Servlet 重定向，请求转发，多个Servlet间的数据共享(ServletContext、Cookie、HttpSession和HttpServletRequest)

请求数据包格式：

响应数据包格式：

多个servlet之间的调用规则

重定向解决方案

关于重定向的缺点：对用户来说虽然只需要手动访问一次服务端，剩下的由浏览器自动完成挺好的，但如果浏览器距离服务器很远，此时往返时间过长会影响用户体验。

请求转发

OneServlet:

public class OneServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("逻辑1");// 通过当前请求对象生成资源文件申请报告对象RequestDispatcher dispatcher = req.getRequestDispatcher("/two");// 将报告对象发送给Tomcatdispatcher.forward(req, resp);}
}

TwoServlet:

public class TwoServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("逻辑2");}
}

xml配置文件：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"version="4.0"><servlet><servlet-name>OneServlet</servlet-name><servlet-class>com.atjx.controller.OneServlet</servlet-class></servlet><servlet><servlet-name>TwoServlet</servlet-name><servlet-class>com.atjx.controller.TwoServlet</servlet-class></servlet><servlet-mapping><servlet-name>OneServlet</servlet-name><url-pattern>/one</url-pattern></servlet-mapping><servlet-mapping><servlet-name>TwoServlet</servlet-name><url-pattern>/two</url-pattern></servlet-mapping>
</web-app>

在浏览器输入：
控制台：

请求转发和重定向的异同

相同点：

都是多个servlet协同完成工作

不同点：

重定向方案中，浏览器要向Tomcat请求多次，而在请求转发方案中，浏览器只请求一次；
重定向设置的location可以是网站内部资源或网站外部资源，而请求转发只能申请网站内部资源(因为请求转发是请求对象代理浏览器向Tomcat发送请求，Tomcat只对本网站的资源有管理权限)；
重定向在代码上实现是：resp.sendRedirect(“myWeb/two”)，即网站名/资源名；而请求转发是：RequestDispatcher dispatcher = req.getRequestDispatcher("/two");dispatcher.forward(req, resp);即/资源名，不用写网站名；
重定向方式是通过响应包的302状态码，进而在浏览器地址栏向服务端请求的资源，因此请求方式是GET，而在请求转发方式中，浏览器只发送一个请求数据包，所有的servlet共享此请求，因此请求方式要看这个请求包的方式。

多个Servlet间的数据共享

ServletContext

全局作用域对象其实占用的是服务器的一块内存，因此向里面插入数据时要小心别爆了。

测试代码，Application1 ：

public class Application1 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {// 通过请求对象向Tomcat索要当前网站的全局作用域对象，通常用application表示全局作用域对象ServletContext application = req.getServletContext();// 将数据添加到全局作用域对象，作为共享数据application.setAttribute("key1", "hello!");}
}

Application2：

public class Application2 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {// 通过请求对象向Tomcat索要当前网站的全局作用域对象，通常用application表示全局作用域对象ServletContext application = req.getServletContext();// 从全局作用域对象得到指定关键字对应的值String res = (String) application.getAttribute("key1");System.out.println(res);}
}

Cookie

Cookie的一个练习，模拟会员卡功能

index.html：

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>注册</title>
</head>
<body><font style="color: red;font-size: 40px">会员办理</font><form action="/myWeb/one"><table border="2"><tr><td>用户名</td><td><input type="text" name="userName"></td></tr><tr><td>预存金额</td><td><input type="text" name="money"></td></tr><tr><td><input type="submit" value="申请办理"></td><td><input type="reset"></td></tr></table></form>
</body>
</html>

OneServlet:

public class OneServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String userName, money;userName = req.getParameter("userName");money = req.getParameter("money");// 设置cookieCookie cookie1 = new Cookie("userName", userName);Cookie cookie2 = new Cookie("money", money);resp.addCookie(cookie1);resp.addCookie(cookie2);// 将会员卡办理后的页面返回给浏览器resp.sendRedirect("/myWeb/index_2.html");
//        req.getRequestDispatcher("/index_2.html").forward(req, resp);}
}

index_2.html:

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>欢迎</title>
</head>
<body>
<font style="color: red;font-size: 40px">会员办理</font><form action="/myWeb/two">食物类型：<input type="radio" name="food" value="饺子"/> 饺子(30元)<input type="radio" name="food" value="面条"/> 面条(20元)<input type="radio" name="food" value="盖饭"/> 盖饭(10元)<input type="submit" value="确认"></form>
</body>
</html>

TwoServlet:

public class TwoServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {int jiaozi = 30;int miantiao = 20;int gaifan = 10;int money = 0;int consume = 0;String userName = null;String food;resp.setContentType("text/html;charset=utf-8");PrintWriter writer = resp.getWriter();Cookie[] cookies = null;Cookie card = null;food = req.getParameter("food");// 读取cookiecookies = req.getCookies();for (Cookie c: cookies) {String key = c.getName();String val = c.getValue();System.out.println(key + " " + val);if ("userName".equals(key)) {userName = key;} else if ("money".equals(key)) {money = Integer.parseInt(val);if ("饺子".equals(food)) {if (jiaozi > money) {writer.print(userName + "饺子余额不足");} else {card = new Cookie("money", "" + (money - jiaozi));consume = jiaozi;}} else if ("面条".equals(food)) {if (miantiao > money) {writer.print(userName + "面条余额不足");} else {card = new Cookie("money", "" + (money - miantiao));consume = miantiao;}} else if ("盖饭".equals(food)) {if (gaifan > money) {writer.print(userName + "盖饭余额不足");} else {card = new Cookie("money", "" + (money - gaifan));consume = gaifan;}}}}// 将cookie返给浏览器resp.addCookie(card);writer.print(userName + " 消费" + consume);}
}

先访问index.html，填完信息提交到OneServlet，然后会设置好cookie：

点击面条后确认可以看到，request带有OneServlet设置好的cookie：

而TwoServlet也重新设置了cookie：

详细看视频：https://www.bilibili.com/video/BV1y5411p7kb?p=25&spm_id_from=pageDriver

Cookie生命周期

如果你在doGet里面设置了存活时间，那么在存活时间结束之前每次访问这个doGet方法都会刷新存活时间。

HttpSession

模拟购物车功能

index.html:

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>首页</title>
</head>
<body><table border="2" align="center"><tr><td>商品名称</td><td>商品单件</td><td>供货商</td><td>放入购物车</td></tr><tr><td>华为nova5 pro</td><td>3999</td><td>华为</td><td><a href="/myWeb/one?goodsName=huawei">放入购物车</a></td></tr><tr><td>苹果</td><td>5</td><td>山东</td><td><a href="/myWeb/one?goodsName=apple">放入购物车</a></td></tr><tr><td>耳机</td><td>1399</td><td>索尼</td><td><a href="/myWeb/one?goodsName=headphone">放入购物车</a></td></tr><tr><td colspan="4"><a href="/myWeb/two">查看购物车</a></td></tr></table>
</body>
</html>

OneServlet:

public class OneServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String name;// 调用请求对象获得用户所选商品名称name = req.getParameter("goodsName");HttpSession session = req.getSession();Integer goodsNum = (Integer) session.getAttribute(name);if (goodsNum == null) {session.setAttribute(name, 1);}else {session.setAttribute(name, goodsNum+1);}}
}

TwoServlet:

public class TwoServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {// 获取当前用户的sessionHttpSession session = req.getSession();// 如果session中数据很多，这时用getAttribute方法一个一个取就太麻烦了，将// 所有的key都拿出来更好Enumeration<String> goodsNames = session.getAttributeNames();while (goodsNames.hasMoreElements()) {String name = goodsNames.nextElement();int goodsNum = (int) session.getAttribute(name);System.out.println("商品名称：" + name + " 商品数量：" + goodsNum);}}
}

Http服务器如何识别不同的浏览器？

getSession的参数

生命周期

HttpServletRequest