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1.HttpServlet介绍（具体方法可参考：java WEB学习笔记08：HttpServletRequest & ServletRequest） HttpServletResponse
1）public abstract class HttpServlet extends GenericServlet implements java.io.Serializable 是继承GenericServlet 的一个针对HTTP 协议定制的Servlet。
2）在service()方法中直接把ServletRequest 和 ServletResponse 转为 HttpServletRequest 和 HttpServletResponse，并且调用了重载的service(HttpServletRequest req, HttpServletResponse resp)
获取请求方式：request.getMethod().根据请求方式又创建了doXxx（）方法（xxx 为具体的请求方式，比如doGet, doPost）
1 @Override
2 public void service(ServletRequest req, ServletResponse res)
3 throws ServletException, IOException {
4
5 HttpServletRequest request;
6 HttpServletResponse response;
7
8 try {
9 request = (HttpServletRequest) req;
10 response = (HttpServletResponse) res;
11 } catch (ClassCastException e) {
12 throw new ServletException("non-HTTP request or response");
13 }
14 service(request, response);
15
16
17 protected void service(HttpServletRequest req, HttpServletResponse resp)
18 throws ServletException, IOException {
19
20 String method = req.getMethod();
21
22 if (method.equals(METHOD_GET)) {
23 long lastModified = getLastModified(req);
24 if (lastModified == -1) {
25 // servlet doesn't support if-modified-since, no reason
26 // to go through further expensive logic
27 doGet(req, resp);
28 } else {
29 long ifModifiedSince;
30 try {
31 ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
32 } catch (IllegalArgumentException iae) {
33 // Invalid date header - proceed as if none was set
34 ifModifiedSince = -1;
35 }
36 if (ifModifiedSince < (lastModified / 1000 * 1000)) {
37 // If the servlet mod time is later, call doGet()
38 // Round down to the nearest second for a proper compare
39 // A ifModifiedSince of -1 will always be less
40 maybeSetLastModified(resp, lastModified);
41 doGet(req, resp);
42 } else {
43 resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
44 }
45 }
46
47 } else if (method.equals(METHOD_HEAD)) {
48 long lastModified = getLastModified(req);
49 maybeSetLastModified(resp, lastModified);
50 doHead(req, resp);
51
52 } else if (method.equals(METHOD_POST)) {
53 doPost(req, resp);
54
55 } else if (method.equals(METHOD_PUT)) {
56 doPut(req, resp);
57
58 } else if (method.equals(METHOD_DELETE)) {
59 doDelete(req, resp);
60
61 } else if (method.equals(METHOD_OPTIONS)) {
62 doOptions(req,resp);
63
64 } else if (method.equals(METHOD_TRACE)) {
65 doTrace(req,resp);
66
67 } else {
68 //
69 // Note that this means NO servlet supports whatever
70 // method was requested, anywhere on this server.
71 //
72
73 String errMsg = lStrings.getString("http.method_not_implemented");
74 Object[] errArgs = new Object[1];
75 errArgs[0] = method;
76 errMsg = MessageFormat.format(errMsg, errArgs);
77
78 resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
79 }
80 }
3.一般在实际开发中，直接继承HttpServlet，并根据请求方法doXxx() 方法接口
优点：直接针对性的覆盖doXxx() 方法；直接使用 HttpServletRequest 和 HttpServletResponse
4.关于Servlet小结
1）了解GenericServlet 原理和实现
2）掌握 HttpServlet 实现的原理 及 源码