1 Accept: text/html,image/*      -- 浏览器接受的数据类型
 2 Accept-Charset: ISO-8859-1     -- 浏览器接受的编码格式
 3 Accept-Encoding: gzip,compress  --浏览器接受的数据压缩格式
 4 Accept-Language: en-us,zh-       --浏览器接受的语言
 5 Host: www.it315.org:80          --（必须的）当前请求访问的目标地址（主机:端口）
 6 If-Modified-Since: Tue, 11 Jul 2000 18:23:51 GMT  --浏览器最后的缓存时间
 7 Referer: http://www.it315.org/index.jsp      -- 当前请求来自于哪里
 8 User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)  --浏览器类型
 9 Cookie:name=eric                     -- 浏览器保存的cookie信息
10 Connection: close/Keep-Alive            -- 浏览器跟服务器连接状态。close: 连接关闭  keep-alive：保存连接。
11 Date: Tue, 11 Jul 2000 18:23:51 GMT      -- 请求发出的时间

 1 HttpSevlet类的源码：
 2 protected void service(HttpServletRequest req, HttpServletResponse resp)
 3         throws ServletException, IOException {
 4        //得到请求方式
 5         String method = req.getMethod();
 6
 7         if (method.equals(METHOD_GET)) {
 8             long lastModified = getLastModified(req);
 9             if (lastModified == -1) {
10                 // servlet doesn't support if-modified-since, no reason
11                 // to go through further expensive logic
12                 doGet(req, resp);
13             } else {
14                 long ifModifiedSince;
15                 try {
16                     ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
17                 } catch (IllegalArgumentException iae) {
18                     // Invalid date header - proceed as if none was set
19                     ifModifiedSince = -1;
20                 }
21                 if (ifModifiedSince < (lastModified / 1000 * 1000)) {
22                     // If the servlet mod time is later, call doGet()
23                     // Round down to the nearest second for a proper compare
24                     // A ifModifiedSince of -1 will always be less
25                     maybeSetLastModified(resp, lastModified);
26                     doGet(req, resp);
27                 } else {
28                     resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
29                 }
30             }
31
32         } else if (method.equals(METHOD_HEAD)) {
33             long lastModified = getLastModified(req);
34             maybeSetLastModified(resp, lastModified);
35             doHead(req, resp);
36
37         } else if (method.equals(METHOD_POST)) {
38             doPost(req, resp);
39
40         } else if (method.equals(METHOD_PUT)) {
41             doPut(req, resp);
42
43         } else if (method.equals(METHOD_DELETE)) {
44             doDelete(req, resp);
45
46         } else if (method.equals(METHOD_OPTIONS)) {
47             doOptions(req,resp);
48
49         } else if (method.equals(METHOD_TRACE)) {
50             doTrace(req,resp);
51
52         } else {
53             //
54             // Note that this means NO servlet supports whatever
55             // method was requested, anywhere on this server.
56             //
57
58             String errMsg = lStrings.getString("http.method_not_implemented");
59             Object[] errArgs = new Object[1];
60             errArgs[0] = method;
61             errMsg = MessageFormat.format(errMsg, errArgs);
62
63             resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
64         }
65     }