Web Frameworks

Application framework

An application framework is a structure that already pre-defines the repetitive and hides the mundane. It should make the underlying technology easier to use.

• Database access, acl, security,
• Data sanitisation, error handling, standards compliance, authentication, templating, localisation
• Control logic, assembling views
• Saves time and effort, has been already tested, may come with tooling

Frameworks encourage

• Separation of business logic from presentation/interface - designed into the interfaces
• Separation of roles - developer/presentation
• Creation of components and the framework typically encourages this to conform to a standard
• Separation of stable code from volatile (GUI typically less stable)

Object Decomposition: Splitting the application into objects and assigning these to tiers

However have less control

Web tier frameworks

• typically based around MVC
  • separation of model (persistence/behaviour), view (display) and controller (business logic/flow)

Model 2 architecture

Separation of servlets (control) and JSP (display) Servlet also single point for logging and security (Model 1 - JSP page contains what page is next i.e. not servlet controller)

Struts/JSF Include a configurable controller servlet Abstract classes for request dispatches [Front Controller Pattern] is also a application of the [Mediator Pattern] as it decouples the views Selection of next page based on model and session state Identifying operation (see Blueprint web application framework)

• Use the servlet name and include a wildcard in the controller servelet mapping url
• Include in a hidden field
• Include in the URL as a parameter

Servlet mapping considered most flexible

Web Tier

• dynamic content
• controls flow
• maintains state
• MIME - many content types
• business logic for low traffic, simple transaction behaviour

Web state

• Spec does not require recovery after crash
  • some containers do provide this feature
  • or persist state in the EIS tier
• Container manages session tracking with url rewriting/cookies - the associated with state

Web state scope

• Application - servlet context
• Session - HttpSession (within the HttpServletRequest)
• Request - ServletRequest

JSPs

• Page - PageContext - active when forwarding/including

When using EJBs recommended to use a stateful EJB to store state

• Thread safety easier in EJB (?)
• Automatic management - pooling, passivating when resources are limited
• Client independent

When not using EJBs use the HttpSession to store state

JSP Servlet shared bean example

cookies

Avoid direct cookie manipulation unless direct need

• cookies can get corrupted
• cookies may be filtered/switched off
• size limitations - 4k

Servlets

• service oriented applications
• control functions - dispatching requests
• binary data
• program centric
• extend javax.servlet.http.HttpServlet
• can form filter chains
• lifecycle - listener interfaces

Distributed Servlets

• constraits on session migration

JSP

• text based mark up - html, svg, wml, xml
• document centric
• must be converted into a servlet
• authoring tools
• JSP typically not well formed XML
  • JSP alternate XML syntax allows checking file before runtime

Custom Tags

• reusable (scripting is not)
• portable
• reduces repetition
• make intent clearer

Java Standard Tag Library JSTL

• resource inclusion
• forwarding
• conditional logic
• iterators
• XSLT transformations
• state access
• internationalisation i8t2n
• forms (html)

upper layer built on lower layer
|------------
|JSTL | JSF |
|-----|--|  |
|JSP     |  |
|-----------|
|Servlets   |
|-----------|

Web container

• Services
  • request dispatching
  • concurrency
  • lifecycle
• API
  • naming
  • transaction
  • email
  • session state

Web application deployment descriptor

• defines deployment configuration

RequestDispatcher

• include - combine multiple pages
• forward - send to another page to handle the response - response must be empty

Servlet

• security
• control logic
  • e.g. routing
• templating
• personalisation
• filters
• composite - include multiple JSPs
• forward to jsp for display
• binary data
• varying data structure

Servlet - services Servlet filter - customisation/extension

binary output

   response.setHeader("Content-type", "image/png");
   OutputStream os = rsp.getOutputStream(); //or PrintWriter

• only one servlet instance per definition
• SingleThreadModel servlets - pooled - discouraged

Distributable

• state may be distributed/failed over - only session state
• Server affinity - load balancing more difficult
• State migration - increases network traffic
• State must be in the session
  • Don't use instance or static variables in servlet
  • Don't rely on the server specific ServletContext - can be used for caching
• context events are not distributed

JSP

• for outputting text
• best for dynamic data but static structure
• HTML, XML, ASCII, postscript
• javax.servlet.jsp.JSPWriter used for output
  • ensures proper encoding
  • prevents binary content
• Use custom tags/servlet or helper calss for complex tasks
  • avoid large use of control tags in JSP - hard to maintain
• should never forward or acting as a controller

<%@ include file="static.jsp" @%> - for static content
<jsp:include page="DynamicServlet"/> - for dynamic (or static) content

include directive

• static - can cause servlet to be large

jsp include

• dynamic - runtime overhead
• encoding can only be set once

Custom tag/Scriptlets

Use custom tag instead of include scripts (scriptlets)

• custom tags can be unit tested
• custom tags help keep separation of logic from presentation
• scripts are help separate role of html editor from programmer
• script errors can be harder to debug than custom tags

Deliver plan for payment system Review use cases and supplier payment documents. Highlight issues, write high level scoping document