细说 HttpHandler 的映射过程

在ASP.NET编程模型中，一个来自客户端的请求要经过一个称为管线的处理过程。在整个处理请求中，相对于其它对象来说，HttpHandler的处理算得上是整个过程的核心部分。由于HttpHandler的重要地位，我前面已经有二篇博客对它过一些使用上的介绍。【用Asp.net写自己的服务框架】中谈到了它的一般使用方法。【细说ASP.NET的各种异步操作】又详细地介绍了异步HttpHandler的使用方式。
今天的博客将着重介绍HttpHandler的配置，创建以及重用过程，还将涉及HttpHandlerFactory的内容。
回顾HttpHandler

HttpHandler其实是一类统称：泛指实现了IHttpHandler接口的一些类型，这些类型有一个共同的功能，那就是可以用来处理HTTP请求。IHttpHandler的接口定义如下：

1. // 定义 ASP.NET 为使用自定义 HTTP 处理程序同步处理 HTTP Web 请求而实现的协定。
2. public interface IHttpHandler
3. {
4. <httpHandlers>
5. <httpHandlers>
6. <httpHandlers>
7.    
8. </httpHandlers>
9. </httpHandlers>
10. </httpHandlers>// 获取一个值，该值指示其他请求是否可以使用 System.Web.IHttpHandler 实例。
11. <httpHandlers>
12. <httpHandlers>
13. <httpHandlers>
14.    
15. </httpHandlers>
16. </httpHandlers>
17. </httpHandlers>//
18. <httpHandlers>
19. <httpHandlers>
20. <httpHandlers>
21.    
22. </httpHandlers>
23. </httpHandlers>
24. </httpHandlers>// 返回结果:
25. <httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>//<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers> 如果 System.Web.IHttpHandler 实例可再次使用，则为 true；否则为 false。
38. <httpHandlers>
39. <httpHandlers>
40. <httpHandlers>
41.    
42. </httpHandlers>
43. </httpHandlers>
44. </httpHandlers>bool IsReusable { get; }
46. <httpHandlers>
47. <httpHandlers>
48. <httpHandlers>
49.    
50. </httpHandlers>
51. </httpHandlers>
52. </httpHandlers>// 通过实现 System.Web.IHttpHandler 接口的自定义 HttpHandler 启用 HTTP Web 请求的处理。
53. <httpHandlers>
54. <httpHandlers>
55. <httpHandlers>
56.    
57. </httpHandlers>
58. </httpHandlers>
59. </httpHandlers>void ProcessRequest(HttpContext context);
60. }

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有关HttpHandler的各类用法，可参考我的博客【用Asp.net写自己的服务框架】，本文将不再重复说明了。它还有一个异步版本：

1. // 摘要:
2. //<httpHandlers>
3. <httpHandlers>
4. <httpHandlers>
5.    
6. </httpHandlers>
7. </httpHandlers>
8. </httpHandlers> 定义 HTTP 异步处理程序对象必须实现的协定。
9. public interface IHttpAsyncHandler : IHttpHandler
10. {
11. <httpHandlers>
12. <httpHandlers>
13. <httpHandlers>
14.    
15. </httpHandlers>
16. </httpHandlers>
17. </httpHandlers>// 摘要:
18. <httpHandlers>
19. <httpHandlers>
20. <httpHandlers>
21.    
22. </httpHandlers>
23. </httpHandlers>
24. </httpHandlers>//<httpHandlers>
25. <httpHandlers>
26. <httpHandlers>
27.    
28. </httpHandlers>
29. </httpHandlers>
30. </httpHandlers> 启动对 HTTP 处理程序的异步调用。
31. <httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>//
38. <httpHandlers>
39. <httpHandlers>
40. <httpHandlers>
41.    
42. </httpHandlers>
43. </httpHandlers>
44. </httpHandlers>// 参数:
45. <httpHandlers>
46. <httpHandlers>
47. <httpHandlers>
48.    
49. </httpHandlers>
50. </httpHandlers>
51. </httpHandlers>//   context:
52. <httpHandlers>
53. <httpHandlers>
54. <httpHandlers>
55.    
56. </httpHandlers>
57. </httpHandlers>
58. </httpHandlers>//<httpHandlers>
59. <httpHandlers>
60. <httpHandlers>
61.    
62. </httpHandlers>
63. </httpHandlers>
64. </httpHandlers> 一个 System.Web.HttpContext 对象，该对象提供对用于向 HTTP 请求提供服务的内部服务器对象（如 Request、Response、Session
65. <httpHandlers>
66. <httpHandlers>
67. <httpHandlers>
68.    
69. </httpHandlers>
70. </httpHandlers>
71. </httpHandlers>//<httpHandlers>
72. <httpHandlers>
73. <httpHandlers>
74.    
75. </httpHandlers>
76. </httpHandlers>
77. </httpHandlers> 和 Server）的引用。
78. <httpHandlers>
79. <httpHandlers>
80. <httpHandlers>
81.    
82. </httpHandlers>
83. </httpHandlers>
84. </httpHandlers>//
85. <httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>//   extraData:
92. <httpHandlers>
93. <httpHandlers>
94. <httpHandlers>
95.    
96. </httpHandlers>
97. </httpHandlers>
98. </httpHandlers>//<httpHandlers>
99. <httpHandlers>
100. <httpHandlers>
101.    
102. </httpHandlers>
103. </httpHandlers>
104. </httpHandlers> 处理该请求所需的所有额外数据。
105. <httpHandlers>
106. <httpHandlers>
107. <httpHandlers>
108.    
109. </httpHandlers>
110. </httpHandlers>
111. </httpHandlers>//
112. <httpHandlers>
113. <httpHandlers>
114. <httpHandlers>
115.    
116. </httpHandlers>
117. </httpHandlers>
118. </httpHandlers>//   cb:
119. <httpHandlers>
120. <httpHandlers>
121. <httpHandlers>
122.    
123. </httpHandlers>
124. </httpHandlers>
125. </httpHandlers>//<httpHandlers>
126. <httpHandlers>
127. <httpHandlers>
128.    
129. </httpHandlers>
130. </httpHandlers>
131. </httpHandlers> 异步方法调用完成时要调用的 System.AsyncCallback。如果 cb 为 null，则不调用委托。
132. <httpHandlers>
133. <httpHandlers>
134. <httpHandlers>
135.    
136. </httpHandlers>
137. </httpHandlers>
138. </httpHandlers>//
139. <httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers>// 返回结果:
146. <httpHandlers>
147. <httpHandlers>
148. <httpHandlers>
149.    
150. </httpHandlers>
151. </httpHandlers>
152. </httpHandlers>//<httpHandlers>
153. <httpHandlers>
154. <httpHandlers>
155.    
156. </httpHandlers>
157. </httpHandlers>
158. </httpHandlers> 包含有关进程状态信息的 System.IAsyncResult。
159. <httpHandlers>
160. <httpHandlers>
161. <httpHandlers>
162.    
163. </httpHandlers>
164. </httpHandlers>
165. </httpHandlers>IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData);
166. <httpHandlers>
167. <httpHandlers>
168. <httpHandlers>
169.    
170. </httpHandlers>
171. </httpHandlers>
172. </httpHandlers>//
173. <httpHandlers>
174. <httpHandlers>
175. <httpHandlers>
176.    
177. </httpHandlers>
178. </httpHandlers>
179. </httpHandlers>// 摘要:
180. <httpHandlers>
181. <httpHandlers>
182. <httpHandlers>
183.    
184. </httpHandlers>
185. </httpHandlers>
186. </httpHandlers>//<httpHandlers>
187. <httpHandlers>
188. <httpHandlers>
189.    
190. </httpHandlers>
191. </httpHandlers>
192. </httpHandlers> 进程结束时提供异步处理 End 方法。
193. <httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers>//
200. <httpHandlers>
201. <httpHandlers>
202. <httpHandlers>
203.    
204. </httpHandlers>
205. </httpHandlers>
206. </httpHandlers>// 参数:
207. <httpHandlers>
208. <httpHandlers>
209. <httpHandlers>
210.    
211. </httpHandlers>
212. </httpHandlers>
213. </httpHandlers>//   result:
214. <httpHandlers>
215. <httpHandlers>
216. <httpHandlers>
217.    
218. </httpHandlers>
219. </httpHandlers>
220. </httpHandlers>//<httpHandlers>
221. <httpHandlers>
222. <httpHandlers>
223.    
224. </httpHandlers>
225. </httpHandlers>
226. </httpHandlers> 包含有关进程状态信息的 System.IAsyncResult。
227. <httpHandlers>
228. <httpHandlers>
229. <httpHandlers>
230.    
231. </httpHandlers>
232. </httpHandlers>
233. </httpHandlers>void EndProcessRequest(IAsyncResult result);
234. }

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IHttpAsyncHandler接口的二个方法该如何使用，可参考我的博客【细说ASP.NET的各种异步操作】，本文也将不再重复讲解。
如果我们创建了一个自定义的HttpHandler，那么为了能让它处理某些HTTP请求，我们还需将它注册到web.config中，就像下面这样：

1. <httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>

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虽然我以前的博客中也曾多次涉及到HttpHandler，但感觉还是没有把它完整地说清楚，今天的博客将继续以前的话题，因为我认为HttpHandler实在是太重要了。
HttpHandler的映射过程

在博客【用Asp.net写自己的服务框架】中，我从MSDN中摘选了一些ASP.NET管线事件，在这些事件中，第10个事件【根据所请求资源的文件扩展名（在应用程序的配置文件中映射），选择实现 IHttpHandler 的类，对请求进行处理】就是本文要介绍的重点事件。这个事件也是HttpHandler创建的地方。
由于IIS6,7在管线事件触发机制上的有一定的差别，本文将以ASP.NET 2.0以及IIS6的运行方式来介绍这个过程，IIS7的集成模式下只是触发机制不同，但事件的绝大部分是一样的。管线事件由HttpApplication控制，由MapHandlerExecutionStep负责封装这个事件的执行过程：

1. internal class MapHandlerExecutionStep : HttpApplication.IExecutionStep{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private HttpApplication _application;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>void HttpApplication.IExecutionStep.Execute()<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>HttpContext context = this._application.Context;<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>HttpRequest request = context.Request;<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>//我删除了一些与本文无关的调用代码<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>context.Handler = this._application.MapHttpHandler(<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>context, request.RequestType, request.FilePathObject, request.PhysicalPathInternal, false);<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers>}

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Execute()最终调用了MapHttpHandler()方法，这个映射过程是由HttpApplication实现的。
MapHttpHandler方法的实现如下：注意代码中我加入的注释。

1. /// /// 根据虚拟路径映射到具体的HttpHandler对象/// /// HttpContext的实例/// GET, POST .../// 一个虚拟路径映射的字符串: /abc.aspx/// 要请求的物理文件的路径/// /// 具体的HttpHandler对象internal IHttpHandler MapHttpHandler(HttpContext context, string requestType,<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers><httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers><httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers> VirtualPath path, string pathTranslated, bool useAppConfig){<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>// 说明：这段代码是我精简后的代码。为了便于理解，我去掉了一些与请求无关的代码行。<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>IHttpHandler handler = (context.ServerExecuteDepth == 0) ? context.RemapHandlerInstance : null;<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>//用于将当前线程的运行帐号切换为web.config指定的帐号<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>using( new ApplicationImpersonationContext() ) {<httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>// 如果之前调用过context.RemapHandler()，则直接返回那个HttpHandler<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers>// 把这个判断放在这里似乎没有意义，应该放在using前面，那样岂不是更快吗？【fish li个人意见】<httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers><httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers>if( handler != null )<httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers><httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers><httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers>return handler;<httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers><httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers>// 到  配置中查找一个能与requestType以及path匹配的配置项<httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers><httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers>HttpHandlerAction mapping = this.GetHandlerMapping(context, requestType, path, useAppConfig);<httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers><httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers>if( mapping == null )<httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers><httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers><httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers> throw new HttpException("Http_handler_not_found_for_request_type");<httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers><httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers>// 获取IHttpHandlerFactory对象，这是比较重要的调用。<httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers><httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers>IHttpHandlerFactory factory = this.GetFactory(mapping);<httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers><httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers>// 尝试转换成IHttpHandlerFactory2对象。设计二个IHttpHandlerFactory的原因与参数的可见类型有关。<httpHandlers>
224. <httpHandlers>
225. <httpHandlers>
226.    
227. </httpHandlers>
228. </httpHandlers>
229. </httpHandlers><httpHandlers>
230. <httpHandlers>
231. <httpHandlers>
232.    
233. </httpHandlers>
234. </httpHandlers>
235. </httpHandlers>IHttpHandlerFactory2 factory2 = factory as IHttpHandlerFactory2;<httpHandlers>
236. <httpHandlers>
237. <httpHandlers>
238.    
239. </httpHandlers>
240. </httpHandlers>
241. </httpHandlers><httpHandlers>
242. <httpHandlers>
243. <httpHandlers>
244.    
245. </httpHandlers>
246. </httpHandlers>
247. </httpHandlers>// 调用IHttpHandlerFactory对象的GetHandler方法获取具体的HttpHandler对象<httpHandlers>
248. <httpHandlers>
249. <httpHandlers>
250.    
251. </httpHandlers>
252. </httpHandlers>
253. </httpHandlers><httpHandlers>
254. <httpHandlers>
255. <httpHandlers>
256.    
257. </httpHandlers>
258. </httpHandlers>
259. </httpHandlers>if( factory2 != null )<httpHandlers>
260. <httpHandlers>
261. <httpHandlers>
262.    
263. </httpHandlers>
264. </httpHandlers>
265. </httpHandlers><httpHandlers>
266. <httpHandlers>
267. <httpHandlers>
268.    
269. </httpHandlers>
270. </httpHandlers>
271. </httpHandlers><httpHandlers>
272. <httpHandlers>
273. <httpHandlers>
274.    
275. </httpHandlers>
276. </httpHandlers>
277. </httpHandlers> handler = factory2.GetHandler(context, requestType, path, pathTranslated);<httpHandlers>
278. <httpHandlers>
279. <httpHandlers>
280.    
281. </httpHandlers>
282. </httpHandlers>
283. </httpHandlers><httpHandlers>
284. <httpHandlers>
285. <httpHandlers>
286.    
287. </httpHandlers>
288. </httpHandlers>
289. </httpHandlers><httpHandlers>
290. <httpHandlers>
291. <httpHandlers>
292.    
293. </httpHandlers>
294. </httpHandlers>
295. </httpHandlers><httpHandlers>
296. <httpHandlers>
297. <httpHandlers>
298.    
299. </httpHandlers>
300. </httpHandlers>
301. </httpHandlers><httpHandlers>
302. <httpHandlers>
303. <httpHandlers>
304.    
305. </httpHandlers>
306. </httpHandlers>
307. </httpHandlers><httpHandlers>
308. <httpHandlers>
309. <httpHandlers>
310.    
311. </httpHandlers>
312. </httpHandlers>
313. </httpHandlers>else<httpHandlers>
314. <httpHandlers>
315. <httpHandlers>
316.    
317. </httpHandlers>
318. </httpHandlers>
319. </httpHandlers><httpHandlers>
320. <httpHandlers>
321. <httpHandlers>
322.    
323. </httpHandlers>
324. </httpHandlers>
325. </httpHandlers><httpHandlers>
326. <httpHandlers>
327. <httpHandlers>
328.    
329. </httpHandlers>
330. </httpHandlers>
331. </httpHandlers> handler = factory.GetHandler(context, requestType, path.VirtualPathString, pathTranslated);<httpHandlers>
332. <httpHandlers>
333. <httpHandlers>
334.    
335. </httpHandlers>
336. </httpHandlers>
337. </httpHandlers><httpHandlers>
338. <httpHandlers>
339. <httpHandlers>
340.    
341. </httpHandlers>
342. </httpHandlers>
343. </httpHandlers>// 将这二个对象都添加到【临时】回收列表。在管线处理的结尾处，会调用HandlerWithFactory.Recycle()<httpHandlers>
344. <httpHandlers>
345. <httpHandlers>
346.    
347. </httpHandlers>
348. </httpHandlers>
349. </httpHandlers><httpHandlers>
350. <httpHandlers>
351. <httpHandlers>
352.    
353. </httpHandlers>
354. </httpHandlers>
355. </httpHandlers>// 进而会调用factory.ReleaseHandler()，最后会丢掉_handlerRecycleList的引用。<httpHandlers>
356. <httpHandlers>
357. <httpHandlers>
358.    
359. </httpHandlers>
360. </httpHandlers>
361. </httpHandlers><httpHandlers>
362. <httpHandlers>
363. <httpHandlers>
364.    
365. </httpHandlers>
366. </httpHandlers>
367. </httpHandlers>// 因此factory的重用与这里无关，它只可能会影响handler<httpHandlers>
368. <httpHandlers>
369. <httpHandlers>
370.    
371. </httpHandlers>
372. </httpHandlers>
373. </httpHandlers><httpHandlers>
374. <httpHandlers>
375. <httpHandlers>
376.    
377. </httpHandlers>
378. </httpHandlers>
379. </httpHandlers>this._handlerRecycleList.Add(new HandlerWithFactory(handler, factory));<httpHandlers>
380. <httpHandlers>
381. <httpHandlers>
382.    
383. </httpHandlers>
384. </httpHandlers>
385. </httpHandlers>}<httpHandlers>
386. <httpHandlers>
387. <httpHandlers>
388.    
389. </httpHandlers>
390. </httpHandlers>
391. </httpHandlers>return handler;}

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今天的代码将着重分析这段代码，以揭示HttpHandler的映射过程。
在这段代码中，有3个主要的调用过程：
1. this.GetHandlerMapping(context, requestType, path, useAppConfig)
2. this.GetFactory(mapping)
3. factory.GetHandler(context, requestType, path.VirtualPathString, pathTranslated)
后面将着重分析这三个调用。
HttpContext.RemapHandler()

在MapHttpHandler()方法的开始处，有这么一段代码，不知您注意没有？

1. IHttpHandler handler = (context.ServerExecuteDepth == 0) ? context.RemapHandlerInstance : null;if( handler != null )<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>return handler;

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这段代码是什么意思呢？
为了能让您较为简单地理解这段代码的意义，请看我准备的一个示例：
我创建了一个TestRemapHandler.ashx文件：

1. using System;using System.Web;public class TestRemapHandler : IHttpHandler {<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers><httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public void ProcessRequest (HttpContext context) {<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>context.Response.ContentType = "text/plain";<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>context.Response.Write("Hello TestRemapHandler");<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>}<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers> public bool IsReusable {<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>get {<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>return false;<httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>}<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers>}}

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至于这个文件在运行时能输出什么，我想我就不用截图了。
接下来，我再创建一个Global.asax文件，并写了一个事件处理方法：

1. //protected void Application_PostResolveRequestCache(object sender, EventArgs e)//{//<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>HttpApplication app = (HttpApplication)sender;//<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>if( app.Request.FilePath.EndsWith(".ashx", StringComparison.OrdinalIgnoreCase) )//<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>app.Context.RemapHandler(new MyTestHandler());//}// 或者是下面这样protected void Application_PostResolveRequestCache(object sender, EventArgs e){<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>HttpApplication app = (HttpApplication)sender;<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>if( app.Request.FilePath.EndsWith("/TestRemapHandler.ashx", StringComparison.OrdinalIgnoreCase) )<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>app.Context.RemapHandler(new MyTestHandler());}

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MyTestHandler是我定义的一个自定义的HttpHandler，后面的示例也会用到它，所以，我现在就将它的实现代码贴出来：

1. public class MyTestHandler : IHttpHandler{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private Counter _counter = new Counter();<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public bool IsReusable<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>get { return true; }<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>}<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>public void ProcessRequest(HttpContext context)<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>{<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>_counter.ShowCountAndRequestInfo(context);<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>}}

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类型Counter是一个简单的计数器，它的实现如下：

1. /// /// 一个简单的计数器/// public class Counter{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private int _count;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public void ShowCountAndRequestInfo(HttpContext context)<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>_count++;<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>context.Response.ContentType = "text/plain";<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>context.Response.Write("count: " + _count.ToString());<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>context.Response.Write("\r\n");<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>context.Response.Write(context.Request.RawUrl);<httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers>}}

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现在，您可以想像一下当我再次访问TestRemapHandler.ashx时，会在浏览器中看到什么？


以下是我看到的结果：

从截图中，我们可以看出，服务端的输出并不是TestRemapHandler.ashx产生的，而是由MyTestHandler产生的。这也说明我调用Context.RemapHandler()确实影响了后面的MapHttpHandler过程。现在我们再回过头来再看一下前面那段代码：

1. IHttpHandler handler = (context.ServerExecuteDepth == 0) ? context.RemapHandlerInstance : null;if( handler != null )<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>return handler;

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这段代码也主要是为配合HttpContext.RemapHandler()一起工作的。这里要补充的是：context.ServerExecuteDepth的判断是说有没有调用HttpServerUtility.Execute()
小结：如果我们在MapHttpHandler事件前调用HttpContext.RemapHandler()，将会直接影响后面的映射过程，或者说，我们可以直接指定一个HttpHandler，而不是让ASP.NET来替我们来决定。
HttpContext.RemapHandler()的另类用途

我在【用Asp.net写自己的服务框架】曾演示过ASP.NET URL路由的设计思路，虽然那个示例能较好地反映微软的Routing组件的工作方式，但对于示例来说，有些代码还是可以简化的。今天我将使用HttpContext.RemapHandler()来简化那段代码，简化后的版本如下：

1. internal class MyServiceUrlRoutingModule2 : IHttpModule{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>public void Init(HttpApplication app)<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>{<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>app.PostResolveRequestCache += new EventHandler(app_PostResolveRequestCache);<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>}<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>private void app_PostResolveRequestCache(object sender, EventArgs e)<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>{<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>HttpApplication app = (HttpApplication)sender;<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>// 根据HttpContext，获取合适的HttpHandler，这个过程俗称路由<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>// 通常就是根据URL去主动寻找一个合适的HttpHandler<httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>// 查找过程仍使用上一版的方法<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers>MyServiceHandler handler = MyServiceUrlRoutingModule.GetHandler(app.Context);<httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers><httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers>if( handler != null )<httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers><httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers><httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers>// 直接设置已找到的HttpHandler<httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers><httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers><httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers>app.Context.RemapHandler(handler);<httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers>}<httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers>public void Dispose()<httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers>{<httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers>}}

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在MyServiceUrlRoutingModule2这个新版本中，基本上跳过了MapHttpHandler()中所有复杂的处理逻辑，因此会更快。
这种方法虽然更简单，但它只能设置一个参数IHttpHandler，因此，如果有其它的参数需要一起传递，则要修改相关的HttpHandler类型，以便容纳需要传递的参数。例如，在【我的服务框架】中，MyServiceHandler类型就专门定义了一个属性ServiceInfo来保存要调用的方法信息。MyServiceUrlRoutingModule.GetHandler()的实现代码如下：

1. internal static MyServiceHandler GetHandler(HttpContext context){<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>NamesPair pair = FrameworkRules.ParseNamesPair(context.Request);<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>if( pair == null )<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>return null;<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>InvokeInfo vkInfo = ReflectionHelper.GetInvokeInfo(pair);<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>if( vkInfo == null )<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>ExceptionHelper.Throw404Exception(context);<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>MyServiceHandler handler = MyServiceHandlerFactory.GetHandler(vkInfo);<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>handler.ServiceInfo = new ServiceInfo(pair, vkInfo);<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>return handler;}

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写到这里，有个问题把我难住了：为什么ASP.NET 3.5 UrlRoutingModule不用RemapHandler()而是采用较复杂的方法呢？难道是为了能保留二个参数吗？但那个【context.Request.Path】没有必要保存呢。实在是没有想通，最后想到ASP.NET 4.0 UrlRoutingModule会不会不一样了？于是，只好再次安装 .net framework 4.0 了（没办法，老家的旧机器上还真没有.net 4.0），结果当我用Reflector.exe找到4.0版本的UrlRoutingModule时，发现它已被移到System.Web.dll中，而且，ASPL.NET 4.0也在使用RemapHandler() ！ 看来微软当初忘记了这个方法。
GetHandlerMapping()

在MapHttpHandler()方法中，有下面这个调用：

1. // 到 <httpHandlers> 配置中查找一个能与requestType以及path匹配的配置项
2. HttpHandlerAction mapping = this.GetHandlerMapping(context, requestType, path, useAppConfig);

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接下来，我们再来看一下这个调用的实现代码：

1. private HttpHandlerAction GetHandlerMapping(HttpContext context, string requestType, VirtualPath path, bool useAppConfig){<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>CachedPathData pathData = null;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>HandlerMappingMemo cachedHandler = null;<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>HttpHandlerAction mapping = null;<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>if (!useAppConfig)<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>{<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>// 先从缓存中查找<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>pathData = context.GetPathData(path);<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>cachedHandler = pathData.CachedHandler;<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>if ((cachedHandler != null) && !cachedHandler.IsMatch(requestType, path))<httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>{<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers><httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers>cachedHandler = null;<httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers><httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers>}<httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers>}<httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers>if (cachedHandler == null)<httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers>{<httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers><httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers>// 根据配置文件，查找映射<httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers><httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers>mapping = (useAppConfig ? RuntimeConfig.GetAppConfig().HttpHandlers<httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers><httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers><httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers><httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers><httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers><httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers><httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers><httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers> : RuntimeConfig.GetConfig(context).HttpHandlers)<httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers><httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers><httpHandlers>
224. <httpHandlers>
225. <httpHandlers>
226.    
227. </httpHandlers>
228. </httpHandlers>
229. </httpHandlers>.FindMapping(requestType, path);<httpHandlers>
230. <httpHandlers>
231. <httpHandlers>
232.    
233. </httpHandlers>
234. </httpHandlers>
235. </httpHandlers><httpHandlers>
236. <httpHandlers>
237. <httpHandlers>
238.    
239. </httpHandlers>
240. </httpHandlers>
241. </httpHandlers>if (!useAppConfig)<httpHandlers>
242. <httpHandlers>
243. <httpHandlers>
244.    
245. </httpHandlers>
246. </httpHandlers>
247. </httpHandlers><httpHandlers>
248. <httpHandlers>
249. <httpHandlers>
250.    
251. </httpHandlers>
252. </httpHandlers>
253. </httpHandlers>{<httpHandlers>
254. <httpHandlers>
255. <httpHandlers>
256.    
257. </httpHandlers>
258. </httpHandlers>
259. </httpHandlers><httpHandlers>
260. <httpHandlers>
261. <httpHandlers>
262.    
263. </httpHandlers>
264. </httpHandlers>
265. </httpHandlers><httpHandlers>
266. <httpHandlers>
267. <httpHandlers>
268.    
269. </httpHandlers>
270. </httpHandlers>
271. </httpHandlers>// 将查找到的结果放入缓存<httpHandlers>
272. <httpHandlers>
273. <httpHandlers>
274.    
275. </httpHandlers>
276. </httpHandlers>
277. </httpHandlers><httpHandlers>
278. <httpHandlers>
279. <httpHandlers>
280.    
281. </httpHandlers>
282. </httpHandlers>
283. </httpHandlers><httpHandlers>
284. <httpHandlers>
285. <httpHandlers>
286.    
287. </httpHandlers>
288. </httpHandlers>
289. </httpHandlers>cachedHandler = new HandlerMappingMemo(mapping, requestType, path);<httpHandlers>
290. <httpHandlers>
291. <httpHandlers>
292.    
293. </httpHandlers>
294. </httpHandlers>
295. </httpHandlers><httpHandlers>
296. <httpHandlers>
297. <httpHandlers>
298.    
299. </httpHandlers>
300. </httpHandlers>
301. </httpHandlers><httpHandlers>
302. <httpHandlers>
303. <httpHandlers>
304.    
305. </httpHandlers>
306. </httpHandlers>
307. </httpHandlers>pathData.CachedHandler = cachedHandler;<httpHandlers>
308. <httpHandlers>
309. <httpHandlers>
310.    
311. </httpHandlers>
312. </httpHandlers>
313. </httpHandlers><httpHandlers>
314. <httpHandlers>
315. <httpHandlers>
316.    
317. </httpHandlers>
318. </httpHandlers>
319. </httpHandlers>}<httpHandlers>
320. <httpHandlers>
321. <httpHandlers>
322.    
323. </httpHandlers>
324. </httpHandlers>
325. </httpHandlers><httpHandlers>
326. <httpHandlers>
327. <httpHandlers>
328.    
329. </httpHandlers>
330. </httpHandlers>
331. </httpHandlers>return mapping;<httpHandlers>
332. <httpHandlers>
333. <httpHandlers>
334.    
335. </httpHandlers>
336. </httpHandlers>
337. </httpHandlers>}<httpHandlers>
338. <httpHandlers>
339. <httpHandlers>
340.    
341. </httpHandlers>
342. </httpHandlers>
343. </httpHandlers>return cachedHandler.Mapping;}

复制代码

代码中的重要阶段，我加了一点注释。我们尤其要注意的FindMapping(requestType, path)的那个调用。
但前面的那个RuntimeConfig.GetAppConfig().HttpHandlers 和RuntimeConfig.GetConfig(context).HttpHandlers)又可以得到什么结果呢？
为了能让您更容易地理解这个调用，我准备了一段代码，可以直观地显示这个调用结果：

1. public partial class TestRuntimeConfig : System.Web.UI.Page{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>protected string HttpHandlers;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>protected void Page_Load(object sender, EventArgs e)<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>string typeName = typeof(HttpRequest).AssemblyQualifiedName<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>.Replace("HttpRequest", "Configuration.RuntimeConfig");<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers>Type type = Type.GetType(typeName);<httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers><httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers>bool useAppConfig = Request.QueryString["useAppConfig"] == "1";<httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers><httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers>// 由于RuntimeConfig类型的可见性是internal，<httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers><httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers>// 所以，我不能直接用它声明变量，只能使用object类型<httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers><httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers>object config = null;<httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers><httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers>if( useAppConfig )<httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers><httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers><httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers>config = type.InvokeMember("GetAppConfig",<httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers><httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers><httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers><httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers>BindingFlags.InvokeMethod | BindingFlags.Static | BindingFlags.NonPublic,<httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers><httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers><httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers><httpHandlers>
224. <httpHandlers>
225. <httpHandlers>
226.    
227. </httpHandlers>
228. </httpHandlers>
229. </httpHandlers>null, null, null);<httpHandlers>
230. <httpHandlers>
231. <httpHandlers>
232.    
233. </httpHandlers>
234. </httpHandlers>
235. </httpHandlers><httpHandlers>
236. <httpHandlers>
237. <httpHandlers>
238.    
239. </httpHandlers>
240. </httpHandlers>
241. </httpHandlers>else<httpHandlers>
242. <httpHandlers>
243. <httpHandlers>
244.    
245. </httpHandlers>
246. </httpHandlers>
247. </httpHandlers><httpHandlers>
248. <httpHandlers>
249. <httpHandlers>
250.    
251. </httpHandlers>
252. </httpHandlers>
253. </httpHandlers><httpHandlers>
254. <httpHandlers>
255. <httpHandlers>
256.    
257. </httpHandlers>
258. </httpHandlers>
259. </httpHandlers>config = type.InvokeMember("GetConfig",<httpHandlers>
260. <httpHandlers>
261. <httpHandlers>
262.    
263. </httpHandlers>
264. </httpHandlers>
265. </httpHandlers><httpHandlers>
266. <httpHandlers>
267. <httpHandlers>
268.    
269. </httpHandlers>
270. </httpHandlers>
271. </httpHandlers><httpHandlers>
272. <httpHandlers>
273. <httpHandlers>
274.    
275. </httpHandlers>
276. </httpHandlers>
277. </httpHandlers><httpHandlers>
278. <httpHandlers>
279. <httpHandlers>
280.    
281. </httpHandlers>
282. </httpHandlers>
283. </httpHandlers>BindingFlags.InvokeMethod | BindingFlags.Static | BindingFlags.NonPublic,<httpHandlers>
284. <httpHandlers>
285. <httpHandlers>
286.    
287. </httpHandlers>
288. </httpHandlers>
289. </httpHandlers><httpHandlers>
290. <httpHandlers>
291. <httpHandlers>
292.    
293. </httpHandlers>
294. </httpHandlers>
295. </httpHandlers><httpHandlers>
296. <httpHandlers>
297. <httpHandlers>
298.    
299. </httpHandlers>
300. </httpHandlers>
301. </httpHandlers><httpHandlers>
302. <httpHandlers>
303. <httpHandlers>
304.    
305. </httpHandlers>
306. </httpHandlers>
307. </httpHandlers>null, null, new object[] { this.Context });<httpHandlers>
308. <httpHandlers>
309. <httpHandlers>
310.    
311. </httpHandlers>
312. </httpHandlers>
313. </httpHandlers><httpHandlers>
314. <httpHandlers>
315. <httpHandlers>
316.    
317. </httpHandlers>
318. </httpHandlers>
319. </httpHandlers>HttpHandlersSection section = (HttpHandlersSection)type.InvokeMember("HttpHandlers",<httpHandlers>
320. <httpHandlers>
321. <httpHandlers>
322.    
323. </httpHandlers>
324. </httpHandlers>
325. </httpHandlers><httpHandlers>
326. <httpHandlers>
327. <httpHandlers>
328.    
329. </httpHandlers>
330. </httpHandlers>
331. </httpHandlers><httpHandlers>
332. <httpHandlers>
333. <httpHandlers>
334.    
335. </httpHandlers>
336. </httpHandlers>
337. </httpHandlers> BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.NonPublic,<httpHandlers>
338. <httpHandlers>
339. <httpHandlers>
340.    
341. </httpHandlers>
342. </httpHandlers>
343. </httpHandlers><httpHandlers>
344. <httpHandlers>
345. <httpHandlers>
346.    
347. </httpHandlers>
348. </httpHandlers>
349. </httpHandlers><httpHandlers>
350. <httpHandlers>
351. <httpHandlers>
352.    
353. </httpHandlers>
354. </httpHandlers>
355. </httpHandlers> null, config, null);<httpHandlers>
356. <httpHandlers>
357. <httpHandlers>
358.    
359. </httpHandlers>
360. </httpHandlers>
361. </httpHandlers><httpHandlers>
362. <httpHandlers>
363. <httpHandlers>
364.    
365. </httpHandlers>
366. </httpHandlers>
367. </httpHandlers>HttpHandlers = string.Format("总共 {0} 个配置项。
368. ", section.Handlers.Count) +<httpHandlers>
369. <httpHandlers>
370. <httpHandlers>
371.    
372. </httpHandlers>
373. </httpHandlers>
374. </httpHandlers><httpHandlers>
375. <httpHandlers>
376. <httpHandlers>
377.    
378. </httpHandlers>
379. </httpHandlers>
380. </httpHandlers><httpHandlers>
381. <httpHandlers>
382. <httpHandlers>
383.    
384. </httpHandlers>
385. </httpHandlers>
386. </httpHandlers>string.Join("
387. ", (<httpHandlers>
388. <httpHandlers>
389. <httpHandlers>
390.    
391. </httpHandlers>
392. </httpHandlers>
393. </httpHandlers><httpHandlers>
394. <httpHandlers>
395. <httpHandlers>
396.    
397. </httpHandlers>
398. </httpHandlers>
399. </httpHandlers><httpHandlers>
400. <httpHandlers>
401. <httpHandlers>
402.    
403. </httpHandlers>
404. </httpHandlers>
405. </httpHandlers><httpHandlers>
406. <httpHandlers>
407. <httpHandlers>
408.    
409. </httpHandlers>
410. </httpHandlers>
411. </httpHandlers>from h in section.Handlers.Cast()<httpHandlers>
412. <httpHandlers>
413. <httpHandlers>
414.    
415. </httpHandlers>
416. </httpHandlers>
417. </httpHandlers><httpHandlers>
418. <httpHandlers>
419. <httpHandlers>
420.    
421. </httpHandlers>
422. </httpHandlers>
423. </httpHandlers><httpHandlers>
424. <httpHandlers>
425. <httpHandlers>
426.    
427. </httpHandlers>
428. </httpHandlers>
429. </httpHandlers><httpHandlers>
430. <httpHandlers>
431. <httpHandlers>
432.    
433. </httpHandlers>
434. </httpHandlers>
435. </httpHandlers>let action = string.Format("path="{0}" verb="{1}" validate="{2}" type="{3}"",<httpHandlers>
436. <httpHandlers>
437. <httpHandlers>
438.    
439. </httpHandlers>
440. </httpHandlers>
441. </httpHandlers><httpHandlers>
442. <httpHandlers>
443. <httpHandlers>
444.    
445. </httpHandlers>
446. </httpHandlers>
447. </httpHandlers><httpHandlers>
448. <httpHandlers>
449. <httpHandlers>
450.    
451. </httpHandlers>
452. </httpHandlers>
453. </httpHandlers><httpHandlers>
454. <httpHandlers>
455. <httpHandlers>
456.    
457. </httpHandlers>
458. </httpHandlers>
459. </httpHandlers><httpHandlers>
460. <httpHandlers>
461. <httpHandlers>
462.    
463. </httpHandlers>
464. </httpHandlers>
465. </httpHandlers><httpHandlers>
466. <httpHandlers>
467. <httpHandlers>
468.    
469. </httpHandlers>
470. </httpHandlers>
471. </httpHandlers>h.Path, h.Verb, h.Validate, h.Type)<httpHandlers>
472. <httpHandlers>
473. <httpHandlers>
474.    
475. </httpHandlers>
476. </httpHandlers>
477. </httpHandlers><httpHandlers>
478. <httpHandlers>
479. <httpHandlers>
480.    
481. </httpHandlers>
482. </httpHandlers>
483. </httpHandlers><httpHandlers>
484. <httpHandlers>
485. <httpHandlers>
486.    
487. </httpHandlers>
488. </httpHandlers>
489. </httpHandlers><httpHandlers>
490. <httpHandlers>
491. <httpHandlers>
492.    
493. </httpHandlers>
494. </httpHandlers>
495. </httpHandlers>select action).ToArray());<httpHandlers>
496. <httpHandlers>
497. <httpHandlers>
498.    
499. </httpHandlers>
500. </httpHandlers>
501. </httpHandlers>}}

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我的示例网站中的web.config中的配置节定义如下：

1. <httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers><httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>

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那么，前面的示例代码的运行结果如下：

通过前面分析可知，代码中的HttpHandlers实际就是在运行时所能访问到的配置集合。而且，我们在网站中所指定的配置会放在集合的前面，ASP.NET的默认配置会在靠后的位置。接下来的FindMapping(requestType, path)的实现如下：

1. internal HttpHandlerAction FindMapping(string verb, VirtualPath path){<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>this.ValidateHandlers();<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>for (int i = 0; i < this.Handlers.Count; i++)<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>HttpHandlerAction action = this.Handlers[i];<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>if (action.IsMatch(verb, path))<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>{<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>return action;<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers>}<httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>}<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers>return null;}

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从代码可以看出，其实所谓的【查找】过程，也就是【逐一匹配】每个在定义的配置项。
至于action.IsMatch()的匹配方式，我们可以从调用所传递的参数看出，其实只是判断【verb, path】这二个参数而已。这个匹配过程的实现代码较长，我就不贴出它们的实现代码了，而是打算通过示例来说明它是如何匹配的。
action.IsMatch()的匹配方式也可以算是GetHandlerMapping()的核心过程。而这里的action的类型其实是HttpHandlerAction，它与配置项一一对应。我们可以先来看一下MSDN是如何解释这个配置项的：

从MSDN的解释中可以看出：verb,path都可以支持【通配符(*)】，并且可以使用逗号(,)来分隔多个预期项。ASP.NET在实现通配符时，内部使用正则表达式的方式，具体过程可以自己去阅读相关代码。
下面，我将通过一个具体的配置项来分析这些配置参数：

1. [/code]1. path="Ajax*.*.aspx,Ajax*/*.aspx"：表示可以接受【Ajax*.*.aspx】或者【Ajax*/*.aspx】这样的URL格式。
2. 2. verb="*": 表示可以接受所有的HTTP调用，如：GET,POST 等等。
3. 3. type="MySimpleServiceFramework.AjaxServiceHandler, MySimpleServiceFramework": 表示当某个请求匹配path,verb时，交给MySimpleServiceFramework程序集中的MySimpleServiceFramework.AjaxServiceHandler来处理请求。
4. 4. validate="true"：表示在第一次读取时，验证type的设置是否有效。如果此项设为false，则表示延迟验证（创建时）。默认值：true
5. 对于这4个属性，有2个比较重要：
6. 1. type:  type中的字符串必须能在运行时找到一个可创建的类型，且该类型必须实现IHttpHandler或者IHttpHandlerFactory接口。
7. 2. path:  在很多资料以及技术书籍中，一般都设置为[b]某类文件扩展名[/b]，如：*.xxx ，事实上，我们完全可以设置为一个URL模式，而且还可以设置多个URL模式。比如上面的配置示例中的path参数，具体在使用时，可以响应来自客户端的以下请求：
8. [code]$(function(){<httpHandlers>
9. <httpHandlers>
10. <httpHandlers>
11.    
12. </httpHandlers>
13. </httpHandlers>
14. </httpHandlers>$("#btnSubmit").click( function(){<httpHandlers>
15. <httpHandlers>
16. <httpHandlers>
17.    
18. </httpHandlers>
19. </httpHandlers>
20. </httpHandlers><httpHandlers>
21. <httpHandlers>
22. <httpHandlers>
23.    
24. </httpHandlers>
25. </httpHandlers>
26. </httpHandlers>$.ajax({<httpHandlers>
27. <httpHandlers>
28. <httpHandlers>
29.    
30. </httpHandlers>
31. </httpHandlers>
32. </httpHandlers><httpHandlers>
33. <httpHandlers>
34. <httpHandlers>
35.    
36. </httpHandlers>
37. </httpHandlers>
38. </httpHandlers><httpHandlers>
39. <httpHandlers>
40. <httpHandlers>
41.    
42. </httpHandlers>
43. </httpHandlers>
44. </httpHandlers><httpHandlers>
45. <httpHandlers>
46. <httpHandlers>
47.    
48. </httpHandlers>
49. </httpHandlers>
50. </httpHandlers>// 注意：下面的二个url地址都是可以使用的。<httpHandlers>
51. <httpHandlers>
52. <httpHandlers>
53.    
54. </httpHandlers>
55. </httpHandlers>
56. </httpHandlers><httpHandlers>
57. <httpHandlers>
58. <httpHandlers>
59.    
60. </httpHandlers>
61. </httpHandlers>
62. </httpHandlers><httpHandlers>
63. <httpHandlers>
64. <httpHandlers>
65.    
66. </httpHandlers>
67. </httpHandlers>
68. </httpHandlers><httpHandlers>
69. <httpHandlers>
70. <httpHandlers>
71.    
72. </httpHandlers>
73. </httpHandlers>
74. </httpHandlers>url: "/AjaxServices/GetMd5.aspx",<httpHandlers>
75. <httpHandlers>
76. <httpHandlers>
77.    
78. </httpHandlers>
79. </httpHandlers>
80. </httpHandlers><httpHandlers>
81. <httpHandlers>
82. <httpHandlers>
83.    
84. </httpHandlers>
85. </httpHandlers>
86. </httpHandlers><httpHandlers>
87. <httpHandlers>
88. <httpHandlers>
89.    
90. </httpHandlers>
91. </httpHandlers>
92. </httpHandlers><httpHandlers>
93. <httpHandlers>
94. <httpHandlers>
95.    
96. </httpHandlers>
97. </httpHandlers>
98. </httpHandlers>//url: "/AjaxServices.GetMd5.aspx",<httpHandlers>
99. <httpHandlers>
100. <httpHandlers>
101.    
102. </httpHandlers>
103. </httpHandlers>
104. </httpHandlers><httpHandlers>
105. <httpHandlers>
106. <httpHandlers>
107.    
108. </httpHandlers>
109. </httpHandlers>
110. </httpHandlers><httpHandlers>
111. <httpHandlers>
112. <httpHandlers>
113.    
114. </httpHandlers>
115. </httpHandlers>
116. </httpHandlers><httpHandlers>
117. <httpHandlers>
118. <httpHandlers>
119.    
120. </httpHandlers>
121. </httpHandlers>
122. </httpHandlers><httpHandlers>
123. <httpHandlers>
124. <httpHandlers>
125.    
126. </httpHandlers>
127. </httpHandlers>
128. </httpHandlers><httpHandlers>
129. <httpHandlers>
130. <httpHandlers>
131.    
132. </httpHandlers>
133. </httpHandlers>
134. </httpHandlers><httpHandlers>
135. <httpHandlers>
136. <httpHandlers>
137.    
138. </httpHandlers>
139. </httpHandlers>
140. </httpHandlers><httpHandlers>
141. <httpHandlers>
142. <httpHandlers>
143.    
144. </httpHandlers>
145. </httpHandlers>
146. </httpHandlers>// 注意：下面的二种type的设置也都是可以使用的。<httpHandlers>
147. <httpHandlers>
148. <httpHandlers>
149.    
150. </httpHandlers>
151. </httpHandlers>
152. </httpHandlers><httpHandlers>
153. <httpHandlers>
154. <httpHandlers>
155.    
156. </httpHandlers>
157. </httpHandlers>
158. </httpHandlers><httpHandlers>
159. <httpHandlers>
160. <httpHandlers>
161.    
162. </httpHandlers>
163. </httpHandlers>
164. </httpHandlers><httpHandlers>
165. <httpHandlers>
166. <httpHandlers>
167.    
168. </httpHandlers>
169. </httpHandlers>
170. </httpHandlers>type: "POST",<httpHandlers>
171. <httpHandlers>
172. <httpHandlers>
173.    
174. </httpHandlers>
175. </httpHandlers>
176. </httpHandlers><httpHandlers>
177. <httpHandlers>
178. <httpHandlers>
179.    
180. </httpHandlers>
181. </httpHandlers>
182. </httpHandlers><httpHandlers>
183. <httpHandlers>
184. <httpHandlers>
185.    
186. </httpHandlers>
187. </httpHandlers>
188. </httpHandlers><httpHandlers>
189. <httpHandlers>
190. <httpHandlers>
191.    
192. </httpHandlers>
193. </httpHandlers>
194. </httpHandlers> //type: "GET",<httpHandlers>
195. <httpHandlers>
196. <httpHandlers>
197.    
198. </httpHandlers>
199. </httpHandlers>
200. </httpHandlers><httpHandlers>
201. <httpHandlers>
202. <httpHandlers>
203.    
204. </httpHandlers>
205. </httpHandlers>
206. </httpHandlers><httpHandlers>
207. <httpHandlers>
208. <httpHandlers>
209.    
210. </httpHandlers>
211. </httpHandlers>
212. </httpHandlers><httpHandlers>
213. <httpHandlers>
214. <httpHandlers>
215.    
216. </httpHandlers>
217. </httpHandlers>
218. </httpHandlers><httpHandlers>
219. <httpHandlers>
220. <httpHandlers>
221.    
222. </httpHandlers>
223. </httpHandlers>
224. </httpHandlers><httpHandlers>
225. <httpHandlers>
226. <httpHandlers>
227.    
228. </httpHandlers>
229. </httpHandlers>
230. </httpHandlers><httpHandlers>
231. <httpHandlers>
232. <httpHandlers>
233.    
234. </httpHandlers>
235. </httpHandlers>
236. </httpHandlers><httpHandlers>
237. <httpHandlers>
238. <httpHandlers>
239.    
240. </httpHandlers>
241. </httpHandlers>
242. </httpHandlers> data: { str: $("#txtInput").val() },<httpHandlers>
243. <httpHandlers>
244. <httpHandlers>
245.    
246. </httpHandlers>
247. </httpHandlers>
248. </httpHandlers><httpHandlers>
249. <httpHandlers>
250. <httpHandlers>
251.    
252. </httpHandlers>
253. </httpHandlers>
254. </httpHandlers><httpHandlers>
255. <httpHandlers>
256. <httpHandlers>
257.    
258. </httpHandlers>
259. </httpHandlers>
260. </httpHandlers><httpHandlers>
261. <httpHandlers>
262. <httpHandlers>
263.    
264. </httpHandlers>
265. </httpHandlers>
266. </httpHandlers>dataType: "text",<httpHandlers>
267. <httpHandlers>
268. <httpHandlers>
269.    
270. </httpHandlers>
271. </httpHandlers>
272. </httpHandlers><httpHandlers>
273. <httpHandlers>
274. <httpHandlers>
275.    
276. </httpHandlers>
277. </httpHandlers>
278. </httpHandlers><httpHandlers>
279. <httpHandlers>
280. <httpHandlers>
281.    
282. </httpHandlers>
283. </httpHandlers>
284. </httpHandlers><httpHandlers>
285. <httpHandlers>
286. <httpHandlers>
287.    
288. </httpHandlers>
289. </httpHandlers>
290. </httpHandlers>success: function(responseText){<httpHandlers>
291. <httpHandlers>
292. <httpHandlers>
293.    
294. </httpHandlers>
295. </httpHandlers>
296. </httpHandlers><httpHandlers>
297. <httpHandlers>
298. <httpHandlers>
299.    
300. </httpHandlers>
301. </httpHandlers>
302. </httpHandlers><httpHandlers>
303. <httpHandlers>
304. <httpHandlers>
305.    
306. </httpHandlers>
307. </httpHandlers>
308. </httpHandlers><httpHandlers>
309. <httpHandlers>
310. <httpHandlers>
311.    
312. </httpHandlers>
313. </httpHandlers>
314. </httpHandlers><httpHandlers>
315. <httpHandlers>
316. <httpHandlers>
317.    
318. </httpHandlers>
319. </httpHandlers>
320. </httpHandlers>$("#md5Result").text(responseText);<httpHandlers>
321. <httpHandlers>
322. <httpHandlers>
323.    
324. </httpHandlers>
325. </httpHandlers>
326. </httpHandlers><httpHandlers>
327. <httpHandlers>
328. <httpHandlers>
329.    
330. </httpHandlers>
331. </httpHandlers>
332. </httpHandlers><httpHandlers>
333. <httpHandlers>
334. <httpHandlers>
335.    
336. </httpHandlers>
337. </httpHandlers>
338. </httpHandlers><httpHandlers>
339. <httpHandlers>
340. <httpHandlers>
341.    
342. </httpHandlers>
343. </httpHandlers>
344. </httpHandlers>}<httpHandlers>
345. <httpHandlers>
346. <httpHandlers>
347.    
348. </httpHandlers>
349. </httpHandlers>
350. </httpHandlers><httpHandlers>
351. <httpHandlers>
352. <httpHandlers>
353.    
354. </httpHandlers>
355. </httpHandlers>
356. </httpHandlers><httpHandlers>
357. <httpHandlers>
358. <httpHandlers>
359.    
360. </httpHandlers>
361. </httpHandlers>
362. </httpHandlers>});<httpHandlers>
363. <httpHandlers>
364. <httpHandlers>
365.    
366. </httpHandlers>
367. </httpHandlers>
368. </httpHandlers><httpHandlers>
369. <httpHandlers>
370. <httpHandlers>
371.    
372. </httpHandlers>
373. </httpHandlers>
374. </httpHandlers>});});

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注意：在path中直接使用某个扩展名的好处是不受配置项的顺序影响，而且容易理解，因为可读性较好。
这里要补充一点：MySimpleServiceFramework是我在博客【用Asp.net写自己的服务框架】中实现的那个框架。
在服务端，可以使用下面的代码来处理前面客户端发出的请求：

1. [MyService]public class AjaxServices{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>[MyServiceMethod]<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public string GetMd5(string str)<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>if( str == null )<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>str = string.Empty;<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>byte[] bb = (new MD5CryptoServiceProvider()).ComputeHash(Encoding.Default.GetBytes(str));<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>return BitConverter.ToString(bb).Replace("-", "");<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>}}

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前面的示例配置中：path="Ajax*.*.aspx,Ajax*/*.aspx" ，我使用了aspx这个扩展名。
我想大家都知道aspx这个扩展名是ASP.NET可处理的扩展名，为什么我还可以使用呢？
其实这个问题的答案在我前面的截图以及FindMapping的实现代码中：由于我们指定的配置要优先于ASP.NET的默认配置，所以先有机会参与匹配，并能匹配成功。
小结：在GetHandlerMapping()过程中，会根据请求的URL地址以及HTTP调用动作（GET,POST），返回一个在定义的配置项（HttpHandlerAction类型）。
GetFactory()

在GetHandlerMapping()返回一个HttpHandlerAction的配置项后，HttpApplication会调用this.GetFactory(mapping);获取一个IHttpHandlerFactory ，本小节将来分析这个过程。
首先，我们还是来看一下GetFactory的实现代码：

1. private Hashtable _handlerFactories = new Hashtable();private IHttpHandlerFactory GetFactory(HttpHandlerAction mapping){<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>// 先尝试从内部缓存中获取<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>HandlerFactoryCache cache = (HandlerFactoryCache) this._handlerFactories[mapping.Type];<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>if (cache == null)<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>{<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>// 创建一个缓存项并保存<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>cache = new HandlerFactoryCache(mapping);<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>this._handlerFactories[mapping.Type] = cache;<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>}<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>// 返回工厂对象<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers>return cache.Factory;}

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代码中做了哪些事情，我在注释中有说明。这个方法的最后一定会返回一个IHttpHandlerFactory的实例。

看到这里，您觉得奇怪吗？
可能我们仅仅只是实现了一个自定义的IHttpHanlder接口，并没有实现IHttpHandlerFactory，那么ASP.NET是如何处理的呢？
请注意 new HandlerFactoryCache(mapping) 这行代码，这可不是简单地创建一个缓存对象，具体实现代码如下：

1. internal class HandlerFactoryCache{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private IHttpHandlerFactory _factory;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>internal HandlerFactoryCache(HttpHandlerAction mapping)<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>// 根据HttpHandlerAction配置中的type字符串创建对应类型的对象<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>object obj2 = mapping.Create();<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>if( obj2 is IHttpHandler ) {<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>// 如果我们的配置是一个IHttpHandler，<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers>// 那么ASP.NET会为我们包装成一个HandlerFactoryWrapper<httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers><httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers>this._factory = new HandlerFactoryWrapper((IHttpHandler)obj2, this.GetHandlerType(mapping));<httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers><httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers>}<httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers><httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers>else {<httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers><httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers><httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers>// 如果不是IHttpHandler以及IHttpHandlerFactory，就抛出异常<httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers><httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers><httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers>if( !(obj2 is IHttpHandlerFactory) ) {<httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers><httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers><httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers><httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers>throw new HttpException("Type_not_factory_or_handler");<httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers><httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers><httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers>}<httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers><httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers><httpHandlers>
224. <httpHandlers>
225. <httpHandlers>
226.    
227. </httpHandlers>
228. </httpHandlers>
229. </httpHandlers>this._factory = (IHttpHandlerFactory)obj2;<httpHandlers>
230. <httpHandlers>
231. <httpHandlers>
232.    
233. </httpHandlers>
234. </httpHandlers>
235. </httpHandlers><httpHandlers>
236. <httpHandlers>
237. <httpHandlers>
238.    
239. </httpHandlers>
240. </httpHandlers>
241. </httpHandlers>}<httpHandlers>
242. <httpHandlers>
243. <httpHandlers>
244.    
245. </httpHandlers>
246. </httpHandlers>
247. </httpHandlers>}

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代码中，我加入的注释已经回答了前面提出的问题：如果是一个IHttpHandler对象，ASP.NET会再创建一个HandlerFactoryWrapper对象来包装IHttpHandler对象，如果是IHttpHandlerFactory对象，则直接返回。
我们再来看一下HandlerFactoryWrapper的实现代码：

1. internal class HandlerFactoryWrapper : IHttpHandlerFactory{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private IHttpHandler _handler;<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>private Type _handlerType;<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>internal HandlerFactoryWrapper(IHttpHandler handler, Type handlerType)<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>{<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>this._handler = handler;<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>this._handlerType = handlerType;<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>}<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>public IHttpHandler GetHandler(HttpContext context,<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers><httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers><httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers> string requestType, string url, string pathTranslated)<httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers>{<httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers><httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers>if( this._handler == null )<httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers><httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers><httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers> this._handler = (IHttpHandler)HttpRuntime.CreateNonPublicInstance(this._handlerType);<httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers><httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers>return this._handler;<httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers>}<httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers>public void ReleaseHandler(IHttpHandler handler)<httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers>{<httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers><httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers>// 一个HttpHandler是否能重用，这里就是一个典型的实现方式<httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers><httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers>if( !this._handler.IsReusable )<httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers><httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers><httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers> this._handler = null;<httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers>}}

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小结：HttpApplication会根据web.config中的配置去查找一个匹配的IHttpHandlerFactory类型, 即使我们配置的是自定义的IHttpHandler类型，而不是IHttpHandlerFactory类型，调用过程依然如此。
GetHandler()

在ASP.NET中，定义了二个版本的HttpHandlerFactory接口，分别为：

1. public interface IHttpHandlerFactory{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>IHttpHandler GetHandler(HttpContext context,<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers><httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers><httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers> string requestType, string url, string pathTranslated);<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers>void ReleaseHandler(IHttpHandler handler);}internal interface IHttpHandlerFactory2 : IHttpHandlerFactory{<httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>IHttpHandler GetHandler(HttpContext context,<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers><httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers><httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers><httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers> string requestType, VirtualPath virtualPath, string physicalPath);}

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设计IHttpHandlerFactory接口的目的是为了在创建和重用IHttpHandler对象时，保留了足够的扩展机会，而IHttpHandlerFactory2则是一个仅供微软使用的内部接口（因为VirtualPath类型的可见性也是internal）。
我们都知道aspx, ashx能直接处理HTTP请求，它们都实现了IHttpHandler接口。它们能处理HTTP请求也因为ASP.NET已经配置过它们。以下是它们的默认配置：

1. <httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers><httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>

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有趣的是：PageHandlerFactory和SimpleHandlerFactory都实现了IHttpHandlerFactory2接口，因此，它们都可以根据要请求的路径创建一个IHttpHandler实例。
从ASP.NET的默认配置，我们也可以看到：type参数是可以设置为一个实现IHttpHandlerFactory接口的类型，而不一定要求是实现IHttpHandler接口的类型。
小结：HttpApplication在处理请求时，并不会直接创建一个IHttpHandler的实例，而是先获取一个IHttpHandlerFactory的对象，再以接口的形式调用GetHandler()方法来获取一个IHttpHandler实例。
IHttpHandler.IsReusable

IHttpHandler接口有个IsReusable属性。MSDN对这个属性的说明也非常简单：
获取一个值，该值指示其他请求是否可以使用 IHttpHandler 实例。
按照这个说明，当我们直接在创建一个实现IHttpHandler的类型，并在web.config中注册到一个自定义的扩展名时，情况会如何呢？
我们再来看一下前面所提过的示例代码（MyTestHandler的）：

1. public class MyTestHandler : IHttpHandler{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private Counter _counter = new Counter();<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public bool IsReusable<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>get { return true; }<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>}<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>public void ProcessRequest(HttpContext context)<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>{<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>_counter.ShowCountAndRequestInfo(context);<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>}}

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web.config中的配置为：

1. <httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>

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当我连续5次访问 http://localhost:51652/abc.test?id=1 时，会在浏览器中看到以下输出结果：

从这个截图来看，显然：MyTestHandler的实例被重用了。

我想很多人都创建过ashx文件，IDE会为我们创建一个实现了IHttpHandler接口的类型，在实现IsReusable属性时，一般都会这样：

1. public bool IsReusable {<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>get {<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers><httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>return false;<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>}}

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有些人看到这个，会想：如果返回true，就可以重用IHttpHandler实例，达到优化性能的目的。但事实上，即使你在ashx中返回true也是无意义的，因为您可以试着这样去实现这个属性：

1. public bool IsReusable{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>get { throw new Exception("这里不起作用。"); }}

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如果您访问那个ashx，会发现：根本没有异常出现！
因此，我们可以得出一个结论：默认情况下，IsReusable不能决定一个ashx的实例是否能重用。
这个结果太奇怪了。为什么会这样呢？

前面我们看到*.ashx的请求交给SimpleHandlerFactory来创建相应的HttpHandler对象，然而当ASP.NET调用SimpleHandlerFactory.GetHandler()方法时，该方法会直接创建并返回我们实现的类型实例。换句话说：SimpleHandlerFactory根本不使用IHttpHandler.IsReusable的属性，因此，这种情况下，想重用ashx的实例是不可能的事，所以，即使我在实现IsReusable属性时，写上抛异常的语句，根本也不会被调用。
同样的事情还发在aspx页面的实例上，所以，在默认情况下，我们不可能重用aspx, ashx的实例。
至于aspx的实例不能重用，除了和PageHandlerFactory有关外，还与Page在实现IHttpHandler.IsReusable有关，以下是Page的实现方式：

1. public bool IsReusable{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>get { return false; }}

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从代码可以看到微软在Page是否重用上的明确想法：就是不允许重用！
由于Page的IsReusable属性我们平时看不到，我想没人对它的重用性有产生过疑惑，但ashx就不同了，它的IsReusable属性的代码是摆在我们面前的，任何人都可以看到它，试想一下：当有人发现把它设为true or false时都不起作用，会是个什么想法？估计很多人会郁闷。
小结：IHttpHandler.IsReusable并不能决定是否重用HttpHanlder ！
实现自己的HttpHandlerFactory

通过前面的示例，我们也看到了，虽然IHttpHandler定义了一个IsReusable属性，但它并不能决定此类型的实例是否能得到重用。重不重用，其实是由HttpHandlerFactory来决定的。ashx的实例不能重用就是一个典型的例子。
下面我就来演示如何实现自己的HttpHandlerFactory来重用ashx的实例。示例代码如下（注意代码中的注释）：

1. internal class ReusableAshxHandlerFactory : IHttpHandlerFactory{<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private Dictionary _cache<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers><httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers> = new Dictionary(200, StringComparer.OrdinalIgnoreCase);<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers>public IHttpHandler GetHandler(HttpContext context,<httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers><httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers><httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers> string requestType, string virtualPath, string physicalPath)<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers>{<httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers>string cacheKey = requestType + virtualPath;<httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers><httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers>// 检查是否有缓存的实例（或者可理解为：被重用的实例）<httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers><httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers>IHttpHandler handler = null;<httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers><httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers>if( _cache.TryGetValue(cacheKey, out handler) == false ) {<httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers><httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers><httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers>// 根据请求路径创建对应的实例<httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers><httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers><httpHandlers>
152. <httpHandlers>
153. <httpHandlers>
154.    
155. </httpHandlers>
156. </httpHandlers>
157. </httpHandlers>Type handlerType = BuildManager.GetCompiledType(virtualPath);<httpHandlers>
158. <httpHandlers>
159. <httpHandlers>
160.    
161. </httpHandlers>
162. </httpHandlers>
163. </httpHandlers><httpHandlers>
164. <httpHandlers>
165. <httpHandlers>
166.    
167. </httpHandlers>
168. </httpHandlers>
169. </httpHandlers><httpHandlers>
170. <httpHandlers>
171. <httpHandlers>
172.    
173. </httpHandlers>
174. </httpHandlers>
175. </httpHandlers>// 确保一定是IHttpHandler类型<httpHandlers>
176. <httpHandlers>
177. <httpHandlers>
178.    
179. </httpHandlers>
180. </httpHandlers>
181. </httpHandlers><httpHandlers>
182. <httpHandlers>
183. <httpHandlers>
184.    
185. </httpHandlers>
186. </httpHandlers>
187. </httpHandlers><httpHandlers>
188. <httpHandlers>
189. <httpHandlers>
190.    
191. </httpHandlers>
192. </httpHandlers>
193. </httpHandlers>if( typeof(IHttpHandler).IsAssignableFrom(handlerType) == false )<httpHandlers>
194. <httpHandlers>
195. <httpHandlers>
196.    
197. </httpHandlers>
198. </httpHandlers>
199. </httpHandlers><httpHandlers>
200. <httpHandlers>
201. <httpHandlers>
202.    
203. </httpHandlers>
204. </httpHandlers>
205. </httpHandlers><httpHandlers>
206. <httpHandlers>
207. <httpHandlers>
208.    
209. </httpHandlers>
210. </httpHandlers>
211. </httpHandlers><httpHandlers>
212. <httpHandlers>
213. <httpHandlers>
214.    
215. </httpHandlers>
216. </httpHandlers>
217. </httpHandlers>throw new HttpException("要访问的资源没有实现IHttpHandler接口。");<httpHandlers>
218. <httpHandlers>
219. <httpHandlers>
220.    
221. </httpHandlers>
222. </httpHandlers>
223. </httpHandlers><httpHandlers>
224. <httpHandlers>
225. <httpHandlers>
226.    
227. </httpHandlers>
228. </httpHandlers>
229. </httpHandlers><httpHandlers>
230. <httpHandlers>
231. <httpHandlers>
232.    
233. </httpHandlers>
234. </httpHandlers>
235. </httpHandlers>// 创建实例，并保存到成员字段中<httpHandlers>
236. <httpHandlers>
237. <httpHandlers>
238.    
239. </httpHandlers>
240. </httpHandlers>
241. </httpHandlers><httpHandlers>
242. <httpHandlers>
243. <httpHandlers>
244.    
245. </httpHandlers>
246. </httpHandlers>
247. </httpHandlers><httpHandlers>
248. <httpHandlers>
249. <httpHandlers>
250.    
251. </httpHandlers>
252. </httpHandlers>
253. </httpHandlers>handler = (IHttpHandler)Activator.CreateInstance(handlerType, true);<httpHandlers>
254. <httpHandlers>
255. <httpHandlers>
256.    
257. </httpHandlers>
258. </httpHandlers>
259. </httpHandlers><httpHandlers>
260. <httpHandlers>
261. <httpHandlers>
262.    
263. </httpHandlers>
264. </httpHandlers>
265. </httpHandlers><httpHandlers>
266. <httpHandlers>
267. <httpHandlers>
268.    
269. </httpHandlers>
270. </httpHandlers>
271. </httpHandlers>// 如果handler要求重用，则保存它的引用。<httpHandlers>
272. <httpHandlers>
273. <httpHandlers>
274.    
275. </httpHandlers>
276. </httpHandlers>
277. </httpHandlers><httpHandlers>
278. <httpHandlers>
279. <httpHandlers>
280.    
281. </httpHandlers>
282. </httpHandlers>
283. </httpHandlers><httpHandlers>
284. <httpHandlers>
285. <httpHandlers>
286.    
287. </httpHandlers>
288. </httpHandlers>
289. </httpHandlers>if( handler.IsReusable )<httpHandlers>
290. <httpHandlers>
291. <httpHandlers>
292.    
293. </httpHandlers>
294. </httpHandlers>
295. </httpHandlers><httpHandlers>
296. <httpHandlers>
297. <httpHandlers>
298.    
299. </httpHandlers>
300. </httpHandlers>
301. </httpHandlers><httpHandlers>
302. <httpHandlers>
303. <httpHandlers>
304.    
305. </httpHandlers>
306. </httpHandlers>
307. </httpHandlers><httpHandlers>
308. <httpHandlers>
309. <httpHandlers>
310.    
311. </httpHandlers>
312. </httpHandlers>
313. </httpHandlers>_cache[cacheKey] = handler;<httpHandlers>
314. <httpHandlers>
315. <httpHandlers>
316.    
317. </httpHandlers>
318. </httpHandlers>
319. </httpHandlers><httpHandlers>
320. <httpHandlers>
321. <httpHandlers>
322.    
323. </httpHandlers>
324. </httpHandlers>
325. </httpHandlers>}<httpHandlers>
326. <httpHandlers>
327. <httpHandlers>
328.    
329. </httpHandlers>
330. </httpHandlers>
331. </httpHandlers><httpHandlers>
332. <httpHandlers>
333. <httpHandlers>
334.    
335. </httpHandlers>
336. </httpHandlers>
337. </httpHandlers>return handler;<httpHandlers>
338. <httpHandlers>
339. <httpHandlers>
340.    
341. </httpHandlers>
342. </httpHandlers>
343. </httpHandlers>}<httpHandlers>
344. <httpHandlers>
345. <httpHandlers>
346.    
347. </httpHandlers>
348. </httpHandlers>
349. </httpHandlers>public void ReleaseHandler(IHttpHandler handler)<httpHandlers>
350. <httpHandlers>
351. <httpHandlers>
352.    
353. </httpHandlers>
354. </httpHandlers>
355. </httpHandlers>{<httpHandlers>
356. <httpHandlers>
357. <httpHandlers>
358.    
359. </httpHandlers>
360. </httpHandlers>
361. </httpHandlers><httpHandlers>
362. <httpHandlers>
363. <httpHandlers>
364.    
365. </httpHandlers>
366. </httpHandlers>
367. </httpHandlers>// 不需要处理这个方法。<httpHandlers>
368. <httpHandlers>
369. <httpHandlers>
370.    
371. </httpHandlers>
372. </httpHandlers>
373. </httpHandlers>}}

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为了能让HttpHandlerFactory能在ASP.NET中运行，还需要在web.config中注册：

1. <httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>

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有了这个配置后，我们可以创建一个Handler2.ashx来测试效果：

1. using System;using System.Web;public class Handler2 : IHttpHandler {<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private Counter _counter = new Counter();<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public bool IsReusable<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>get { return true; }<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers>}<httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers>public void ProcessRequest(HttpContext context)<httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>{<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers>_counter.ShowCountAndRequestInfo(context);<httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>}}

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在多次访问Handler2.ashx后，我们可以看到以下效果：

再来看看按照IDE默认生成的IsReusable会在运行时出现什么结果。示例代码：

1. using System;using System.Web;public class Handler1 : IHttpHandler {<httpHandlers>
2. <httpHandlers>
3. <httpHandlers>
4.    
5. </httpHandlers>
6. </httpHandlers>
7. </httpHandlers>private Counter _counter = new Counter();<httpHandlers>
8. <httpHandlers>
9. <httpHandlers>
10.    
11. </httpHandlers>
12. </httpHandlers>
13. </httpHandlers>public bool IsReusable<httpHandlers>
14. <httpHandlers>
15. <httpHandlers>
16.    
17. </httpHandlers>
18. </httpHandlers>
19. </httpHandlers>{<httpHandlers>
20. <httpHandlers>
21. <httpHandlers>
22.    
23. </httpHandlers>
24. </httpHandlers>
25. </httpHandlers><httpHandlers>
26. <httpHandlers>
27. <httpHandlers>
28.    
29. </httpHandlers>
30. </httpHandlers>
31. </httpHandlers>get {<httpHandlers>
32. <httpHandlers>
33. <httpHandlers>
34.    
35. </httpHandlers>
36. </httpHandlers>
37. </httpHandlers><httpHandlers>
38. <httpHandlers>
39. <httpHandlers>
40.    
41. </httpHandlers>
42. </httpHandlers>
43. </httpHandlers><httpHandlers>
44. <httpHandlers>
45. <httpHandlers>
46.    
47. </httpHandlers>
48. </httpHandlers>
49. </httpHandlers>// 如果在配置文件中启用ReusableAshxHandlerFactory，那么这里将会被执行。<httpHandlers>
50. <httpHandlers>
51. <httpHandlers>
52.    
53. </httpHandlers>
54. </httpHandlers>
55. </httpHandlers><httpHandlers>
56. <httpHandlers>
57. <httpHandlers>
58.    
59. </httpHandlers>
60. </httpHandlers>
61. </httpHandlers><httpHandlers>
62. <httpHandlers>
63. <httpHandlers>
64.    
65. </httpHandlers>
66. </httpHandlers>
67. </httpHandlers>// 可以尝试切换下面二行代码测试效果。<httpHandlers>
68. <httpHandlers>
69. <httpHandlers>
70.    
71. </httpHandlers>
72. </httpHandlers>
73. </httpHandlers><httpHandlers>
74. <httpHandlers>
75. <httpHandlers>
76.    
77. </httpHandlers>
78. </httpHandlers>
79. </httpHandlers><httpHandlers>
80. <httpHandlers>
81. <httpHandlers>
82.    
83. </httpHandlers>
84. </httpHandlers>
85. </httpHandlers>//throw new Exception("这里不起作用。");<httpHandlers>
86. <httpHandlers>
87. <httpHandlers>
88.    
89. </httpHandlers>
90. </httpHandlers>
91. </httpHandlers><httpHandlers>
92. <httpHandlers>
93. <httpHandlers>
94.    
95. </httpHandlers>
96. </httpHandlers>
97. </httpHandlers><httpHandlers>
98. <httpHandlers>
99. <httpHandlers>
100.    
101. </httpHandlers>
102. </httpHandlers>
103. </httpHandlers>return false;<httpHandlers>
104. <httpHandlers>
105. <httpHandlers>
106.    
107. </httpHandlers>
108. </httpHandlers>
109. </httpHandlers><httpHandlers>
110. <httpHandlers>
111. <httpHandlers>
112.    
113. </httpHandlers>
114. </httpHandlers>
115. </httpHandlers>}<httpHandlers>
116. <httpHandlers>
117. <httpHandlers>
118.    
119. </httpHandlers>
120. </httpHandlers>
121. </httpHandlers>}<httpHandlers>
122. <httpHandlers>
123. <httpHandlers>
124.    
125. </httpHandlers>
126. </httpHandlers>
127. </httpHandlers>public void ProcessRequest(HttpContext context)<httpHandlers>
128. <httpHandlers>
129. <httpHandlers>
130.    
131. </httpHandlers>
132. </httpHandlers>
133. </httpHandlers>{<httpHandlers>
134. <httpHandlers>
135. <httpHandlers>
136.    
137. </httpHandlers>
138. </httpHandlers>
139. </httpHandlers><httpHandlers>
140. <httpHandlers>
141. <httpHandlers>
142.    
143. </httpHandlers>
144. </httpHandlers>
145. </httpHandlers>_counter.ShowCountAndRequestInfo(context);<httpHandlers>
146. <httpHandlers>
147. <httpHandlers>
148.    
149. </httpHandlers>
150. </httpHandlers>
151. </httpHandlers>}}

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此时，无论我访问Handler1.ashx多少次，浏览器始终显示如下结果：

如果我启用代码行 throw new Exception("这里不起作用。"); 将会看到以下结果：

终于，我们期待的黄页出现了。
此时，如果我在web.config中将ReusableAshxHandlerFactory的注册配置注释起来，发现Handler1.ashx还是可以访问的。
回想一下前面我们看到的IHttpHandlerFactory接口，它还定义了一个ReleaseHandler方法，这个方法又是做什么的呢？对于这个方法，MSDN也有一句简单的说明：
使工厂可以重用现有的处理程序实例。
对于这个说明，我认为并不恰当。如果按照HandlerFactoryWrapper的实现方式，那么这个解释是正确的。但我前面的示例中，我在实现这个方法时，没有任何代码，但一样可以达到重用HttpHandler的目的。因此，我认为重用的方式取决于具体的实现方式。
小结：IHttpHandler.IsReusable并不能完全决定HttpHandler的实例是否能重用，它只起到一个指示作用。HttpHandler如何重用，关键还是要由HttpHandlerFactory来实现。
是否需要IsReusable = true ？

经过前面文字讲解以及示例演示，有些人可能会想：我在实现IHttpHandler的IsReusable属性时，要不要返回true呢？（千万别模仿我的示例代码抛异常哦。）
如果返回true，则HttpHandler能得到重用，或许某些场合下，是可以达到性能优化的目的。
但是，它也可能会引发新的问题：HttpHandler实例的一些状态会影响后续的请求。
也正是由于这个原因，aspx, ashx 的实例在默认情况下，都是不重用的。
有些人还可能会担心：被重用的HttpHandler是否有线程安全问题？
理论上，在ASP.NET中，只要使用static的数据成员都会有这个问题。不过，这里所说的被重用的单个HttpHandler实例在处理请求过程中，只会被一个线程所调用，因此，它的实例成员还是线程安全的。但有一点需要注意：在HttpHandlerFactory中实现重用HttpHandler时，缓存HttpHandler的容器要保证是线程安全的。
如果您希望重用HttpHandler来提升程序性能，那么我建议应该考虑以下问题：
HttpHandler的所有数据成员都能在处理请求前初始化。（通常会在后期维护时遗忘，尤其是多人维护时）
小结：在通常情况下，当实现IsReusable时返回false，虽然性能上不是最优，但却是最安全的做法。
HttpHandlerFactory的主要用途

前面示例演示了如何使用HttpHandlerFactory来重用HttpHandler，但设计HttpHandlerFactory并不是完全为了这个目的，它的主要用途还是如何创建HttpHandler，而且定义IHttpHandlerFactory的主要目的是为了扩展性。
我想很多人也许使用过 Web Service ，它运行在ASP.NET平台上，自然也有对应的HttpHandler，我们来看看asmx这个扩展名是如何映射的。

1. [/code]接着找WebServiceHandlerFactory，最后发现是这样创建的HttpHandler :
2. [code]internal IHttpHandler CoreGetHandler(Type type,<httpHandlers>
3. <httpHandlers>
4. <httpHandlers>
5.    
6. </httpHandlers>
7. </httpHandlers>
8. </httpHandlers><httpHandlers>
9. <httpHandlers>
10. <httpHandlers>
11.    
12. </httpHandlers>
13. </httpHandlers>
14. </httpHandlers> HttpContext context, HttpRequest request, HttpResponse response){<httpHandlers>
15. <httpHandlers>
16. <httpHandlers>
17.    
18. </httpHandlers>
19. </httpHandlers>
20. </httpHandlers>// ..... 已删除一些无关的代码<httpHandlers>
21. <httpHandlers>
22. <httpHandlers>
23.    
24. </httpHandlers>
25. </httpHandlers>
26. </httpHandlers>bool isAsync = protocol.MethodInfo.IsAsync;<httpHandlers>
27. <httpHandlers>
28. <httpHandlers>
29.    
30. </httpHandlers>
31. </httpHandlers>
32. </httpHandlers>bool enableSession = protocol.MethodAttribute.EnableSession;<httpHandlers>
33. <httpHandlers>
34. <httpHandlers>
35.    
36. </httpHandlers>
37. </httpHandlers>
38. </httpHandlers>if( isAsync ) {<httpHandlers>
39. <httpHandlers>
40. <httpHandlers>
41.    
42. </httpHandlers>
43. </httpHandlers>
44. </httpHandlers><httpHandlers>
45. <httpHandlers>
46. <httpHandlers>
47.    
48. </httpHandlers>
49. </httpHandlers>
50. </httpHandlers>if( enableSession ) {<httpHandlers>
51. <httpHandlers>
52. <httpHandlers>
53.    
54. </httpHandlers>
55. </httpHandlers>
56. </httpHandlers><httpHandlers>
57. <httpHandlers>
58. <httpHandlers>
59.    
60. </httpHandlers>
61. </httpHandlers>
62. </httpHandlers><httpHandlers>
63. <httpHandlers>
64. <httpHandlers>
65.    
66. </httpHandlers>
67. </httpHandlers>
68. </httpHandlers>return new AsyncSessionHandler(protocol);<httpHandlers>
69. <httpHandlers>
70. <httpHandlers>
71.    
72. </httpHandlers>
73. </httpHandlers>
74. </httpHandlers><httpHandlers>
75. <httpHandlers>
76. <httpHandlers>
77.    
78. </httpHandlers>
79. </httpHandlers>
80. </httpHandlers>}<httpHandlers>
81. <httpHandlers>
82. <httpHandlers>
83.    
84. </httpHandlers>
85. </httpHandlers>
86. </httpHandlers><httpHandlers>
87. <httpHandlers>
88. <httpHandlers>
89.    
90. </httpHandlers>
91. </httpHandlers>
92. </httpHandlers>return new AsyncSessionlessHandler(protocol);<httpHandlers>
93. <httpHandlers>
94. <httpHandlers>
95.    
96. </httpHandlers>
97. </httpHandlers>
98. </httpHandlers>}<httpHandlers>
99. <httpHandlers>
100. <httpHandlers>
101.    
102. </httpHandlers>
103. </httpHandlers>
104. </httpHandlers>if( enableSession ) {<httpHandlers>
105. <httpHandlers>
106. <httpHandlers>
107.    
108. </httpHandlers>
109. </httpHandlers>
110. </httpHandlers><httpHandlers>
111. <httpHandlers>
112. <httpHandlers>
113.    
114. </httpHandlers>
115. </httpHandlers>
116. </httpHandlers>return new SyncSessionHandler(protocol);<httpHandlers>
117. <httpHandlers>
118. <httpHandlers>
119.    
120. </httpHandlers>
121. </httpHandlers>
122. </httpHandlers>}<httpHandlers>
123. <httpHandlers>
124. <httpHandlers>
125.    
126. </httpHandlers>
127. </httpHandlers>
128. </httpHandlers>return new SyncSessionlessHandler(protocol);}

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这才是Factory嘛！
老实说，看到这几句话，我是眼前一亮：用HttpHandlerFactory来动态处理【是否支持Session】实在是太合适了。
这里有必要补充一下：

1. internal class SyncSessionHandler : SyncSessionlessHandler, IRequiresSessionState
2. {
3. }

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小结：HttpHandlerFactory用途并非是为了专门处理HttpHandler的重用，它只是一个Factory，WebServiceHandlerFactory从另一个角度向我们展示了HttpHandlerFactory在扩展性方面所体现的重要作用。

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