"Web" a simple Web server

How does the WEB server work?   HTTP (hypertext Transfer Protocol) is a set of rules for computers to communicate over a network. Computer experts design HTTP to enable HTTP clients (such as Web browsers) to request information and services from an HTTP server (Web server). HTTP Current protocol version is 1.1.HTTP is a stateless protocol, stateless refers to the Web browser and the Web server does not need to establish a persistent connection, which means when a client makes a request to the server, and then the Web server returns a response (response), The connection is closed and information about the connection is not maintained on the server side. HTTP follows the request/answer (Response) model. The Web browser sends a request to the Web server, and the Web server processes the request and returns the appropriate answer. All HTTP connections are constructed as a set of requests and responses. HTTP uses content types, which are the types of files that the Web server returns to a Web browser. All of these types are modeled on the MIME Internet Mail protocol, where the Web server tells the Web browser what kind of file it has, whether it is an HTML document, a GIF format image, a sound file, or a standalone application. Most Web browsers have a series of configurable helper applications that tell the browser how to handle the various types of content that the Web server sends over. The HTTP communication mechanism is that during a complete HTTP communication, the following 7 steps will be completed between the Web browser and the Web server: (1)      establish a TCP connection before HTTP work begins, The Web browser first establishes a connection over the network to the Web server, which is done through TCP, which is built on the Internet with the IP protocol, known as the TCP/IP protocol family, so the internet is also known as a TCP/IP network. HTTP is a higher level of application layer protocol than TCP, according to the rules, only the lower layer protocol is established before the connection can be more protocol, so the first to establish a TCP connection, the port number of the general TCP connection is (2)      Web browser sends a request command to a Web server once a TCP connection is established, the Web browser sends a request command to the Web server for example: get/sample/hello.jsp http/1.1 (3)      After the Web browser sends the request header information The browser sends its request command, it also sends some other information to the Web server in the form of header information, after which the browser sends a blankLine to notify the server that it has ended sending the header information. (4) After the     web server answers the client to make a request to the server, the server will reply back to the client, the first part of http/1.1 OK answer is the protocol version number and the response status Code (5)      web the server sends the reply header information as the client sends information about itself along with the request, the server also sends the user with the answer about its own data and the requested document. (6) The     web server sends a data to the browser after the Web server sends the header information to the browser, it sends a blank line to indicate that the header message is sent to this end, and then It sends the actual data requested by the user in the format described in the Content-type reply header information (7)     web server shuts down TCP connections in general, once the Web server sends the request data to the browser, It closes the TCP connection and then if the browser or server joins this line of code in its header connection:keep-alivetcp the connection will remain open after it is sent, the browser can continue to send the request through the same connection.　　　　Maintaining a connection saves the time it takes to establish a new connection for each request and also saves network bandwidth. HTTP request format when the browser makes a request to the Web server, it passes a block of data to the server, which is the request information, into the:l    request method URI Protocol/version l    request header Header) l    request body Below is an example of an HTTP request: get/sample.jsphttp/1.1accept:image/gif.image/jpeg,*/* accept-language:zh-cnconnection:keep-alivehost:localhostuser-agent:mozila/4.0 (compatible; MSIE5.01; Window NT5.0) accept-encoding:gzip,deflate username=jinqiao&password=1234  (1)          Request Method URI Protocol/version The first line of the request is "method URL negotiation/Version": get/sample.jsp http/1.1 the "GET" in the code above represents the request method, "/sample.jsp" represents the URI, "http/1.1 represents the version of the Protocol and Protocol. HTTP requests can use a variety of request methods, depending on the HTTP standard. For example: HTTP1.1 supports 7 methods of request: GET, POST, HEAD, OPTIONS, PUT, delete, and Tarce. In Internet applications, the most common method is get and post. The URL completely specifies the network resource to be accessed, usually with a relative directory relative to the root of the server, always beginning with a "/", and finally, the version of the Protocol that declares the use of HTTP during communication. (2) The request header request header contains many useful information about the client environment and the request body. For example, the request header can declare the language used by the browser, the length of the request body, and so on. accept:image/gif.image/jpeg.*/*accept-language:zh-cnconnection:keep-alivehost:localhostuser-agent:mozila/4.0 ( Compatible:msie5.01:windows NT5.0) accept-encoding:gzip,deflate. (3) There is a blank line between the request body request header and the request body, which is very important, which indicates that the request header has ended. Next is the request body. The request body can contain query string information submitted by the customer: username=jinqiao&password=1234 in the HTTP request for the example above, the body of the request has only one line of content. Of course, in real-world applications, the HTTP request body can contain more content. HTTP request method I only discuss the Get method with the Post method L         get method The Get method is the default HTTP request method, We use the Get method to submit the form data on a daily basis, but the form data submitted with the Get method is simply encoded, and it is sent to the Web server as part of the URL, so there is a security risk if you use the Get method to submit the form data. For example Http://127.0.0.1/login.jsp?Name=zhangshi&Age=30&Submit=%cc%E+%BD%BB from the URL request above, it is easy to identify the content submitted by the form. （？ After the contents) in addition due toThe data submitted by the Get method is part of the URL request so the amount of data submitted cannot be too large l          The Post method post method is an alternative to the Get method, which is primarily to submit form data to the Web server, especially large batches of data. The Post method overcomes some of the drawbacks of the Get method. When submitting form data through the Post method, the data is not sent as part of the URL request but as standard data to the Web server, which overcomes the drawback that the information in the Get method is not confidential and the amount of data is too small.　　Therefore, for security reasons and respect for user privacy, the Post method is usually used for form submission. From a programmatic point of view, if a user submits data through a GET method, the data is stored in the QUERY_STRING environment variable, and the data submitted by the Post method can be obtained from the standard input stream. HTTP replies are similar to HTTP requests, and HTTP responses are made up of 3 parts, namely: L Protocol Status Version Code description L response Header (Response header) L response body Below is an example of an HTTP response: http/1.1 okserver: Apache tomcat/5.0.12date:mon,6oct2003 13:23:42 gmtcontent-length:112 The Protocol status code describes the first line of the HTTP response similar to the first line of the HTTP request, which indicates that the protocol used by the HTTP1.1 server has successfully processed the client-issued request (200 indicates success): http/1.1 OK
The response header (Response header) also contains many useful information, such as server type, datetime, content type, and length, as well as the request header: Server:apache tomcat/5.0.12date:mon,6oct2003　　13:13:33 gmtcontent-type:text/htmllast-moified:mon,6 OCT 2003 13:23:42 gmtcontent-length:112 Response Body Response body is the HTML page returned by the server:

Web A simple Web server