J2EE Servlets

1
J2EE Servlets

2
Agenda

• Overview
• Servlet Interface
• Servlet Context
• Request
• Response
• Sample Servlet
• Sessions
• Dispatching Request
• Web Applications
• Deployment Descriptor

3
What is a Sevlet?

• Web component managed by servlet container
• Generates dynamic content, usually HTML
• Small, platform-independent classes compiled to
  bytecode
• Loaded dynamically and run by a container in
  conjunction with a web server
• Interacts with a web client via request -
  response model based on the the HTTP protocol
• Can access databases and other storage systems

4
What is a Servlet Container?

• Execution context of the servlet
• Provides network services over which requests and
  responses are exchanged
• Contains and manages servlets through their
  lifecycle
• Can be either built-in into a web server or web
  enabled application server or installed as an
  add-on component
• Must support the HTTP Protocol (at minimum
  HTTP/1.0)
• May place security restrictions on the
  environment that a servlet executes in (e.g.
  Number of simultenously running threads)

5
Benefits of Servlets

• Performance
• Scalability
• Distributed environment (distributable web
  applications)
• Multithreaded environment
• Portability
• Platform independence
• Ease of development
• Part of J2EE (Servlet API is a required API of
  J2EE)
• Standard supported by many vendors and web servers

6
Typical Scenario

• Request is submitted from a web browser
• Request is converted to a name-value list by the
  servlet container
• The service() method of a servlet is called with
  the name-value list together with the response as
  arguments
• The servlet processes the request by invoking
  EJBs, performing other business logic, connecting
  to databases or application server extensions
• The servlet prepares HTML code to return either
  directly printing to the response stream

7
The Servlet Interface

• void init(ServletConfig config)
• void service(ServletRequest req, ServletResponse
  res)
• void destroy()
• ServletConfig getServletConfig()
• java.lang.String getServletInfo()
• Central abstraction of the Servlet API
• Interface implemented either directly or by
  extending

8
Servlet Life Cycle

• Loading and instantiating
• At start time
• Delayed until the service method of the servlet
  is requested
• The servlet container load the servlet class
  (file system, remote file system, other network
  services)
• The Servlet Container instantiates an object
  instance
• There can be more than one instance
  (SingleThreadModel) of a given servlet (several
  servlet definitions which point to the same
  class) or only one instance (MultipleThreadModel)
• Initialization
• Point where servlet can read any configuration
  data, persistent data, make a connection to a
  database and other costly resources, perform
  one-time activities
• The Servlet Container calls init method of the
  servlet and passes ServletConfig - access to
  name-value initialization parameters
• On error throw ServletException or
  UnavailableException

9
Servlet Life Cycle

• End of service
• A servlet instance may be kept active in a
  container for any amount of time
• The container allow any threads executing service
  method to complete
• The container executes destroy method - point
  where a servlet may release any resources and
  save its state to any persistance storage
• No requests can be routed to this instance of the
  servlet
• The instance is released so that it can be
  garbage collected

10
Servlet Life Cycle

• Request handling
• Each request represented by ServletRequest object
• Servlet can create the response by using object
  of type ServletResponse
• ServletRequest and ServletResponse are provided
  as parameters to service
• HTTP - HttpServletRequest, HttpServletResponse
• Errors during request handling
• ServletException
• UnavailableException
• Permanent - servlet should be removed from the
  service (destroy is called)
• Temporary - SERVICE_UNAVAILABLE (503) is returned
  with Retry-After header

11
Multithreading Requests

• MultipleThreadModel (default) One instance
  of a servlet class per servlet definition
• SingleThreadModel (marker interface)
• Multiple instances of the servlet. Ensures that
  servlets handle only one request at a time.
• No two threads will execute concurrently in the
  servlet's service method. The servlet container
  can make this guarantee by synchronizing access
  to a single instance of the servlet, or by
  maintaining a pool of servlet instances and
  dispatching each new request to a free servlet.
• The servlet is thread safe. SingleThreadedModel
  does not prevent synchronization problems that
  result from servlets accessing shared resources
  (static class variables, classes outside the
  scope of the servlet).

12
The Servlet Context

• A servlet's view of the web application within
  which it is running
• One instance per web application (per VM for
  distributed applications)
• The container implements the ServletContext
  interface
• ServletContext
• allows a servlet to set and store attributes, log
  events, obtain URL references to resources
• provides a set of methods that a servlet uses to
  communicate with its servlet container
• Context attributes - available to any other
  servlet of the web application (exits only
  locally in the VM)
• Resources - direct access to static documents
  (HTML, GIF, JPEG)

13
The Request

• Encapsulates information from the client
• HttpServletRequest - HTTP headers and message
  body
• Parameters
• getParameter, getParameterNames,
  getParameterValues
• String parameters stored as name-value pairs
• Possible multiple parameter values
• Attributes
• getAttribute, getAttributeNames,
  getAttributeValues
• Objects associated with a request set by the
  container or a servlet to communicate with
  another servlet
• Single values
• Cookies
• getCookies

14
Path Elements Translation

• Request path is composed of
• Context Path getContextPath
• Servlet Path getServletPath
• Following is always true
• RequestURI contextPath servletPath
• Pattern /garden
• Servlet GardenServlet
• /catalog/garden/implements -gt ContextPath
  /catalog
• ServletPath /garden

15
The Response

• Encapsulates information to be returned to the
  client
• HttpServletResponse - HTTP headers and message
  body
• Cookies
• Helper methods
• sendRedirect
• sendError
• Typical use
• Either getOutputStream (binary data) or getWriter
  (text)

16
Sample Servlet

• public class SampleServlet extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws IOException, ServletException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• out.println("lthtmlgtltbodygt")
• out.println("Hello World!")
• out.println("lt/bodygtlt/htmlgt")

17
Sessions

• HTTP - stateless protocol
• Different requests from the same client should be
  associated with each other
• Strategies for session tracking
• Hidden variables
• URL rewriting
• Cookies
• In servlet environment
• Session tracking implemented by servlet container
• HttpSession provided as an convenient interface
• A session persists for a specified period of time

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HttpSession

• View and manipulate information about a session,
  such as the session identifier, creation time,
  last accessed time
• Bind objects to sessions, allowing user
  information to persist across multiple user
  connections
• Session information is scoped only to the current
  web application (ServletContext)
• Binding attributes into a session
• getAttribute, setAttribute, removeAttribute
• HttpSessionBindingListener - causes an object to
  be notified when it is bound to or unbound from a
  session
• Session timeouts because there is no explicit
  client termination

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Session Semantics

• Multiple servlets may have access to a single
  session object
• Synchronization is the responsibility of a
  developer
• For applications marked as distributable session
  objects must implement java.io.Serializable
  interface
• Scalability - in distributed environment session
  objects can be moved from any active node to some
  other node of the system
• All windows of a client are participating in the
  same session

20
Dispatching Requests

• forward - pass processing to another servlet
• Including output of another servlet in the
  response
• RequestDispatcher can be obtained from
  ServletContext
• include
• Can be called at any time
• Included servlet cannot set headers or call any
  method that affects the headers of the response
• forward
• Can only be called if no output has been commited
  to a client

21
Web Applications

• Collection of servlets, JSPs, HTML pages and
  other resources bundled and run on possibly
  multiple containers
• Rooted at a specific path eg. http//www.company.c
  om/catalog
• By default one instance must only be run on one
  VM
• Distributable web applications - components
  distributed across multiple containers
• 11 mapping between web application and
  ServletContext
• Representation
• Structured hierarchy of directories
• Archive file

22
Open Directory Structure

• Root directory eg. /catalog
• Special WEB-INF directory - not a part of public
  document tree
• Sample directory structure
• /index.html
• /howto.jsp
• /feedback.jsp
• /images/logo.gif
• /WEB-INF/web.xml
• /WEB-INF/lib/jspbeans.jar
• /WEB-INF/classes/com/dsrg/servlets/Test.class
• /WEB-INF/classes/com/dsrg/util/Helper.class

23
Deployment Descriptor

• Elements and configuration information of a web
  application
• Contents of a deployment descriptor
• ServletContext init parameters
• Session configuration
• Servlet/JSP definitions
• Servlet/JSP mappings
• Mime Type mappings
• Welcome file list
• Error pages
• Security info
• XML document