Web Technology

HTTP

HTTPis a protocol which allows the fetching of resources, such as HTML documents. It is the foundation of any data exchange on the Web and it is a client-server protocol, which means requests are initiated by the recipient, usually the Web browser. It is an application layer protocol that is sent over TCP, or over a TLS-encrypted TCP connection, though any reliable transport protocol could theoretically be used. A complete document is reconstructed from the different sub-documents fetched, for instance text, layout description, images, videos, scripts, and more.

Experiment:

request

GET / HTTP/1.1
Host: developer.mozilla.org
Accept-Language: fr

response

HTTP/1.1 200 OK
Date: Sat, 09 Oct 2010 14:28:02 GMT
Server: Apache
Last-Modified: Tue, 01 Dec 2009 20:18:22 GMT
ETag: "51142bc1-7449-479b075b2891b"
Accept-Ranges: bytes
Content-Length: 29769
Content-Type: text/html
    
<!DOCTYPE html... (here comes the 29769 bytes of the requested web page)

RESTFul

Representational State Transfer
GET –> read
POST –> create, update
PUT –> update
DELETE –> delete

About XML

eXtensible Markup Language, 可扩展标记语言,是一种程序与程序之间传输数据的标记语言

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE note SYSTEM "book.dtd">
<book id="1">
    <name>Java</name>
    <author>Cay S. Horstmann</author>
    <isbn lang="CN">1234567</isbn>
    <tags>
        <tag>Java</tag>
        <tag>Network</tag>
    </tags>
    <pubDate/>
</book>

XML Downside
- 文件格式庞大复杂,内存占用大传输占用带宽
- 服务端课客户端需要大量的代码解析
- 不同浏览器之间解析XML的方式不一样

XML的解析API

Document Object Model (DOM)：一次性读取XML，并在内存中表示为树形结构

InputStream input = Main.class.getResourceAsStream("/book.xml");
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(input);

Simple API for XML (SAX)：它是一种基于流的解析方式，边读取XML边解析，并以事件回调的方式让调用者获取数据。因为是一边读一边解析，所以无论XML有多大，占用的内存都很小。
```
InputStream input = Main.class.getResourceAsStream("/book.xml");
SAXParserFactory spf = SAXParserFactory.newInstance();
SAXParser saxParser = spf.newSAXParser();
saxParser.parse(input, new MyHandler());
```

JSON

javascript object notation, 新型的轻量级数据交换格式

{
    "id": 1,
    "name": "Java核",
    "author": {
        "firstName": "Abc",
        "lastName": "Xyz"
    },
    "isbn": "1234567",
    "tags": ["Java", "Network"]
}

Upside
- JSON只允许使用UTF-8编码，不存在编码问题
- JSON只允许使用双引号作为key，特殊字符用\转义，格式简单；
- 格式是压缩的,占用带宽小
- 浏览器内置JSON支持，如果把数据用JSON发送给浏览器，可以用JavaScript直接处理
- 支持多种语言
- 能够直接被服务端的代码使用,简化代码开发量

JSON的解析API

Jackson

InputStream input = Main.class.getResourceAsStream("/book.json");
ObjectMapper mapper = new ObjectMapper();
// 反序列化时忽略不存在的JavaBean属性:
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
Book book = mapper.readValue(input, Book.class);

CURL

以命令的方式来执行HTTP协议的请求工具
可以通过CURL操作HTTP的GET/POST/PUT/DELETE方法

访问一个网页

curl www.google.com     //default is get

现实http response的头信息
```
curl -i www.google.com
```
显示一次http请求的通信过程
```
curl -v www.google.com
```

执行GET/POST/PUT/DELETE方法

curl -X GET/POST/PUT/DELETE www.google.com