1. Overview of Servlets and JSP
Overview of HTTP
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2. Overview of Servlets and JSP
HTTP
Hypertext Transfer Protocol
Specification:
Set of rules for transferring resources (files, images, etc.) over the web
A resource on the web is usually identified by a URL (Uniform Resource Locator)
Characteristics
simple
stateless
text-based (originally, but not HTTP/2)
request/response protocol
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3. Structure of an HTTP Transaction
Overview of Servlets and JSP
Structure of an HTTP Transaction
HTTP client (e.g., a browser) opens a connection and sends a request message to an HTTP server.
Server formulates and returns a response message, usually containing the resource that was requested.
In HTTP/1.0, after delivering the response, the server closed the connection. Connection information was not maintained between transactions.
HTTP/1.1 added persistent connections
connection can remain open after receiving a response and can be re-used for the next request
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4. HTTP Example Using HTTP 1.0
Overview of Servlets and JSP
HTTP Example Using HTTP 1.0
From the command prompt, enter the following commands:
telnet
set localecho
open macs.citadel.edu 80
GET /moorej/hello.html HTTP/1.0
<enter>
Note: Telnet is not installed by default on Windows. To
turn it on, go to Control Panel -> All Control Panel Items
-> Programs and Features. Click on “Turn Windows
features on or off” and check the box for Telnet Client.
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5. HTTP Example Using HTTP 1.1
Overview of Servlets and JSP
HTTP Example Using HTTP 1.1
From the command prompt, enter the following commands:
telnet
set localecho
open macs.citadel.edu 80
GET /moorej/hello.html HTTP/1.1
Host: macs.citadel.edu
<enter>
With HTTP 1.1, a single web server can host several
domains. Therefore, every HTTP request must specify
the host name (and possibly port if different from 80).
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6. Overview of Servlets and JSP
HTTP Message Format
Both request and response messages have a similar format:
an initial line
zero or more lines known as headers
a blank line
an optional message body (e.g. a file, query data, binary data, etc.)
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7. Overview of Servlets and JSP
Initial Request Line
The initial line of a request has three parts, separated by spaces:
Method name
GET is the most common HTTP method
Other methods include POST and HEAD
Method names are always uppercase
Local path of the requested resource
the request URI (a URI is like a URL, but more general)
Version of HTTP being used
Always takes the form “HTTP/x.x”
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8. Initial Response Line (a.k.a. the Status Line)
Overview of Servlets and JSP
Initial Response Line (a.k.a. the Status Line)
The initial line of a response also has three parts, separated by spaces:
The HTTP version
same format as the initial request line, “HTTP/x.x”
A response status code that gives the result of the request
Examples: 200 or 404
An English phrase describing the status code.
Examples: “OK” or “Not Found”
Example: HTTP/ OK
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9. Overview of Servlets and JSP
Status Code
Three-digit integer
First digit identifies the general category of response
1xx indicates an informational message only
2xx indicates success of some kind
3xx redirects the client to another URL
4xx indicates an error on the client's part
5xx indicates an error on the server's part
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10. Overview of Servlets and JSP
Common Status Codes
200 OK
request succeeded; resource is returned in the message body
404 Not Found
requested resource doesn’t exist
301 Moved Permanently
302 Moved Temporarily
303 See Other (HTTP 1.1 only)
resource has moved to the URL given in the “Location” response header; should be automatically retrieved by the client
500 Server Error
unexpected server error
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11. Overview of Servlets and JSP
HTTP Methods
HTTP defines a set of request messages called methods (not to be confused with Java methods)
GET
POST
PUT
DELETE
HEAD
Others (CONNECT, TRACE. etc.)
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12. Overview of Servlets and JSP
The HTTP GET Method
Requests information from a web server
Example
GET /HelloUser.jsp?name=John HTTP/1.1
Host:
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: ) Gecko/ Firefox/
Accept: text/xml,application/xml,application/xhtml+xml, text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO ,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Note: Each header is on a single line (no line breaks).
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13. Using HTTP GET Requests
Overview of Servlets and JSP
Using HTTP GET Requests
The handling of a GET method is expected to be
safe: it does not have any side effects for which users are held responsible
idempotent: it can safely be repeated. This allows a client to repeat a GET request without penalty.
Intuitively, GET should be “looking without touching.”
If processing has side effects, use another HTTP method, such as POST.
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14. Important Limitation on HTTP GET Requests
Overview of Servlets and JSP
Important Limitation on HTTP GET Requests
Most web servers limit how much data can be passed as part of the URL name (usually a few hundred bytes).
If more data must be passed between the client and the server, the HTTP POST method can be used instead.
data as part of the URL
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15. Example: HTTP Response to a GET Method
Overview of Servlets and JSP
Example: HTTP Response to a GET Method
HTTP/ Document follows Date: Tue, 14 Apr :25:19 PST Server: JWS/1.1 Last-modified: Mon, 17 Jun :53:08 GMT Content-type: text/html Content-length: <4435 bytes worth of data -- the document body>
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16. Overview of Servlets and JSP
The HEAD Method
Similar to the HTTP GET method
Server returns only the header information.
HEAD is often used to check the following:
last-modified date of a document on the server for caching purposes
size of a document before downloading
server type
type of the requested document
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17. Overview of Servlets and JSP
The POST Method
Allows a client to send data to the server
Passes all of its parameter data in an input stream
Unlike the GET method, POST
may cause side effects
is not required to be repeatable
Several purposes
posting information to a newsgroup
adding entries to a web site's guest book
passing more information than a GET request allows
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18. Overview of Servlets and JSP
Example: POST Request
POST /servlet/MyServlet HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE 4.01; Windows NT)
Host:
Accept: image/gif, image/x-xbitmap, image/jpeg, */
Content-type: application/x-www-form-urlencoded
Content-length: 39
name=John&company=SoftMoore%20Consulting
Note: A blank line signals the end of the POST request header and the beginning of the extended information.
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19. HTTP/2 Second major version of the HTTP network protocol
Approved in February 2015 as the successor to HTTP 1.1. which was standardized in 1997.
Based on SPDY, an HTTP-compatible protocol developed by Google and supported in most browsers.
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20. HTTP/2 Features Binary, multiplexed network protocol
no longer text-based
Basic protocol unit of HTTP/2 is a frame. Frames are exchanged over a TCP connection instead of text-based messages.
Negotiation mechanism allows use of HTTP 1.1, 2.0, etc.
Maintains high-level compatibility with HTTP 1.1
HTTP methods, status codes, default port numbers, and semantics are not changed.
Loads page elements in parallel over a single TCP connection
HTTP/1.1 used multiple parallel connections for downloading all the elements on a given web page (e.g., images).
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