Java Servlet学习笔记——5.Cookies

写在前面

这一部分学习Cookies。学了几天，感觉效率还是不太高，应该是拘泥于太多细节了。有一些东西现在看不太懂，但是后面慢慢就应该自然而然懂了。但是这样最起码没坏处，没坏处就一直做下去。

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。

Cookies的创建

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);

例如，创建一个语言属性的Cookies，示例代码如下：

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。

Cookies的发送与读取

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。

获取与删除特定Cookies

获取特定Cookies需要遍历一遍getCookies方法获得的Cookies数组，例如，查询名为maxRecords的cookies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;
        }
    }
}

要直接删除Cookies也还没有此类方法，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);

实例代码

PreferenceServlet类向浏览器输出一个表单，用户提交表单时，Web服务器会会调用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。

表单上方有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，分别对应后面的CookiesClassServlet与CookiesInfoServlet类。

CookieClassServlet 和CookieInfoServlet 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。

可通过URL调用PreferenceServlet。

PreferenceServlet

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);

        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>");
    }
}

CookieClassServlet

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

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。

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

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>");
    }
}