JSTL and Web Development at Universities

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JSTL and Web Development at Universities

• Shawn Bayern
• Research programmer, Yale University
• JSTL reference-implementation lead
• Author, Web Development with JavaServer Pages
• Author, The JSP Standard Tag Library (upcoming)

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Agenda

• JSP overview scripting and tag libraries
• JSTL basics
• Origins
• Design
• Features
• Examples of design principles in use
• Yales network-registration application
• HTML scraping for portals
• QA

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JavaServer Pages

• Scriptlets
• getFoo(request)
• printFoo(out)
• String a goat
• Oink!

• Java (and more?) embedded within template text
• Access to implicit objects request, response,
  etc.
• Conditional blocks, loopsmanually constructed

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JavaServer Pages

• Tag libraries
• c is true
• Round and round we go

• HTML-like tags (can be well-formed XML)
• Invoke Java logic behind the scenes.
• May access body, e.g., for iteration, conditional
  exclusionor just as arbitrary parameter
• May access PageContext
• Libraries and prefixes

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Advantages of tag libraries

• Abstraction, abstraction, abstraction.
• Abstraction.
• Separation of logic from presentation and
  content.
• Simple, familiar interface for page authors.
• Implicit (versus explicit) arguments.

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Tags Abstracting logic
JSP page
Back-end Java code
Tag library
Database
Tag library
Directory
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What is a JSP tag?

• Standard actions e.g.,
• Part of JSP spec
• Java class implementing Tag interface
• Accepts attributes as JavaBean properties
  (setXXX())
• Answers callbacks associated with
• Start of tag
• End of body
• End of tag
• and so on
• Why so specific? (No closure for body.)
• Knows about its page (PageContext)
• Knows about its parent tags
• findAncestorWithClass()
• Optionally knows about its body (BodyContent)

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Tag handler
Tag handler
doStartTag()
doEndTag()
Tag body
Tag attributes
?
doCatch()
doFinally()
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Where do tag libraries come from?

• Java developers like us
• To support ourselves, our page authors
• Vendorse.g., Allaire, BEA, iPlanet
• Standard tag library
• Java Community Process (JSR-052)
• Input from familiar vendors, motivated
  individuals
• Under Suns leadership
• Implementation distributed through Apache

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JSTL design principles

• JSTL 1.0 Keep it simple!
• Targeted tags
• Could have a single tag
• Instead, single-purpose tags, tightly focused
• Design motivated by page author
• Perhaps something of a fantasy, like the legal
  reasonable person. But a helpful guide
  nonetheless.

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The parable of Mike and Phillipe
Mike
Phillipe
Credit Pierre Delisle (spec lead)
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Potential limitations

• Tag abstraction may be excessive for tiny
  applications (but JSP 1.3)
• Page authors need technical skills! They must
  understand
• Flow of application
• Programmatic control flow within page
• Variable reference (expression language)
• Domain-specific languages (SQL, XPath) and
  associated principles

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JSTL 1.0 features

• Conditional logic
• Iteration
• Text retrieval (URL, RequestDispatcher)
• Text formatting and parsing (i18n)
• XML manipulation (XPath, XSLT)
• Database access
• Last but certainly not least
• Expression language

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JSTL 1.0 libraries

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JSTL featuresexpression language

• JSTLs major structural innovation
• Replaces rtexprvalues
• Allows simple retrieval of scoped attributes
• sessionduck
• and other data
• cookiecrumb
• paramemail

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JSP history progress

• Scriptlets
• Beans and standard actions
• /
• Traditional tag libraries
• etid)) /
• Expression-language capable library

rtexprvalue
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State of expression language

• Syntax currently being debated
• EA2 supports pluggable languages for
  experimentation. Candidates include
• SPEL (straw man)
• ECMAScript
• JXPath
• Comments?

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Advantages of expression language

• Scripting, not programming. Not even scripting,
  really.
• Having said that, I personally hope that the
  EL(s) we
• standardize on are so simple as to require almost
  no
• education at all.
• 2
• Typeless reference
• Simplified syntax

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JSTL featurescore tags (1)

• Iteration
• begin5 end20 step4
• varitem
• var convention and variable access
• paging

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JSTL featurescore tags (2)

• Conditional evaluation
• a equals b

• Mutually exclusive conditionals
• a equals b
• a equals c
• I dont know what aequals.

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JSTL featurescore tags (3)

• URL retrieval
• urlhttp//www.cnn.com/cnn.rss/
• Data exposed as String or Reader
• All core URLs supported (HTTP, FTP, HTTPS with
  JSSE)
• Local, cross-context imports supported

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JSTL featurestext formatting

• Formatting and parsing
• Numbers
• Dates
• Internationalization
• Message bundles
• Locale support
• Message argument substitution
• Hi 0. I would like to 1 your money today. I
  will use it to buy myself a big 2.

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JSTL featuresXML manipulation

• Use of XSLT, XPath to access, display pieces of
  XML documents
• varcnn/
• Example later

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JSTL featuresdatabase manipulation

• Tags for
• Queries
• ResultSet caching
• Updates / inserts
• Transactions
• Parametric (PreparedStatement) argument
  substitution
• DataSource-based connection management

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SQL tags the debate
JSP page
Back-end Java code
Tag library
Database
Tag library
JSTL SQL tags
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JSTL programmer support

• JSTL also supports Java developers
• Simplifies tag development
• IteratorTagSupport, ConditionalTagSupport
• Instead of writing whole tag handler
  (doStartTag(), doEndTag()), simply override a few
  methods
• protected boolean condition()
• protected Object next()

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JSTL schedule

• Currently in Early Access Release 2
• No promises, but many hints.
• http//jakarta.apache.org/taglibs/ ? Standard
• EA3 mid December
• Release of 1.0 in 2002
• JSTL 1.0 to include core features
• Later versions fill in other convenient but less
  general functionality.
• Perhaps more J2EE-specific support.

Applies to this presentation too!
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JSP/JSTL future

• Finalize JSTL
• JSP 1.3
• Universalize expression language
• Support creation of tags in JSP itself
• Works nicely with JSTLs focused tags
• Books, educational materials on JSTL to assist
  page authors (and us developers vicariously).

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Yale example Netreg

• Student network registration (Netreg) application
• Used by 5900 students to register computers for
  network use
• Discovers EA via SNMP router query
• Managed by Student Computing group at Yale my
  group provided support
• Developed in less than a month
• Student Computing group (nonprogrammers)
  maintains user interface

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Yale example Netreg

• HTML, JSP code produced and maintained by
  nonprogrammers
• Application uses early-access standard plus
  locally written tags.

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Yale example Netreg

• Example code
• Retrieve full name from database
• var"name netidnetid"/
• Conduct CAS authentication
• Discover Ethernet address
• onError"discoveryFailed.jsp"/

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Yale example Netreg

• Custom registration tags documented, available
  to page authors.
• Via attributes, the data these tags expose is
  accessible to JSTL tags.

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Yale example XML/XPath

• Portals, other summary web sites often need to
  retrieve content from other pages.
• JSTL provides a flexible, standard way to handle
  this task.
• Example retrieve headlines from student
  newspaper, display in custom format.

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Yale example XML/XPath

• com/"/
• select"dom//a_at_class'leadheadline/
• Sample XML
• Headl
  ine

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Advantages of JSTL XML/XPath support

• Why not always use XSLT?
• JSTL provides XPath support but doesnt preclude
  convenient, standard access to Java/JSP code.
• E.g., parse an article URL out of a document,
  then follow the URL and parse its contents.
• JSP/JSTL may be more familiar and convenient for
  simple tasks.

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Summary

• JSP supports scriptlets (poor abstraction) and
  tags (useful abstraction)
• JSTL is the JSP Standard Tag Library.
• JSTL and JSP tags support separation of content
  from presentation

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URLs

• Jakarta Taglibs
• http//jakarta.apache.org/taglibs/
• JSTL JSR
• http//www.jcp.org/jsr/detail/052.jsr
• JSP 1.3 JSR
• http//www.jcp.org/jsr/detail/152.jsr
• Official email address for JSTL comments
• mailtojsr052-comments_at_sun.com
• My upcoming JSTL books site
• http//www.jstlbook.com/
• My email address
• mailtoshawn.bayern_at_yale.edu