Servlet Basic Concepts

The Java Servlet API

HTTP

HyperText Transfer Protocol

• Stateless request/response client-server protocol

Requests:

• Method: GET, POST, HEAD, TRACE, OPTIONS, PUT, DELETE
• URI (required in HTTP/1.1)
• Header Fields
• E.g. how the response should be returned, under what conditions, identification and characterization of client, accounting data
• Body
• POST data
• Empty for GET
• Response:
• Status code (machine), reason (human)
• Header
• Metadata, e.g. Content-Type (Media type), Content-Length, Last-Modified, Etag
• Body
• (X)HTML, other XML, text, binary data … 

URL Connections

• java.net also -- connections extend Socket
• Encapsulates HTTP and FTP connections
• URI, URL, URLConnection, HttpURLConnection

Servlets Definition

Server side component in a client server model (now the browser is the client )
Reside in a servlet container, assigned to a certain URL pattern.
Provide mechanisms for maintaining state over the stateless HTTP protocol

Servlet Model

Servlet API

Interfaces:

• HttpServletRequest
• HttpServletResponse
• HttpSession
• HttpBindingSession
• HttpSessionContext

Interfaces are implemented by server providers and can be used out of the box


Classes

• Cookie
• HttpServlet
• HttpSessionBindingEvent
• HttpUtils

Servlet Lifecycle

• Multithreaded access (usually default)
• init called first time only (by the container)
• zero to many calls to service
• destroy called

init (ServletConfig)

• call super.init (config), or just use init ()
• Called once
• Prior to any call to service
• Don’t worry about multithreading issues here
• Sometimes used to get resources needed for the lifetime of the servlet

service (req, resp)

• Not usually overridden
• Default impl. determines what request handler to call (based on HTTP request type), calls itService method will call doGet, doPost, doPut, etc. based on service type.
• Default implementations provided for doHead, doTrace, doOptions

doPost, doGet, etc.

• doPost (HttpServletRequest req, HttpServletResponse resp)
• Implement this to handle POSTs
• Read from req, build resp
• Multithreaded access by default (depending on server config)
• Beware instance variables, shared data
• config and context are shared, session is usually safe, req/resp are not
• Use locks and/or synchronized data structures if shared data is an issue

destroy ()

called once

• Servlet timeout, servlet reload, container shutdown
• Other threads may still be processing service requests, no further requests will be processed
• Release resources, write data, etc.