Servlet - 会话跟踪

系统管理员 2022-08-22 15:23 169阅读 0赞

## 会话跟踪 ##

> HTTP本身是“无状态”协议,它不保存连接交互信息,一次响应完成之后即连接断开,下一次请求需要重新建立连接,服务器不记录上次连接的内容.因此如果判断两次连接是否是同一用户, 就需要使用会话跟踪技术来解决.常见的会话跟踪技术有如下几种:

*  URL重写: 在URL结尾附加会话ID标识,服务器通过会话ID识别不同用户.
 *  隐藏表单域: 将会话ID埋入HTML表单隐藏域提交到服务端(会话ID不在浏览器页面显示).
 *  Cookie: 第一次请求时服务器主动发一小段信息给浏览器(即Cookie),下次请求时浏览器自动附带该段信息发送给服务器,服务器读取Cookie识别用户.
 *  Session: 服务器为每个用户创建一个Session对象保存到内存,并生成一个sessionID放入Cookie发送给浏览器,下次访问时sessionID会随Cookie传回来,服务器再根据sessionID找到对应Session对象(Java领域特有).

> Session机制依赖于Cookie,如果Cookie被禁用Session也将失效.

--------------------

## Cookie ##

Cookie是识别当前用户,实现持久会话的最好方式.最初由网景公司开发,但现在所有主流浏览器都支持.以至于HTTP协议为他定义了一些新的HTTP首部.

> URL重写与隐藏表单域两种技术都有一定的局限,细节可参考博客[四种会话跟踪技术][Link 1]

*  Cookie规范   
    
    
     *  Cookie通过请求头/响应头在服务器与客户端之间传输, 大小限制为4KB;
     *  一台服务器在一个客户端最多保存20个Cookie;
     *  一个浏览器最多保存300个Cookie;

> Cookie的key/value均不能保存中文,如果需要,可以在保存前对中文进行编码, 取出时再对其解码.

--------------------

### Java-Cookie ###

在Java中使用Cookie, 必须熟悉`javax.servlet.http.Cookie`类, 以及`HttpServletRequest`/`HttpServletResponse`接口提供的几个方法:

<table style="width:922.727px; border-collapse:collapse; border-spacing:0px; border:1px solid rgb(238,238,238); color:rgb(0,0,0); font-family:'microsoft yahei'; font-size:14px; line-height:26px"> 
 <thead style=""> 
 <tr style=""> 
 <th align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Cookie</th> 
 <th align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> 描述</th> 
 </tr> 
 </thead> 
 <tbody style=""> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">Cookie(String name, String value)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Constructs a cookie with the specified name and value.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">String getName()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Returns the name of the cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">String getValue()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Gets the current value of this Cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void setValue(String newValue)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Assigns a new value to this Cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void setMaxAge(int expiry)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Sets the maximum age in seconds for this Cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">int getMaxAge()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Gets the maximum age in seconds of this Cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void setPath(String uri)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Specifies a path for the cookie to which the client should return the cookie.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void setDomain(String domain)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Specifies the domain within which this cookie should be presented.</td> 
 </tr> 
 </tbody> 
</table>

*  示例: 获取上次访问时间   
    从Request中获取Cookie: last\_access\_time, 如果没有则新建,否则显示last\_access\_time内容, 并更新为当前系统时间, 最后放入Response:

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">@Override
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 Cookie[] cookies = request.getCookies();
 Cookie latCookie = null;
 if (cookies != null){
 for (Cookie cookie : cookies){
 if (cookie.getName().equals(L_A_T)){
 latCookie = cookie;
 break;
 }
 }
 }
 
 // 已经访问过了
 if (latCookie != null){
 printResponse("您上次访问的时间是" + latCookie.getValue(), response);
 latCookie.setValue(new Date().toString());
 } else{
 printResponse("您还是第一次访问", response);
 latCookie = new Cookie(L_A_T, new Date().toString());
 }
 
 response.addCookie(latCookie);
 }
 
 private void printResponse(String data, HttpServletResponse response) throws IOException {
 response.setContentType("text/html; charset=utf-8");
 response.getWriter().print("<H1>" + data + "</H1>");
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li></ul>

--------------------

### 有效期 ###

Cookie的Max-Age决定了Cookie的有效期,单位为秒.`Cookie`类通过`getMaxAge()`与`setMaxAge(int maxAge)`方法来读写Max-Age属性:

--------------------

### 域属性 ###

服务器可向Set-Cookie响应首部添加一个Domain属性来控制哪些站点可以看到该Cookie, 如

<code class="hljs sql has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Set-Cookie: last_access_time="xxx"; Domain=.fq.com</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

该响应首部就是在告诉浏览器将Cookie `last_access_time="xxx"`发送给域”.fq.com”中的所有站点(如www.fq.com, mail.fq.com).

`Cookie`类通过`setDomain()`方法设置域属性.

> 如果没有指定域, 则Domain默认为产生Set-Cookie响应的服务器主机名.

--------------------

### 路径属性 ###

Cookie规范允许用户将Cookie与部分Web站点关联起来.该功能可通过向Set-Cookie响应首部添加Path属性来实现:

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Set-Cookie:last_access_time="Tue Apr 26 19:35:16 CST 2016"; Path=/servlet/</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

这样如果访问[http://www.example.com/hello\_http\_servlet.do][http_www.example.com_hello_http_servlet.do]就不会获得`last_access_time`,但如果访问[http://www.example.com/servlet/index.html][http_www.example.com_servlet_index.html], 就会带上这个Cookie.

`Cookie`类中通过`setPath()`方法设置路径属性.

> 如果没有指定路径, Path默认为产生Set-Cookie响应的URL的路径.

--------------------

## Session ##

在所有的会话跟踪技术中, Session是功能最强大,最多的. 每个用户可以没有或者有一个HttpSession对象, 并且只能访问他自己的Session对象.

> 与URL重写, 隐藏表单域和Cookie不同, Session是保存在服务器内存中的数据,在达到一定的阈值后, Servlet容器会将Session持久化到辅助存储器中, 因此最好将使保存到Session内的对象实现`java.io.Serializable`接口.

使用Session, 必须熟悉`javax.servlet.http.HttpSession`接口, 以及`HttpServletRequest`接口中提供的几个方法:

<table style="width:922.727px; border-collapse:collapse; border-spacing:0px; border:1px solid rgb(238,238,238); color:rgb(0,0,0); font-family:'microsoft yahei'; font-size:14px; line-height:26px"> 
 <thead style=""> 
 <tr style=""> 
 <th align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> HttpSession</th> 
 <th align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> 描述</th> 
 </tr> 
 </thead> 
 <tbody style=""> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void setAttribute(String name, Object value)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Binds an object to this session, using the name specified.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">Object getAttribute(String name)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Returns the object bound with the specified name in this session, or null if no object is bound under the name.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void invalidate()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Invalidates this session then unbinds any objects bound to it.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">Enumeration&lt;String&gt; getAttributeNames()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Returns an Enumeration of String objects containing the names of all the objects bound to this session.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">void removeAttribute(String name)</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Removes the object bound with the specified name from this session.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">String getId()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Returns a string containing the unique identifier assigned to this session.</td> 
 </tr> 
 <tr style=""> 
 <td align="center" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> <code style="font-family:'Source Code Pro',monospace; padding:2px 4px; font-size:12.6px; color:rgb(63,63,63); white-space:nowrap">boolean isNew()</code></td> 
 <td align="left" style="padding:8px; line-height:20px; vertical-align:top; border:1px solid rgb(238,238,238)"> Returns true if the client does not yet know about the session or if the client chooses not to join the session.</td> 
 </tr> 
 </tbody> 
</table>

--------------------

### 示例-购物车 ###

*  domain

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">/**
 * @author jifang.
 * @since 2016/5/1 20:14.
 */
 public class Product implements Serializable {
 
 private int id;
 private String name;
 private String description;
 private double price;
 
 public Product(int id, String name, String description, double price) {
 this.id = id;
 this.name = name;
 this.description = description;
 this.price = price;
 }
 
 // ...
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

*  商品列表页面(/jsp/products.jsp)

*  商品详情(/jsp/product\_details.jsp)

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><%@ page import="com.fq.web.domain.Product" %>
 <%@ page import="com.fq.web.util.ProductContainer" %>
 <%@ page contentType="text/html;charset=UTF-8" language="java" %>
 <html>
 <head>
 <title>Product Details</title>
 </head>
 <body>
 <h2>Product Details</h2>
 <%
 int id = Integer.parseInt(request.getParameter("id"));
 Product product = ProductContainer.getProduct(id);
 assert product != null;
 %>
 <form action="${pageContext.request.contextPath}/session/add_to_card.do" method="post">
 <input type="hidden" name="id" value="<%=id%>"/>
 <table>
 <tr>
 <td>Name:</td>
 <td><%=product.getName()%>
 </td>
 </tr>
 <tr>
 <td>Price:</td>
 <td><%=product.getPrice()%>
 </td>
 </tr>
 <tr>
 <td>Description:</td>
 <td><%=product.getDescription()%>
 </td>
 </tr>
 <tr>
 <td><input type="text" name="quantity"></td>
 <td><input type="submit" value="Buy"></td>
 </tr>
 <tr>
 <td><a href="${pageContext.request.contextPath}/jsp/products.jsp">Products</a></td>
 <td><a href="${pageContext.request.contextPath}/jsp/shopping_cart.jsp">Shopping Cart</a></td>
 </tr>
 </table>
 </form>
 </body>
 </html></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li></ul>

*  加入购物车(AddCardServlet)

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">@WebServlet(name = "AddCardServlet", urlPatterns = "/session/add_to_card.do")
 public class AddCardServlet extends HttpServlet {
 
 @SuppressWarnings("All")
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 int id = Integer.parseInt(request.getParameter("id"));
 Product product = ProductContainer.getProduct(id);
 int quantity = Integer.parseInt(request.getParameter("quantity"));
 
 HttpSession session = request.getSession();
 List<ShoppingItem> items = (List<ShoppingItem>) session.getAttribute(SessionConstant.CART_ATTRIBUTE);
 if (items == null) {
 items = new ArrayList<ShoppingItem>();
 session.setAttribute(SessionConstant.CART_ATTRIBUTE, items);
 }
 items.add(new ShoppingItem(product, quantity));
 
 request.getRequestDispatcher("/jsp/products.jsp").forward(request, response);
 }
 
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 doPost(request, response);
 }
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li></ul>

*  购物车(/jsp/shopping\_card.jsp)

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><%@ page import="com.fq.web.constant.SessionConstant" %>
 <%@ page import="com.fq.web.domain.ShoppingItem" %>
 <%@ page import="java.util.List" %>
 <%@ page contentType="text/html;charset=UTF-8" language="java" %>
 <html>
 <head>
 <title>Shopping Cart</title>
 </head>
 <body>
 <h2>Shopping Cart</h2>
 <a href="${pageContext.request.contextPath}/jsp/products.jsp">Products</a>
 <table>
 <tr>
 <td style="width: 150px">Quantity</td>
 <td style="width: 150px">Product</td>
 <td style="width: 150px">Price</td>
 <td>Amount</td>
 </tr>
 <%
 List<ShoppingItem> items = (List<ShoppingItem>) session.getAttribute(SessionConstant.CART_ATTRIBUTE);
 if (items != null) {
 double total = 0.0;
 for (ShoppingItem item : items) {
 double subtotal = item.getQuantity() * item.getProduct().getPrice();
 %>
 <tr>
 <td><%=item.getQuantity()%>
 </td>
 <td><%=item.getProduct().getName()%>
 </td>
 <td><%=item.getProduct().getPrice()%>
 </td>
 <td><%=subtotal%>
 </td>
 </tr>
 <%
 total += subtotal;
 }%>
 <tr>
 <td>Total: <%=total%>
 </td>
 </tr>
 <%
 }
 %>
 
 </table>
 </body>
 </html></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li></ul>

--------------------

### 有效期 ###

Session有一定的过期时间: 当用户长时间不去访问该Session,就会超时失效,虽然此时sessionID可能还在Cookie中, 只是服务器根据该sessionID已经找不到Session对象了.   
Session的超时时间可以在web.xml中配置, 单位为分钟:

另外一种情况: 由于sessionID保存在Cookie中且Max-Age为`-1`,因此当用户重新打开浏览器时已经没有sessionID了, 此时服务器会再创建一个Session,此时新的会话又开始了.而原先的Session会因为超时时间到达而被销毁.

--------------------

## 字符编码 ##

字符编码就是以二进制的数字来对应字符集的字符,常见字符编码方式有:ISO-8859-1(不支持中文),GB2312,GBK,UTF-8等.在JavaWeb中, 经常遇到的需要编码/解码的场景有响应编码/请求编码/URL编码:

--------------------

### 响应编码 ###

服务器发送数据给客户端由Response对象完成,如果响应数据是二进制流,就无需考虑编码问题.如果响应数据为字符流,那么就一定要考虑编码问题:

> `response.getWriter()`默认使用ISO-889-1发送数据,而该字符集不支持中文,因此遇到中文就一定会乱码.

在需要发送中文时, 需要使用:

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">response.setCharacterEncoding("UTF-8");
 // getWriter() ...</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

设置编码方式,由于在`getWriter()`输出前已经设置了UTF-8编码,因此输出字符均为UTF-8编码,但我们并未告诉客户端使用什么编码来读取响应数据,因此我们需要在响应头中设置编码信息(使用Content-Type):

<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">response.setContentType("text/html;charset=UTF-8");
 // getWriter() ...</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

> 注意: 这句代码不只在响应头中添加了编码信息,还相当于调用了一次`response.setCharacterEncoding("UTF-8");`

--------------------

### 请求编码 ###

#### 1. 浏览器地址栏编码 ####

在浏览器地址栏书写字符数据,由浏览器编码后发送给服务器,因此如果在地址栏输入中文,则其编码方式由浏览器决定:

#### 2. 页面请求 ####

如果通过页面的超链接/表单向服务器发送数据,那么其编码方式由当前页面的编码方式确定:

#### 3. GET ####

当客户端发送GET请求时,无论客户端发送的数据编码方式为何,服务端均已ISO-8859-1解码(Tomcat8.x之后改用UTF-8),这就需要我们在`request.getParameter()`获取数据后再转换成正确的编码:

#### 4. POST ####

当客户端发送POST请求时,服务端也是默认使用IOS-8859-1解码,但POST的数据是通过请求体传送过来,因此POST请求可以通过`request.setCharacterEncoding()`来指定请求体编码方式:

<code class="hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">private Map<String, String> convertToParameterMap(HttpServletRequest request) throws IOException {
 Map<String, String> parameters = new HashMap<String, String>();
 if (request.getMethod().equals("POST")) {
 request.setCharacterEncoding("UTF-8");
 Enumeration<String> names = request.getParameterNames();
 while (names.hasMoreElements()) {
 String key = names.nextElement();
 parameters.put(key, request.getParameter(key));
 }
 } else {
 Enumeration<String> names = request.getParameterNames();
 while (names.hasMoreElements()) {
 String key = names.nextElement();
 String value = request.getParameter(key);
 parameters.put(key, new String(value.getBytes("ISO-8859-1"), "UTF-8"));
 }
 }
 
 return parameters;
 }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

--------------------

### URL编码 ###

网络标准RFC 1738规定：

> “…Only alphanumerics `[0-9a-zA-Z]`, the special characters `"$-_.+!*'(),"` \[not including the quotes - ed\], and reserved characters used for their reserved purposes may be used unencoded within a URL.”   
> “只有字母和数字`[0-9a-zA-Z]`、一些特殊符号`"$-_.+!*'(),"`\[不包括双引号\]、以及某些保留字，才可以不经过编码直接用于URL。”

如果URL中有汉字,就必须编码后使用, 而URL编码过程其实很简单:

> 首先需要指定一种字符编码,把字符串解码后得到`byte[]`,然后把小于0的字节+256,再将其转换成16进制,最后前面再添加一个%.

这个编码过程在Java中已经封装成了现成的库, 可直接使用:

> 注: 在Web中Tomcat容器会自动识别URL是否已经编码并自动解码.

[Link 1]: http://blog.csdn.net/xh16319/article/details/8033552
[http_www.example.com_hello_http_servlet.do]: http://www.example.com/hello_http_servlet.do
[http_www.example.com_servlet_index.html]: http://www.example.com/servlet/index.html