        Cookie[] cookies = request.getCookies();
        Cookie maxRecordsCookie = null;
        if (cookies != null) {
            for (Cookie cookie : cookies) {
                if (cookie.getName().equals("maxRecords")) {
                    maxRecordsCookie = cookie;
                    break;
                }
            }
        }

        int maxRecords = 5; // default
        if (maxRecordsCookie != null) {
            try {
                maxRecords = Integer.parseInt(
                        maxRecordsCookie.getValue());
            } catch (NumberFormatException e) {
                // do nothing, use maxRecords default value
            }
        }

        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.print("<html><head>" + "<title>Cookie Class</title>"
                + "</head><body>"
                + PreferenceServlet.MENU
                + "<div>Here are some of the methods in " +
                           "javax.servlet.http.Cookie");
        writer.print("<ul>");

        for (int i = 0; i < maxRecords; i++) {
            writer.print("<li>" + methods[i] + "</li>");
        }
        writer.print("</ul>");
        writer.print("</div></body></html>");
    }
}

CookieInfoServlet 类读取titleFontSize、titleFontWeight和titleFontStyle 三个cookie值，并写入到如下发给浏览器的CSS中，其中x、y和z分别为如上所提的cookie。

.title {
    font-size: x;
    font-weight: y;
    font-style: z;
}

该style应用在一个div元素中，并格式化文字“Session Management with Cookies:”。

清单2.6　CookieInfoServlet类

package app02a.cookie;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(name = "CookieInfoServlet", urlPatterns = { "/cookieInfo" })
public class CookieInfoServlet extends HttpServlet {
    private static final long serialVersionUID = 3829L;

    @Override
    public void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException,
            IOException {

        Cookie[] cookies = request.getCookies();
        StringBuilder styles = new StringBuilder();
        styles.append(".title {");
        if (cookies != null) {
            for (Cookie cookie : cookies) {
                String name = cookie.getName();
                String value = cookie.getValue();
                if (name.equals("titleFontSize")) {
                    styles.append("font-size:" + value + ";");
                } else if (name.equals("titleFontWeight")) {
                    styles.append("font-weight:" + value + ";");
                } else if (name.equals("titleFontStyle")) {
                    styles.append("font-style:" + value + ";");
                }
            }
        }
        styles.append("}");
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.print("<html><head>" + "<title>Cookie Info</title>"
                + "<style>" + styles.toString() + "</style>"
                + "</head><body>" + PreferenceServlet.MENU
                + "<div class='title'>"
                + "Session Management with Cookies:</div>");
        writer.print("<div>");

        // cookies will be null if there's no cookie
        if (cookies == null) {
            writer.print("No cookie in this HTTP response.");
        } else {
            writer.println("<br/>Cookies in this HTTP response:");
            for (Cookie cookie : cookies) {
                writer.println("<br/>" + cookie.getName() + ":"
                        + cookie.getValue());
            }
        }
        writer.print("</div>");
        writer.print("</body></html>");
    }
}

可以通过如下URL来访问CookieClassServlet：

http://localhost:8080/app02a/cookieClass
可以通过如下URL来访问CookieInfoServlet：

http://localhost:8080/app02a/cookieInfo
图2.7和图2.8分别展示了CookieClassServlet和CookieInfoServlet的显示界面。

screenshot

图2.7　CookieClassServlet输出

screenshot

图2.8　CookieInfoServlet输出

《Servlet、JSP和Spring MVC初学指南》——2.3　Cookies

2.3　Cookies

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