JSP数据传递详解

第一节 JSP 内置对象

1. JSP 内置对象的概念

Web容器：能装在web应用的容器，比如我们所学的Tomcat（webapps文件夹下就能够装载我们的web应用）

JSP 内置对象是 Web 容器创建的一组对象，在页面中可以直接使用。JSP常用内置对象入下图所示：

2. 内置对象 out

2.1 out 对象的数据类型

javax.servlet.jsp.JspWriter

2.2 作用

向Web浏览器内输出信息，负责管理对客户端的输出

2.3 用法

<span style="background-color:#f8f8f8"><span style="color:#333333"><%
     <span style="color:#aa5500">//在页面上输出Hello JSP</span>
    <span style="color:#000000">out</span>.<span style="color:#000000">println</span>(<span style="color:#aa1111">"Hello JSP"</span>);
%></span></span>

3. 内置对象 request

3.1 request 对象的数据类型

javax.servlet.http.HttpServletRequest

3.2 作用

获取客户端的参数和数据流

3.3 常用方法

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//根据表单组件名称获取提交数据  </span>
<span style="color:#008855">String</span>  <span style="color:#0000ff">getParameter</span>(<span style="color:#008855">String</span>  <span style="color:#000000">name</span>);
<span style="color:#aa5500">//获取表单组件对应多个值时的请求数据</span>
<span style="color:#008855">String</span>[]  <span style="color:#0000ff">getParameterValues</span>(<span style="color:#008855">String</span> <span style="color:#000000">name</span>);
<span style="color:#aa5500">//指定请求的编码</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setCharacterEncoding</span>(<span style="color:#008855">String</span> <span style="color:#000000">charset</span>);
<span style="color:#aa5500">//返回一个RequestDispatcher对象，该对象的forward()方法用于转发请求  </span>
<span style="color:#000000">RequestDispatcher</span> <span style="color:#0000ff">getRequestDispatcher</span>(<span style="color:#008855">String</span>  <span style="color:#000000">path</span>);
<span style="color:#aa5500">//获取客户端cookie  为什么能够获取cookie，主要是浏览器无条件的会将cookie中所有的数据在发送请求的</span>
<span style="color:#aa5500">//携带上，这样，我们就可以从请求中获取cookie的信息了</span>
<span style="color:#000000">Cookie</span>[] <span style="color:#0000ff">getCookies</span>();
<span style="color:#aa5500">//获取请求中所有参数和参数值的映射  </span>
<span style="color:#000000">Map</span><span style="color:#981a1a"><</span><span style="color:#008855">String</span>,<span style="color:#008855">String</span>[]<span style="color:#981a1a">></span> <span style="color:#0000ff">getParameterMap</span>();
<span style="color:#aa5500">//获取当前会话  </span>
<span style="color:#000000">HttpSession</span> <span style="color:#0000ff">getSession</span>();</span></span>

webapp目录就是我们访问视图时的根目录，根目录默认是一条斜杠，如果还存在子目录，访问的时候直接将子目录名称加上即可。如果只是访问视图，直接使用视图名即可。

“WEB-INF”这个目录下的所有内容都是受服务器保护的，不能够直接访问，只能通过服务器处理才能访问。

3.4 案例

使用内置对象 request 完成注册信息显示

register.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#117700"><</span><span style="color:#117700">form</span> <span style="color:#0000cc">action</span>=<span style="color:#aa1111">"info.jsp"</span> <span style="color:#0000cc">method</span>=<span style="color:#aa1111">"post"</span><span style="color:#117700">></span>
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">><</span><span style="color:#117700">span</span><span style="color:#117700">></span>用户名<span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"text"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"username"</span><span style="color:#117700">></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">><</span><span style="color:#117700">span</span><span style="color:#117700">></span>密码<span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"password"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"password"</span><span style="color:#117700">></</span><span style="color:#117700">div</span><span style="color:#117700">></span> 
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>
        <span style="color:#117700"><</span><span style="color:#117700">span</span><span style="color:#117700">></span>信息来源<span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">></span>
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"checkbox"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"channel"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"报刊"</span><span style="color:#117700">></span>报刊
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"checkbox"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"channel"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"网络"</span><span style="color:#117700">></span>网络
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"checkbox"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"channel"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"朋友推荐"</span><span style="color:#117700">></span>朋友推荐
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"checkbox"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"channel"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"电视"</span><span style="color:#117700">></span> 电视
    <span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"submit"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"注册"</span><span style="color:#117700">></span>
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"reset"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"重置"</span><span style="color:#117700">></span>
    <span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
<span style="color:#117700"></</span><span style="color:#117700">form</span><span style="color:#117700">></span></span></span>

info.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%
    <span style="color:#008855">String</span> <span style="color:#000000">name</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"username"</span>);<span style="color:#aa5500">//获取参数username的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">password</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"password"</span>);<span style="color:#aa5500">//获取参数password的值</span>
    <span style="color:#008855">String</span>[] <span style="color:#000000">channels</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameterValues</span>(<span style="color:#aa1111">"channel"</span>);<span style="color:#aa5500">//获取参数channel的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">channelInfo</span> <span style="color:#981a1a">=</span> <span style="color:#000000">Arrays</span>.<span style="color:#000000">toString</span>(<span style="color:#000000">channels</span>).<span style="color:#000000">replaceAll</span>(<span style="color:#aa1111">"[\\[\\]]"</span>,<span style="color:#aa1111">""</span>);
%>
<span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>用户名： <%<span style="color:#981a1a">=</span> <span style="color:#000000">name</span> %><span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
<span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>密码： <%<span style="color:#981a1a">=</span> <span style="color:#000000">password</span> %><span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
<span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>信息来源： <%<span style="color:#981a1a">=</span> <span style="color:#000000">channelInfo</span> %><span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span></span></span>

访问 register.jsp ，然后点击 注册 按钮，跳转后的页面中显示呈现乱码，如何解决呢？

在小脚本的第一行代码前面添加如下代码：

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//在从请求中获取参数值之前，先对该请求的字符集编码进行设置</span>
<span style="color:#000000">request</span>.<span style="color:#000000">setCharacterEncoding</span>(<span style="color:#aa1111">"UTF-8"</span>);</span></span>

再次测试，显示正常。但如果将 form 表单的提交方式修改为 get ，再测试，又出现了乱码，这如何解释呢？

这是因为 request.setCharacterEncoding("UTF-8"); 只能解决 post 请求方式的出现的乱码，而 get 请求方式的乱码需要使用字符串转码的方式来解决。

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//使用字符串的构造方法进行转码</span>
<span style="color:#770088">new</span> <span style="color:#008855">String</span>(<span style="color:#000000">字符串</span>.<span style="color:#000000">getBytes</span>(<span style="color:#aa1111">"ISO-8859-1"</span>), <span style="color:#aa1111">"UTF-8"</span>);</span></span>

转发可以访问WEB-INF下的资源，转发还可以共享请求中的参数

3.5 GET 和 POST 请求的区别

• GET 请求的参数在URL中，而 POST 请求的参数在请求体（body） 中
• GET 请求有数据长度限制，这个长度限制是浏览器或者服务器为了提升处理效率而做出的限制，而POST 请求没有。（HTTP协议本质就是一个有规则的字符串）
• GET 请求的安全性低，因为参数在URL中，直接暴露了信息，而 POST 请求的安全性高，因为 POST 请求的参数在请求体（body） 中，隐藏了信息

4. 内置对象 response

4.1 response对象的数据类型

javax.servlet.http.HttpServletResponse

4.2 作用

对客户端请求做出响应

4.3 常用方法

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//添加cookie</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">addCookie</span>(<span style="color:#000000">Cookie</span> <span style="color:#000000">c</span>);
<span style="color:#aa5500">//重新定位新的资源，也叫重定向</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">sendRedirect</span>(<span style="color:#008855">String</span> <span style="color:#000000">url</span>);
<span style="color:#aa5500">//设置响应状态码</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setStatus</span>(<span style="color:#008855">int</span> <span style="color:#000000">status</span>);
<span style="color:#aa5500">//获取打印流，主要用于向页面传输数据</span>
<span style="color:#000000">PrintWriter</span> <span style="color:#0000ff">getWriter</span>();
<span style="color:#aa5500">//获取输出流，主要用于图片传输、下载等功能</span>
<span style="color:#000000">ServletOutputStream</span> <span style="color:#0000ff">getOutputStream</span>();
<span style="color:#aa5500">//设置向页面输出的数据的字符集编码</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setCharacterEncoding</span>(<span style="color:#008855">String</span> <span style="color:#000000">charset</span>);</span></span>

4.4 案例

实现登录页面跳转功能，并在跳转的页面中显示登录信息

login.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#117700"><</span><span style="color:#117700">form</span> <span style="color:#0000cc">action</span>=<span style="color:#aa1111">"process.jsp"</span> <span style="color:#0000cc">method</span>=<span style="color:#aa1111">"post"</span><span style="color:#117700">></span>
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">><</span><span style="color:#117700">span</span><span style="color:#117700">></span>用户名<span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"text"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"username"</span><span style="color:#117700">></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">><</span><span style="color:#117700">span</span><span style="color:#117700">></span>密码<span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"password"</span> <span style="color:#0000cc">name</span>=<span style="color:#aa1111">"password"</span><span style="color:#117700">></</span><span style="color:#117700">div</span><span style="color:#117700">></span> 
    <span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>
        <span style="color:#117700"><</span><span style="color:#117700">input</span> <span style="color:#0000cc">type</span>=<span style="color:#aa1111">"submit"</span> <span style="color:#0000cc">value</span>=<span style="color:#aa1111">"登录"</span><span style="color:#117700">></span>
    <span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
<span style="color:#117700"></</span><span style="color:#117700">form</span><span style="color:#117700">></span></span></span>

process.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">page</span> <span style="color:#000000">contentType</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"text/html;charset=UTF-8"</span> <span style="color:#000000">language</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"java"</span>%>
<%
    <span style="color:#008855">String</span> <span style="color:#000000">username</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"username"</span>);<span style="color:#aa5500">//获取参数username的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">password</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"password"</span>);<span style="color:#aa5500">//获取参数password的值</span>
    <span style="color:#770088">if</span>(<span style="color:#aa1111">"admin"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">username</span>) <span style="color:#981a1a">&&</span> <span style="color:#aa1111">"123456"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">password</span>)){
        <span style="color:#aa5500">//页面重定向至主页面</span>
        <span style="color:#000000">response</span>.<span style="color:#000000">sendRedirect</span>(<span style="color:#aa1111">"main.jsp"</span>);
    }
%></span></span>

main.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">page</span> <span style="color:#000000">contentType</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"text/html;charset=UTF-8"</span> <span style="color:#000000">language</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"java"</span>%>
<%
    <span style="color:#008855">String</span> <span style="color:#000000">username</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"username"</span>);<span style="color:#aa5500">//获取参数username的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">password</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"password"</span>);<span style="color:#aa5500">//获取参数password的值</span>
%>
<span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>用户名： <%<span style="color:#981a1a">=</span> <span style="color:#000000">username</span> %><span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span>
<span style="color:#117700"><</span><span style="color:#117700">div</span><span style="color:#117700">></span>密码： <%<span style="color:#981a1a">=</span> <span style="color:#000000">password</span> %><span style="color:#117700"></</span><span style="color:#117700">div</span><span style="color:#117700">></span></span></span>

访问 login.jsp ，然后点击 登录 按钮，查看地址栏信息与页面信息。地址栏信息发生了变化，说明重定向发生在客户端，相当于客户端再发了一次请求，重新定位了新的资源。由于这次请求是新的请求，与之前的登录请求完全独立，因此页面信息中展示全是null

思考：如何才能将登录信息在跳转的页面中显示呢？

可以通过请求转发来实现，修改 process.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">page</span> <span style="color:#000000">contentType</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"text/html;charset=UTF-8"</span> <span style="color:#000000">language</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"java"</span>%>
<%
    <span style="color:#008855">String</span> <span style="color:#000000">username</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"username"</span>);<span style="color:#aa5500">//获取参数username的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">password</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"password"</span>);<span style="color:#aa5500">//获取参数password的值</span>
    <span style="color:#770088">if</span>(<span style="color:#aa1111">"admin"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">username</span>) <span style="color:#981a1a">&&</span> <span style="color:#aa1111">"123456"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">password</span>)){
        <span style="color:#aa5500">//页面转发</span>
        <span style="color:#000000">request</span>.<span style="color:#000000">getRequestDispatcher</span>(<span style="color:#aa1111">"main.jsp"</span>).<span style="color:#000000">forward</span>(<span style="color:#000000">request</span>, <span style="color:#000000">response</span>);
    }
%></span></span>

访问 login.jsp ，然后点击 登录 按钮，查看地址栏信息与页面信息。地址栏信息未发生变化，而页面进行了跳转，说明转发发生在服务器，由服务器完成。转发后，页面能够展示登录信息，说明转发可以共享请求的参数

5. 内置对象 session

5.1 session 的概念

session 就是浏览器与服务器之间的一次通话

5.2 为什么会有 session

HTTP 协议是一种无状态协议，用户在访问服务器时，服务器无法感知到用户是哪一个用户，也就无法追踪用户的后续操作。为了解决这一问题，服务器端设计了一个类 HttpSession 来感知用户，这个类产生的对象就是 session。在用户第一次访问服务器时，服务器就会为该用户生成了一个 session 对象，session 对象一产生就会生成了一个唯一标识符 JSESSIONID， 并将这个唯一标识符使用Cookie存储在浏览器中，用户后续进行的每一个操作都将携带这个唯一标识符，服务器就根据这个唯一标识符追踪用户。session 对象产生时就有一个过期时间，主要用于检测用户是否还在进行有效的操作。如果用户具有有效的操作，那么每一次用户的有效操作都将重置该session的过期时间。这个过期时间就是检测用户登录超时的依据。除此之外，session 还可以存储数据。

5.3 session 对象的数据类型

javax.servlet.http.HttpSession

5.4 常用方法

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//以key/value的形式保存对象值</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setAttribute</span>(<span style="color:#008855">String</span> <span style="color:#000000">key</span>, <span style="color:#008855">Object</span> <span style="color:#000000">value</span>);
<span style="color:#aa5500">//通过key获取对象值</span>
<span style="color:#008855">Object</span> <span style="color:#0000ff">getAttribute</span>(<span style="color:#008855">String</span> <span style="color:#000000">key</span>);
<span style="color:#aa5500">//设置session对象失效</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">invalidate</span>();
<span style="color:#aa5500">// 获取sessionid</span>
<span style="color:#008855">String</span> <span style="color:#0000ff">getId</span>();
<span style="color:#aa5500">//设定session的非活动时间</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setMaxInactiveInterval</span>(<span style="color:#000000">intinterval</span>);
<span style="color:#aa5500">//获取session的有效非活动时间(以秒为单位)</span>
<span style="color:#008855">int</span> <span style="color:#0000ff">getMaxInactiveInterval</span>();
<span style="color:#aa5500">//从session中删除指定名称(key)所对应的对象</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">removeAttribute</span>(<span style="color:#008855">String</span> <span style="color:#000000">key</span>);</span></span>

5.5 案例

使用 session 完成登录成功后页面显示登录信息，要求登录处理使用重定向。

process.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">page</span> <span style="color:#000000">contentType</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"text/html;charset=UTF-8"</span> <span style="color:#000000">language</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"java"</span> %>
<%
    <span style="color:#aa5500">//获取参数username的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">username</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"username"</span>);
    <span style="color:#aa5500">//获取参数password的值</span>
    <span style="color:#008855">String</span> <span style="color:#000000">password</span> <span style="color:#981a1a">=</span> <span style="color:#000000">request</span>.<span style="color:#000000">getParameter</span>(<span style="color:#aa1111">"password"</span>);
    <span style="color:#770088">if</span> (<span style="color:#aa1111">"admin"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">username</span>) <span style="color:#981a1a">&&</span> <span style="color:#aa1111">"123456"</span>.<span style="color:#000000">equals</span>(<span style="color:#000000">password</span>)) {
        <span style="color:#aa5500">//将用户名存储在session中，方便后面使用的时候获取</span>
        <span style="color:#000000">session</span>.<span style="color:#000000">setAttribute</span>(<span style="color:#aa1111">"user"</span>, <span style="color:#000000">username</span>);
        <span style="color:#aa5500">//页面重定向</span>
        <span style="color:#000000">response</span>.<span style="color:#000000">sendRedirect</span>(<span style="color:#aa1111">"main.jsp"</span>);
    }
%></span></span>

main.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#117700"><</span><span style="color:#117700">nav</span><span style="color:#117700">><</span><span style="color:#117700">span</span><span style="color:#117700">></span>欢迎您，<%<span style="color:#981a1a">=</span> <span style="color:#000000">session</span>.<span style="color:#000000">getAttribute</span>(<span style="color:#aa1111">"user"</span>)%><span style="color:#117700"></</span><span style="color:#117700">span</span><span style="color:#117700">></</span><span style="color:#117700">nav</span><span style="color:#117700">></span></span></span>

5.6 include 指令

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500"><!-- 语法 --></span>
<%<span style="color:#555555">@</span> <span style="color:#000000">include</span> <span style="color:#000000">file</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"文件名"</span> %></span></span>

在开发过程中，开发的页面数量总是很多，如何确保用户的每一次操作都是有效操作呢？所谓的有效操作是指在登录没有超时的情况下进行的操作。

可以编写一个检测登录超时的页面，然后使用 include 指令引入至每一个页面中

timeout.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">page</span> <span style="color:#000000">contentType</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"text/html;charset=UTF-8"</span> <span style="color:#000000">language</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"java"</span> %>
<%
    <span style="color:#008855">String</span> <span style="color:#000000">username</span> <span style="color:#981a1a">=</span> (<span style="color:#008855">String</span>)<span style="color:#000000">session</span>.<span style="color:#000000">getAttribute</span>(<span style="color:#aa1111">"user"</span>);
    <span style="color:#770088">if</span> (<span style="color:#000000">username</span> <span style="color:#981a1a">==</span> <span style="color:#221199">null</span>)
        <span style="color:#000000">response</span>.<span style="color:#000000">sendRedirect</span>(<span style="color:#aa1111">"login.jsp"</span>);
%></span></span>

main.jsp

<span style="background-color:#f8f8f8"><span style="color:#333333"><%<span style="color:#555555">@</span> <span style="color:#000000">include</span> <span style="color:#000000">file</span><span style="color:#981a1a">=</span><span style="color:#aa1111">"timeout.jsp"</span> %></span></span>

5.7 session 超时设置

第一种方式： Tomcat 中的 web.xml

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#117700"><</span><span style="color:#117700">session-config</span><span style="color:#117700">></span>
    <span style="color:#aa5500"><!-- 单位：分钟 --></span>
    <span style="color:#117700"><</span><span style="color:#117700">session-timeout</span><span style="color:#117700">></span>30<span style="color:#117700"></</span><span style="color:#117700">session-timeout</span><span style="color:#117700">></span>
<span style="color:#117700"></</span><span style="color:#117700">session-config</span><span style="color:#117700">></span></span></span>

第二种方式： 工程中的 web.xml

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#117700"><</span><span style="color:#117700">session-config</span><span style="color:#117700">></span>
    <span style="color:#aa5500"><!-- 单位：分钟 --></span>
    <span style="color:#117700"><</span><span style="color:#117700">session-timeout</span><span style="color:#117700">></span>30<span style="color:#117700"></</span><span style="color:#117700">session-timeout</span><span style="color:#117700">></span>
<span style="color:#117700"></</span><span style="color:#117700">session-config</span><span style="color:#117700">></span></span></span>

第三种方式： Java 代码实现

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">//设置会话超时时间，单位：秒</span>
<span style="color:#000000">session</span>.<span style="color:#000000">setMaxInactiveInterval</span>(<span style="color:#116644">15</span> <span style="color:#981a1a">*</span> <span style="color:#116644">60</span>);</span></span>

6. 内置对象 application

6.1 application 对象的数据类型

javax.servlet.ServletContext

6.2 作用

实现用户数据共享，将信息保存在服务器中，直到服务器关闭

6.3 常用方法

<span style="background-color:#f8f8f8"><span style="color:#333333"><span style="color:#aa5500">// 以key/value的形式保存对象值</span>
<span style="color:#008855">void</span> <span style="color:#0000ff">setAttribute</span>(<span style="color:#008855">String</span> <span style="color:#000000">key</span>,<span style="color:#008855">Object</span> <span style="color:#000000">value</span>);
<span style="color:#aa5500">//通过key获取对象值</span>
<span style="color:#008855">Object</span> <span style="color:#0000ff">getAttribute</span>(<span style="color:#008855">String</span>  <span style="color:#000000">key</span>);
<span style="color:#aa5500">//返回相对路径的真实路径</span>
<span style="color:#008855">String</span> <span style="color:#0000ff">getRealPath</span>(<span style="color:#008855">String</span>  <span style="color:#000000">path</span>);</span></span>