Java web basics 2-http protocol, javaweb

Java web basics 2-http protocol, javaweb
Java web basics 2-http protocol

 

Because java web is based on B/S, http is the most basic and important knowledge of java web. HTTP is used to define the format of communication between the client and the web server. Obtaining a web resource from a web server or receiving client requests from a web server must comply with a certain communication format, that is, the http protocol.

 

 

I. What is HTTP?

HTTP protocol is the cornerstone of learning java web and an essential foundation for managing and maintaining complex WEB sites. If you want to deeply understand the implementation of java web servers, you need to master it.

HTTP is short for hypertext transfer protocol (hypertext transfer protocol). It is an application layer protocol over TCP/IP. The default port is 80, defines the format and specifications for data exchange between a WEB browser and a WEB server.

HTTP is a stateless protocol because a web server does not save any information about the client. In the B/S architecture, web servers always open with fixed IP addresses and serve thousands of different browsers at the same time.

The HTTP protocol has two versions: HTTP/1.0 and HTTP/1.1. Currently, HTTP/1.1 is basically used. There is an important difference between them: In HTTP1.0 Protocol, only one web resource can be obtained after the client establishes a connection with the web server. HTTP1.1 protocol, allows the client to establish a connection with the web server and then obtain multiple web resources in a connection. That is, persistent connections.

Ii. Differences between persistent connections and non-persistent connections

A web page is also called a document. It is composed of objects. Simply put, objects are files, such as html files and image files. Most web pages contain a basic html file and multiple referenced objects. For example, if a web page contains a basic html file and two image files, the web page has three objects. The basic html file references the object through the object URL. The URL address consists of the Host Name of the server that stores the object and the path name of the object relative to the host. For example:

The c.csdnimg.cn in the http://c.csdnimg.cn/www/images/pic_foot_BNIA.png is the host name,

/Www/images/pic_foot_BNIA.png is the path name.

A non-persistent connection means that when the client requests a web page, the request/response to each object is sent through a separate TCP connection. For example, if a web page contains a basic html file and five JPEG image files. When a non-persistent connection is used, the user requests this page to establish six TCP connections.

Persistent connections are requests to a page. Even when multiple web pages on the same server are sent, they can be transmitted over a single persistent TCP connection. For example, in the preceding example, only a persistent TCP connection is required. In general, if a connection is not used after a certain period of time, the HTTP server will close the connection.

 

 

3. HTTP requests

After the browser of the client connects to the web server, it will request a web resource from the server, which is called an HTTP request sent by the client to the server. A complete HTTP request includes a request line, some message headers, and entity content. Some of the message headers and entity content are optional, the message header and entity content must be separated by blank lines.

 

1. Request Line

In the request line, you must note the following request methods:

POST, GET, HEAD, OPTIONS, DELETE, TRACE, PUT

We usually use POST and GET. If there are few others, we will not discuss them here.

Both POST and GET are used for the client to request a certain WEB resource from the server. They can all bring some data to the server, but they have some differences:

If the request method is GET, you can use? To the server. Multiple Data are separated by "&". For example:

GET/aaa/bbb? Name = ccc & password = maid HTTP/1.1

GET Mode features: the parameters attached to the URL address are limited, and the data capacity cannot exceed 1 K. As you can see, when using the GET request method, parameters are displayed in the input box of the browser, so they are visible.

When the request method is POST, you can send data to the server in the request's entity content, for example:

POST/user/loginHTTP/1.1

Host:

Content-Type: application/x-www-form-urlencoded

Content-Length: 28

 

Name = aaa & password = bbb

Post Mode features: There is no size limit on the amount of data transmitted, and the parameters are not displayed in the address box of the browser.

 

2. Message Header

The http Request Header must exist. It carries a lot of important information, such as the browser ID of the client. Below are common headers in HTTP requests:

Accept: text/html, image /*

Accept-Charset: ISO-8859-1

Accept-Encoding: gzip, compress

Accept-Language: en-us, zh-cn

Host: http://www.csdn.net: 80

If-Modified-Since: Tue, 11 Jul 2010 18:23:51 GMT

Referer: http: // http://www.csdn.net

User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)

Cookie

Connection: close/Keep-Alive

Date: Tue, 11 Jul 2010 18:23:51 GMT

 

 

Iv. HTTP response

In response to HTTP requests, an HTTP response represents the data the server sends back to the client. It also includes three parts: a status line, several message headers, and entity content, some message headers and entity content are optional. A blank line is used as the interval between the message header and entity content.

The status code is the server's processing result for the request. It is a three-digit decimal number. The response status codes are divided into five categories:

 

100 ~ 199	The request is successfully received, and the client is required to submit the next request to complete the entire process.
200 ~ 299	Indicates that the request is successfully received and the entire processing process has been completed.
300 ~ 399	To complete the request, the customer needs to further refine the request. For example, the requested resource has moved a new address, which is usually 302, 307, and 304. For example, 304 redirection.
400 ~ 499	Client request errors, such as 403 forbidden access and 404 cannot find resources
500 ~ 599	An error occurs on the server. For example, if an exception is thrown by a server program, 500 is usually used.

 

 

Like HTTP request headers, HTTP also has some common response headers, as shown below:

Location: http: // www.csdn.net

Server: apache tomcat

Content-Encoding: gzip

Content-Length: 80

Content-Language: zh-cn

Content-Type: text/html; charset = GB2312

Last-Modified: Tue, 11 Jul 2010 18:23:51 GMT

Refresh: 1, url = http://www.csdn.net

Content-Disposition: attachment; filename=aaa.zip

Transfer-Encoding: chunked

Set-Cookie: SS = Q0 = 5Lb_nQ; path =/aaa

Expires:-1

Cache-Control: no-cache

Pragma: no-cache

Connection: close/Keep-Alive

Date: Tue, 11 Jul 2000 18:23:51 GMT

 

Note that some common message headers can be used for both requests and responses, such:

Cache-Control: no-cache is used to specify whether the Cache is required

Pragma: no-cache is compatible with http 1.0. Cache-Control: no-cache is the newly added message header of http 1.1.

Connection: close/Keep-Alive, which controls whether a persistent Connection or a non-persistent Connection.

Date: Tue, 11Jul 2000 18:23:51 GMT