Servlet&HTTP&Reques&Response Quick Start

servlet related configuration

URLPattern: servlet access path

A servlet can define multiple access paths
```
@WebServlet({"/d4","/dd4","/ddd4"})
```
Path rules:
- /xxx: path matching
- /xxx/xx: multi-layer path, directory structure
- *.do: extension match

HTTP

concept

Hyper Text Transfer Protocol hypertext transfer protocol. What is a protocol? It defines the format for transmitting information between two devices.
features
- The default port number is 80
- Based on the request/response model: one request corresponds to one response
- Stateless, each request is independent of each other, and data cannot be exchanged

3. version

1.0: A new connection will be established for each request response
1.1: multiplexing connection

Request message data format
- request line
```
GET /login.html	HTTP/1.1
```
  There are 7 request methods in the HTTP protocol, and 2 are commonly used
  
  GET
  1. The request parameters are in the request line, after the url? connect
  2. The requested url length is limited
  3. not perfect
  POST
  1. The request parameters are in the request body
  2. There is no limit to the length of the requested url
  3. relatively safe
- request header
  1. User-agent
    
    The browser tells the server the version information of my browser
  2. Refer
    
    where do i come from
    
    It can prevent chain hotlinking and carry out statistical work
- request blank line
  
  Blank line, used to separate post request header and request body.
- request body (text)
  
  The post request method requests the place where the data is stored
- Example:
```
POST /login.html	HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-CN,zh;q=0.8,zh-TW;q=0.7,zh-HK;q=0.5,en-US;q=0.3,en;q=0.2
Accept-Encoding: gzip, deflate
Referer: http://localhost/login.html
Connection: keep-alive
Upgrade-Insecure-Requests: 1

username=zhangsan	
```
Response message data format (omitted)

Request

Request function

Primary user gets request message data

Get request line data GET /day14/demo1?name=zhangsan HTTP/1.1
- getMethod get request method GET
- getContextPath gets the virtual path /day14
- getServletPath gets the servlet path/demo1
- getQueryString gets the Get request parameter name=zhangsan
- getRequestURI Get request URI/day14/demo1
- getProtocol get protocol and version HTTP/1.1
- getRemoteAddr gets the client IP address
Get request header data
- getHeader(String name)
- getHeaderNames(): Get all request header names
Get request body data

getReader obtains after obtaining the stream object

Other functions

General method for getting request parameters
1. String getParameter(String name): Obtain the parameter value according to the parameter name username=zs&password=123
2. String[] getParameterValues(String name): Obtain the array of parameter values according to the parameter name hobby=xx&hobby=game
3. Enumeration getParameterNames(): Get the parameter names of all requests
4. Map<String,String[]> getParameterMap(): Get the map collection of all parameters

Chinese garbled problem

request.setCharacterEncoding("utf-8");

request forwarding

request.getRequestDispatcher.forward(ServletRequest request,ServletResponse response)

features
1. Retweet is just a request
2. It can only be forwarded to the internal resources of the current server
3. The browser address bar does not change
share data
1. Domain objects: share data within a certain scope
2. request domain: the scope of data sharing in a request
```
setAttribute() 存储数据
getAttribute()获取数据
removeAttribute移除数据
```

ServletContext

(Learn in detail under Response)

ServletContext getServletContext()

case-login

Link: Case - Login

Response

HTTP protocol response

Request message: the data sent by the client to the server
- request line
- request header
- request blank line
- request body
Response message: the message sent by the server to the client
- response line
  1. composition
    
    Protocol/Version Response Status Code Status Code Description
  2. response status code
    
    1xx: The server accepts the message from the client, but has not completed the acceptance. After waiting for a while, it sends 1xx multi-status codes
    
    2xx: Success, 200
    
    3xx: Redirection. 302 redirect. 304 access cache
    
    4xx: client error
    
    404 The request path does not have a corresponding resource
    
    405 The request method does not have a corresponding doxxx method
    
    5xx: server error 500 server internal exception
- response header
  1. Content-Type: The server tells the client the data format and encoding format of the response body
  2. Content-disposition: In what format does the server-side Gaoshu client open the corresponding data
    
    –inlinedefault
    
    –attachment;filename=xxx Open as an attachment
```
        response.setHeader("content-disposition","attachment;filname=111");
```
- response empty line
- Response body: the transmitted data
- Response Content Example
```
HTTP/1.1 200 OK
Content-Type: text/html;charset=UTF-8
Content-Length: 101
Date: Wed, 06 Jun 2018 07:08:42 GMT

<html>
<head>
<title>$Title$</title>
</head>
<body>
hello , response
</body>
</html>
```

Response object

set response message
- set response line
  
  Format: HTTP/1.1 200 OK
  
  Set status code: setStatus
- set response header
  
  setHeader(String name, String value)
- set response body
  
  1. Get the output stream getWriter or getOutputStream
  
  2. Use the output stream

redirect

//response.setStatus(302);//1. 设置状态码为302
//      response.setHeader("location","/day/servletDemo3"); //2.设置响应头location

//上面的写法的简介
response.sendRedirect("/day/servletDemo3");

The difference between redirection and forwarding

redirection feature
- address bar changes
- Redirects to other sites
- The redirection is two requests, and the request domain class cannot be used to share data
forwarding features
- Forwarding address path unchanged
- is a request
- Forwarding can only access resources on the current server

access path

relative path

For example./index.html

./Current directory

.../Back one directory
absolute path

For example http://localhost/day14/demo

or /day14/demo
path usage
- client use
  
  A virtual directory needs to be added, and the virtual directory is dynamically obtained
```
request.getContextPath()
```
- server use
  
  No need to add a virtual directory, generally used when forwarding

The server outputs character data to the browser

step

Get character output stream -> output data

Garbled problem

set response header

response.setContentType("text/html;charset=utf-8");

Case - Captcha

Link: verification code case

ServletContext object

concept

Represents the entire web application and can communicate with the program's container (server)

Obtain

request get
```
request.getServletContext();
```
HttpServlet get
```
this.getServletContext();
```

Function

Get the MIME type

MIME type: A file data type defined in the Internet communication process

Format: major type/small type

text/html image/jpeg

method of obtaining
```
getMimeType(String file)
```
Domain Objects: Shared Data
- setAttribute
- getAttribute
- removeAttribute
The scope of the ServletContext object: the data requested by all users, personal understanding is that all servlets share data

Get the real (server) path of the file

String b = context.getRealPath("/b.txt");//web目录下资源访问
System.out.println(b);
String c = context.getRealPath("/WEB-INF/c.txt");//WEB-INF目录下的资源访问
System.out.println(c);
String a = context.getRealPath("/WEB-INF/classes/a.txt");//src目录下的资源访问
System.out.println(a);

Case-File Download

File Download Requirements
- page display hyperlink
- After clicking the hyperlink, the download prompt box will pop up
- Complete image file download
Link File Download-Case