What is HTTP ? How does HTTP work ?

 " Hypertext Transfer Protocol " is a request - response protocol which is used by webmasters and other clients to access a web server.

Basics of HTTP

• HTTP ( Hypertext Transfer Protocol ) is a protocol which is used by a www ( e.g a Browser ) to send a request to a web server.
• HTTP is a Request - Response oriented protocol.
• A HTTP request consists of 

1. Request method
2. URI
3. Header fields
4. Body ( which can be empty )

•  A HTTP response consists of

1.  Result code
2. Header fields
3. Body

 Overall Process

• The service method of HttpServlet dispatches a request to different java methods for different HTTP request methods.
• It recognizes the standard HTTP/1.1 methods and It should not be overridden in subclasses unless you need to implement additional methods.
• The recognized methods are GET, HEAD, PUT, POST, DELETE, OPTIONS and TRACE. Other methods are answered with a Bad request HTTP error.
• An HTTP method xxx is dispatched to a Java method doxxx ie. GET ==> doGET.
• The doOptions & doTrace methods hava suitable default implementations & are usually not overridden.
• The HEAD method ( similar to GET method ) is performed by calling doGet & ignores any o/p that is written by this method.
• This leaves us with doGet, doPut, doPost & doDelete. whose default implementations return a Bad request in HttpServlet.
• HttpServlet overrides one or more of the remaining methods by its subclass to provide a meaningful implementation.

The Request data is passed to all methods through the first argument of type HttpServletRequest ( A subclass of ServletRequest class ).

The Response data created is created with methods of the second argument of type HttpServletResponse ( A subclass of ServletResponse class ).

 How GET and POST methods work ?

when a client request a URL through a web browser, the GET method is used for the request.

 GET method

• A GET request does not have a body ( i.e  the body is empty ).
• where as the response should contain a body with the response data & header fields which describe the body ( like content-type & content-encoding )
• when we send a HTML form, using GET method( request) the parameters are encoded in the URL ( i.e The data is sent through URL ).

PUT method

• PUT method(request) is used by HTML editors & Upload tools to upload resources to a web server.
• when we send a HTML form, using PUT method(request) the parameters are transmitted in the body ( i.e The data is sent through Body ).