HTTP(HyperText Transfer Protocol)是一套計算機通過網絡進行通信的規則。計算機專家設計出HTTP，使HTTP客戶（如Web浏覽器）能夠從HTTP伺服器(Web伺服器)請求資訊和服務，HTTP目前協定的版本是1.1.HTTP是一種無狀态的協定，無狀态是指Web浏覽器和Web伺服器之間不需要建立持久的連接配接，這意味着當一個用戶端向伺服器端送出請求，然後Web伺服器傳回響應(response)，連接配接就被關閉了，在伺服器端不保留連接配接的有關資訊.HTTP遵循請求(Request)/應答(Response)模型。Web浏覽器向Web伺服器發送請求，Web伺服器處理請求并傳回适當的應答。所有HTTP連接配接都被構造成一套請求和應答。
HTTP 使用内容類型，是指Web伺服器向Web浏覽器傳回的檔案都有與之相關的類型。所有這些類型在MIME Internet郵件協定上模型化，即Web服務 器告訴Web浏覽器該檔案所具有的種類，是HTML文檔、GIF格式圖像、聲音檔案還是獨立的應用程式。大多數Web浏覽器都擁有一系列的可配置的輔助應 用程式，它們告訴浏覽器應該如何處理Web伺服器發送過來的各種内容類型。
HTTP通信機制是在一次完整的HTTP通信過程中，Web浏覽器與Web伺服器之間将完成下列7個步驟：
（1） 建立TCP連接配接
在HTTP 工作開始之前，Web浏覽器首先要通過網絡與Web伺服器建立連接配接，該連接配接是通過TCP來完成的，該協定與IP協定共同建構Internet，即著名的 TCP/IP協定族，是以Internet又被稱作是TCP/IP網絡。HTTP是比TCP更高層次的應用層協定，根據規則，隻有低層協定建立之後才能， 才能進行更層協定的連接配接，是以，首先要建立TCP連接配接，一般TCP連接配接的端口号是80
（2） Web浏覽器向Web伺服器發送請求指令
一旦建立了TCP連接配接，Web浏覽器就會向Web伺服器發送請求指令
例如：GET/sample/hello.jsp HTTP/1.1
（3） Web浏覽器發送請求頭資訊
浏覽器發送其請求指令之後，還要以頭資訊的形式向Web伺服器發送一些别的資訊，之後浏覽器發送了一空白行來通知伺服器，它已經結束了該頭資訊的發送。
（4） Web伺服器應答
客戶機向伺服器送出請求後，伺服器會客戶機回送應答，
HTTP/1.1 200 OK
應答的第一部分是協定的版本号和應答狀态碼
（5） Web伺服器發送應答頭資訊
正如用戶端會随同請求發送關于自身的資訊一樣，伺服器也會随同應答向使用者發送關于它自己的資料及被請求的文檔。
（6） Web伺服器向浏覽器發送資料
Web伺服器向浏覽器發送頭資訊後，它會發送一個空白行來表示頭資訊的發送到此為結束，接着，它就以Content-Type應答頭資訊所描述的格式發送使用者所請求的實際資料
（7） Web伺服器關閉TCP連接配接
一般情況下，一旦Web伺服器向浏覽器發送了請求資料，它就要關閉TCP連接配接，然後如果浏覽器或者伺服器在其頭資訊加入了這行代碼
Connection:keep-alive
TCP連接配接在發送後将仍然保持打開狀态，于是，浏覽器可以繼續通過相同的連接配接發送請求。保持連接配接節省了為每個請求建立新連接配接所需的時間，還節約了網絡帶寬。
HTTP請求格式
當浏覽器向Web伺服器送出請求時，它向伺服器傳遞了一個資料塊，也就是請求資訊，HTTP請求資訊由3部分組成：
l 請求方法URI協定/版本
l 請求頭(Request Header)
l 請求正文
下面是一個HTTP請求的例子：
GET/sample.jspHTTP/1.1
Accept:image/gif.image/jpeg,**
Accept-Language:zh-cn
Connection:Keep-Alive
Host:localhost
User-Agent:Mozila/4.0(compatible:MSIE5.01:Windows NT5.0)
Accept-Encoding:gzip,deflate.
（3） 請求正文
請求頭和請求正文之間是一個空行，這個行非常重要，它表示請求頭已經結束，接下來的是請求正文。請求正文中可以包含客戶送出的查詢字元串資訊：
username=jinqiao&password=1234
在以上的例子的HTTP請求中，請求的正文隻有一行内容。當然，在實際應用中，HTTP請求正文可以包含更多的内容。
HTTP請求方法我這裡隻讨論GET方法與POST方法
l GET方法
GET方法是預設的HTTP請求方法，我們日常用GET方法來送出表單資料，然而用GET方法送出的表單資料隻經過了簡單的編碼，同時它将作為URL的一部分向Web伺服器發送，是以，如果使用GET方法來送出表單資料就存在着安全隐患上。例如
Http://127.0.0.1/login.jsp?Name=zhangshi&Age=30&Submit=%cc%E+%BD%BB
從上面的URL請求中，很容易就可以辯認出表單送出的内容。（？之後的内容）另外由于GET方法送出的資料是作為URL請求的一部分是以送出的資料量不能太大
l POST方法
POST 方法是GET方法的一個替代方法，它主要是向Web伺服器送出表單資料，尤其是大批量的資料。POST方法克服了GET方法的一些缺點。通過POST方法 送出表單資料時，資料不是作為URL請求的一部分而是作為标準資料傳送給Web伺服器，這就克服了GET方法中的資訊無法保密和資料量太小的缺點。是以， 出于安全的考慮以及對使用者隐私的尊重，通常表單送出時采用POST方法。
從程式設計的角度來講，如果使用者通過GET方法送出資料，則資料存放在QUERY＿STRING環境變量中，而POST方法送出的資料則可以從标準輸入流中擷取。
HTTP應答與HTTP請求相似，HTTP響應也由3個部分構成，分别是：
l 協定狀态版本代碼描述
l 響應頭(Response Header)
l 響應正文
下面是一個HTTP響應的例子：
HTTP/1.1 200 OK
Server:Apache Tomcat/5.0.12
Date:Mon,6Oct2003 13:23:42 GMT
Content-Length:112
<html>
<head>
<title>HTTP響應示例<title>
</head>
<body>
Hello HTTP!
</body>
</html>
協定狀态代碼描述HTTP響應的第一行類似于HTTP請求的第一行，它表示通信所用的協定是HTTP1.1伺服器已經成功的處理了用戶端發出的請求（200表示成功）:
HTTP/1.1 200 OK
響應頭(Response Header)響應頭也和請求頭一樣包含許多有用的資訊，例如伺服器類型、日期時間、内容類型和長度等：
Server:Apache Tomcat/5.0.12
Date:Mon,6Oct2003 13:13:33 GMT
Content-Type:text/html
Last-Moified:Mon,6 Oct 2003 13:23:42 GMT
Content-Length:112
響應正文響應正文就是伺服器傳回的HTML頁面：
<html>
<head>
<title>HTTP響應示例<title>
</head>
<body>
Hello HTTP!
</body>
</html>
響應頭和正文之間也必須用空行分隔。
l HTTP應答碼
HTTP應答碼也稱為狀态碼，它反映了Web伺服器處理HTTP請求狀态。HTTP應答碼由3位數字構成，其中首位數字定義了應答碼的類型：
1XX－資訊類(Information),表示收到Web浏覽器請求，正在進一步的進行中
2XX－成功類（Successful）,表示使用者請求被正确接收，了解和處理例如：200 OK
3XX-重定向類(Redirection),表示請求沒有成功，客戶必須采取進一步的動作。
4XX-用戶端錯誤(Client Error)，表示用戶端送出的請求有錯誤 例如：404 NOT
Found，意味着請求中所引用的文檔不存在。
5XX-伺服器錯誤(Server Error)表示伺服器不能完成對請求的處理：如 500
對于我們Web開發人員來說掌握HTTP應答碼有助于提高Web應用程式調試的效率和準确性。
安全連接配接
Web 應用最常見的用途之一是電子商務，可以利用Web伺服器端程式使人們能夠網絡購物，需要指出一點是，預設情況下，通過Internet發送資訊是不安全 的，如果某人碰巧截獲了你發給朋友的一則消息，他就能打開它，假想在裡面有你的信用卡号碼，這會有多麼糟糕，幸運的是，很多Web伺服器以及Web浏覽器 都有創立安全連接配接的能力，這樣它們就可以安全的通信了。
通過Internet 提供安全連接配接最常見的标準是安全套接層(Secure Sockets layer,SSl)協定。SSL協定是一個應用層協定(和HTTP一樣)，用于安全方式在Web上交換資料，SSL使用公開密鑰編碼系統。從本質講，這 意味着業務中每一方都擁有一個公開的和一個私有的密鑰。當一方使用另一方公開密鑰進行編碼時，隻有擁有比對密鑰的人才能對其解碼。簡單來講，公開密鑰編碼 提供了一種用于在兩方之間交換資料的安全方法，SSL連接配接建立之後，客戶和伺服器都交換公開密鑰，并在進行業務聯系之前進行驗證，一旦雙方的密鑰都通過驗 證，就可以安全地交換資料。
-
GET
通過請求URI得到資源
-
POST,
用于添加新的内容
-
PUT
用于修改某個内容
-
DELETE,
删除某個内容
-
CONNECT,
用于代理進行傳輸，如使用SSL
-
OPTIONS
詢問可以執行哪些方法
-
PATCH,
部分文檔更改
-
PROPFIND, (wedav)
檢視屬性
-
PROPPATCH, (wedav)
設定屬性
-
MKCOL, (wedav)
建立集合（檔案夾）
-
COPY, (wedav)
拷貝
-
MOVE, (wedav)
移動
-
LOCK, (wedav)
加鎖
-
UNLOCK (wedav)
解鎖
-
TRACE
用于遠端診斷伺服器
-
HEAD
類似于GET, 但是不傳回body資訊，用于檢查對象是否存在，以及得到對象的中繼資料
apache2中，可使用Limit，LimitExcept進行通路控制的方法包括：
GET
,
POST
,
PUT
,
DELETE
,
CONNECT
,
OPTIONS
,
PATCH
,
PROPFIND
,
PROPPATCH
,
MKCOL
,
COPY
,
MOVE
,
LOCK
, 和
UNLOCK
.
其中, HEAD GET POST OPTIONS PROPFIND是和讀取相關的方法，MKCOL PUT DELETE LOCK UNLOCK COPY MOVE PROPPATCH是和修改相關的方法
part of Hypertext Transfer Protocol -- HTTP/1.1
RFC 2616 Fielding, et al.
9 Method Definitions
The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers.
The Host request-header field (section 14.23) MUST accompany all HTTP/1.1 requests.
9.1 Safe and Idempotent Methods
9.1.1 Safe Methods
Implementors should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they might take which may have an unexpected significance to themselves or others.
In particular, the convention has been established that the GET and HEAD methods SHOULD NOT have the significance of taking an action other than retrieval. These methods ought to be considered "safe". This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.
9.1.2 Idempotent Methods
Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request. The methods GET, HEAD, PUT and DELETE share this property. Also, the methods OPTIONS and TRACE SHOULD NOT have side effects, and so are inherently idempotent.
However, it is possible that a sequence of several requests is non- idempotent, even if all of the methods executed in that sequence are idempotent. (A sequence is idempotent if a single execution of the entire sequence always yields a result that is not changed by a reexecution of all, or part, of that sequence.) For example, a sequence is non-idempotent if its result depends on a value that is later modified in the same sequence.
A sequence that never has side effects is idempotent, by definition (provided that no concurrent operations are being executed on the same set of resources).
9.2 OPTIONS
The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.
Responses to this method are not cacheable.
If the OPTIONS request includes an entity-body (as indicated by the presence of Content-Length or Transfer-Encoding), then the media type MUST be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server. A server that does not support such an extension MAY discard the request body.
If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server in general rather than to a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 compliance (or lack thereof).
If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource.
A 200 response SHOULD include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g., Allow), possibly including extensions not defined by this specification. The response body, if any, SHOULD also include information about the communication options. The format for such a
body is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation MAY be used to select the appropriate response format. If no response body is included, the response MUST include a Content-Length field with a field-value of "0".
The Max-Forwards request-header field MAY be used to target a specific proxy in the request chain. When a proxy receives an OPTIONS request on an absoluteURI for which request forwarding is permitted, the proxy MUST check for a Max-Forwards field. If the Max-Forwards field-value is zero ("0"), the proxy MUST NOT forward the message; instead, the proxy SHOULD respond with its own communication options. If the Max-Forwards field-value is an integer greater than zero, the proxy MUST decrement the field-value when it forwards the request. If no Max-Forwards field is present in the request, then the forwarded request MUST NOT include a Max-Forwards field.
9.3 GET
The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.
The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.
The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in section 14.35. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.
The response to a GET request is cacheable if and only if it meets the requirements for HTTP caching described in section 13.
See section 15.1.3 for security considerations when used for forms.
9.4 HEAD
The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.
The response to a HEAD request MAY be cacheable in the sense that the information contained in the response MAY be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.
9.5 POST
The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions:
- Annotation of existing resources;
- Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
- Providing a block of data, such as the result of submitting a
form, to a data-handling process;
- Extending a database through an append operation.
The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.
The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.
If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see section 14.30).
Responses to this method are not cacheable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable resource.
POST requests MUST obey the message transmission requirements set out in section 8.2.
See section 15.1.3 for security considerations.
9.6 PUT
The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server MUST inform the user agent via the 201 (Created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. The recipient of the entity MUST NOT ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and MUST return a 501 (Not Implemented) response in such cases.
If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.
The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource might be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI,
it MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request.
A single resource MAY be identified by many different URIs. For example, an article might have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI might result in several other URIs being defined by the origin server.
HTTP/1.1 does not define how a PUT method affects the state of an origin server.
PUT requests MUST obey the message transmission requirements set out in section 8.2.
Unless otherwise specified for a particular entity-header, the entity-headers in the PUT request SHOULD be applied to the resource created or modified by the PUT.
9.7 DELETE
The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.
A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include an entity.
If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries SHOULD be treated as stale. Responses to this method are not cacheable.
9.8 TRACE
The TRACE method is used to invoke a remote, application-layer loop- back of the request message. The final recipient of the request SHOULD reflect the message received back to the client as the entity-body of a 200 (OK) response. The final recipient is either the
origin server or the first proxy or gateway to receive a Max-Forwards value of zero (0) in the request (see section 14.31). A TRACE request MUST NOT include an entity.
TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (section 14.45) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.
If the request is valid, the response SHOULD contain the entire request message in the entity-body, with a Content-Type of "message/http". Responses to this method MUST NOT be cached.
9.9 CONNECT
This specification reserves the method name CONNECT for use with a proxy that can dynamically switch to being a tunnel (e.g. SSL tunneling [44]).
![一場春雨，一個帶着泥土芬芳的記憶[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)