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2.3 Cookies

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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HttpServletResponse两个接口。

创建Cookies

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： 

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Cookie cookie = new Cookie(name, value);

如下是一个创建语言选择的cookie示例：

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Cookie languageSelectionCookie = new Cookie("language", "Italian");

Cookie属性

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。

服务器如何发送Cookie

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

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httpServletResponse.addCookie(cookie);

浏览器如何发送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。

服务器端如何读取浏览器提交的Cookie

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.

查找Cookie

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：

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//获取浏览器提交的所有Cookie数组
Cookie[] cookies = request.getCookies();
//引用,用来保存你想要得到的某个cookie对象
Cookie maxRecordsCookie = null;
if (cookies != null) { 
	for (Cookie cookie : cookies) 
	{
		//查找名称为maxRecords的cookie对象
		if (cookie.getName().equals("maxRecords")) 
		{
			//保存找到的这个cookie对象
			maxRecordsCookie = cookie; 
			break;
		}
	}
}

目前，还没有类似于getCookieByName这样的方法来帮助简化工作。

删除Cookie

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,覆盖掉原来的Cookie即可。如下为删除一个名为userName的cookie代码：

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

demo

工具类

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package session.management.cookies;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
public class CookieTools
{
    /**
     * 从浏览器提交的Cookie中找到名称为name
	 * 的Cookie对象.
     * 
     * @param request
     *            请求.
     * @param name
     *            要查找的Cookie的名称.
     * @return Cookie对象,如果找到该对象,
	 *				则返回该对象,没找到则返回null.
     */
    public static Cookie findCookieByName(
		HttpServletRequest request,String name)
    {
        Cookie cookies[] = request.getCookies();
        for (Cookie cookie : cookies) {
            if (cookie.getName().equals(name)) {
                return cookie;
            }
        }
        return null;
    }
}

设置Cookie的Servlet

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package session.management.cookies;
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 = "SetCookieByForm",
    urlPatterns =
    {"/setcookie"}
)
public class SetCookieByForm extends HttpServlet
{
    private static final long serialVersionUID = 1L;
    // 设置一个菜单栏
    public static final String MENU = 
        "<div style='background:#e8e8e8;"
        + "padding:15px'>" 
        + "<a href='setcookie'>Set Cookie</a>&nbsp;&nbsp;"
        + "<a href='showfruits'>Show Fruits</a>&nbsp;&nbsp;"
        + "<a href='showcss'>Show CSS</a>&nbsp;&nbsp;" 
        + "</div>";
    @Override
    protected void doGet(HttpServletRequest req,
        HttpServletResponse resp)
            throws ServletException, IOException
    {
        // 以UTF-8编码响应
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        PrintWriter writer = resp.getWriter();
        writer.print("<html>\r\n" 
            + "<head>\r\n"
            + "<title>Preference</title>\r\n"
            + "</head>\r\n"
            + "<body>"
            + MENU
            + "<form method=\"post\">\r\n"
            + "<hr>\r\n"
            + "<table>\r\n"
            + "<tbody>\r\n"
            + "<tr>\r\n"
            + "<td>设置字体大小: </td>\r\n"
            + "<td>\r\n"
            + "<select name=\"cookieFontSize\">\r\n"
            + "<option>large</option>\r\n"
            + "<option>x-large</option>\r\n"
            + "<option>xx-large</option>\r\n"
            + "</select>\r\n"
            + "</td>\r\n"
            + "</tr>\r\n"
            + "</table><hr><table>\r\n"
            + "<tbody>\r\n"
            + "<tr>\r\n"
            + "<td>显示的水果数量:</td>\r\n"
            + "<td>\r\n"
            + "<select name=\"fruitNum\">\r\n");
        for (int i = 0; 
                i < ShowFruitsByCookie.fruits.length;
                    i++) 
        {
            writer.print(
                "<option>" + (i + 1) 
                + "</option>\r\n");
        }
        writer.print(
            "</select>\r\n"
            + "</td>\r\n"
            + "</tr>\r\n"
            + "<tr>\r\n"
            + "<td rowspan=\"2\">\r\n"
            + "<input type=\"submit\" value=\"Set\">\r\n"
            + "</td>\r\n"
            + "</tr>\r\n"
            + "</tbody>\r\n"
            + "</table>\r\n"
            + "</form>\r\n"
            + "</body>\r\n"
            + "</html>");
    }
    @Override
    protected void doPost(HttpServletRequest request,
        HttpServletResponse response)
            throws ServletException, IOException
    {
        // 以utf-8发送响应
        response.setCharacterEncoding("utf-8");
        // 读取表单提交的值
        String fruitNum =
            request.getParameter("fruitNum");
        // 创建一个Cookie,设置为用户在表单中输入的值,
        // 然后发送给浏览器
        response.addCookie(
            new Cookie("fruitNum", fruitNum));
        // 获取表单提交的值
        String cookieFontSize =
            request.getParameter("cookieFontSize");
        // 存储到Cookie中.
        response.addCookie(
            new Cookie("cookieFontSize", cookieFontSize));
        // 响应post请求
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.println(
            "<html><head>"
            + "<title>Preference</title>"
            + "</head><body>"
            + MENU
            + "<ul><li>你想要显示 <strong>"
            + fruitNum
            + "</strong> 种水果。</li>"
            + "<li>你想要设置CSS选择器<strong>"
            +".cookieStyle</strong>的字体大小为:<strong>"
            + cookieFontSize
            + "</strong></li></ul></body></html>");
    }
}

这个Servlet会把表单输入转换为Cookie发送到浏览器,对应的URL为http://localhost:8080/SessionManagement/setcookie,
当浏览器再次访问http://localhost:8080/SessionManagement/这个Java项目下的资源时,浏览器会把之前服务器发给他的Cookie,再原封不动的提交给服务器。这样服务器就可以根据Cookie来进行响应的操作。
在上面代码中,服务器分别发送fruitNum和cookieFontSize这两个cookie给浏览器,下面将再创建两个Servlet来使用者连个Cookie。
填写表单,如下所示:
这里有一张图片
然后点击Set按钮,提交表单,得到提示如下:
这里有一张图片

根据Cookie显示数组中的元素

ShowFruitsByCookie这个Servlet会根据fruitNum这个Cookie的值,来显示打印数组中的元素,如下所示:

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package session.management.cookies;
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 = "ShowFruitByCookie",
    urlPatterns =
    {"/showfruits"}
)
public class ShowFruitsByCookie extends HttpServlet
{
    private static final long serialVersionUID =
        7275643908439277071L;
    static String[] fruits = {
            "苹果", "葡萄", "芒果", "草莓",
            "荔枝", "香蕉", "乌梅", "菠萝",
            "樱桃", "枇杷", "芒果", "橙子",
            "桔子", "柠檬", "柚子", "杨梅", 
            "木瓜", "桂圆", "榴莲", "西瓜",
            "石榴", "山楂", "椰子", "山竹",
            "橄榄", "柿子","李子", "桑葚", 
            "红枣", "柑橘", "话梅", "乌梅", 
            "红提", "桃子", "甜瓜", "香瓜",
            "荸荠","龙眼", "沙果", "佛手",
            "芭乐", "蓝莓", "西梅", "释迦", 
            "黄皮", "莲雾", "杏子", "槟榔",
            "酸橙", "黑莓", "栗子", "板栗",
            "金桔", "山梨", "毛桃", "白果", 
            "银杏", "青梅", "蜜桃","脐橙",
            "沙枣", "凤梨", "椰枣", "油桃", 
            "鳄梨", "酸莓", "蛇果", "鸭梨"};
    @Override
    protected void doGet(HttpServletRequest request,
        HttpServletResponse response) 
            throws ServletException, IOException
    {
        Cookie fruitNumCookie = 
            CookieTools.findCookieByName(request, "fruitNum");
        if (fruitNumCookie != null) {
            // 取出Cookie中的值.
            String cookieValue = fruitNumCookie.getValue();
            int fruitNum = Integer.parseInt(cookieValue);
            if (fruitNum <= fruits.length) {
                response.setCharacterEncoding("utf-8");
                response.setContentType("text/html");
                PrintWriter writer = response.getWriter();
                writer.print(
                    "<html>\r\n"
                    + "<head>\r\n"
                    + "<title>showFruits</title>\r\n"
                    + "</head>\r\n"
                    + "<body>\r\n"
                    + SetCookieByForm.MENU
                    + "为你显示 <strong>"
                    + fruitNum
                    + "</strong> 种水果,如下所示:\r\n"
                    + "<ul>");
                for (int i = 0; i < fruitNum; i++) {
                    writer.print("<li>" + fruits[i] + "</li>");
                }
                writer.println("</ul>\r\n" 
                    + "</body>\r\n" + "</html>");
            }
        }
    }
}

点击工具条上的Show Fruits超链接,效果如下:
这里有一张图片

根据Cookie设置CSS

SetCSSByCookie这个Servlet根据cookieFontSize这个Cookie中的值来来设置元素的CSS样式,如下所示:

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package session.management.cookies;
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 = "SetCSSByCookie",
    urlPatterns =
    {"/showcss"}
)
public class SetCSSByCookie extends HttpServlet
{
    private static final long serialVersionUID =
        -8190275536448562340L;
    @Override
    protected void doGet(HttpServletRequest request,
        HttpServletResponse response)
            throws ServletException, IOException
    {
        response.setCharacterEncoding("utf-8");
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        Cookie cookieFontSize =
            CookieTools.findCookieByName(request,
                "cookieFontSize");
        writer.print(
            "<html>\r\n"
            + "<head>\r\n"
            + "<title>根据Cookie设置CSS</title>\r\n"
            + "<style>");
        writer.print(".cookieStyle{");
        if (cookieFontSize != null) {
            writer.print("font-size: "
                + cookieFontSize.getValue() + ";");
        }
        writer.print("}");
        writer.println(
            "</style>\r\n"
            + "</head>\r\n"
            + "<body>"
            + SetCookieByForm.MENU);
        writer.println(
            "<div class=\"cookieStyle\">这句话的样式通过Cookie设置.</div>");
        writer.println("</body>\r\n"
                + "</html>");
    }
}

点击工具条上的Show CSS超链接,显示效果如下:
这里有一张图片

原文链接: 2.3 Cookies