613
《Servlet、JSP和Spring MVC初学指南》——2.3 Cookies
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着，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> "
+ "<a href='cookieInfo'>Cookie Info</a> "
+ "<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。
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：
https://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：
https://localhost:8080/app02a/cookieClass
可以通过如下URL来访问CookieInfoServlet：
https://localhost:8080/app02a/cookieInfo
图2.7和图2.8分别展示了CookieClassServlet和CookieInfoServlet的显示界面。
图2.7 CookieClassServlet输出
图2.8 CookieInfoServlet输出
最后更新：2017-05-27 18:01:30