JSTL and Web Development at Universities Shawn Bayern Research programmer, Yale University JSTL reference-implementation lead Author, Web Development with

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)

Agenda

JSP overview: scripting and tag libraries JSTL basics

Origins Design Features

Examples of design principles in use Yale’s network-registration application HTML “scraping” for portals

Q&A

JavaServer Pages

Scriptlets

<%

getFoo(request);

printFoo(out);

String a = “goat”;

%>

<%if (a.equals(“pig”) {%>

Oink!

<%}%>

Java (and more?) embedded within template text

Access to implicit objects: request, response, etc.

Conditional blocks, loops—manually constructed

JavaServer Pages

Tag libraries

<foo:bar/><c:if test=“c”> c is true</c:if><c:forEach> Round and round

we go</c:forEach>

HTML-like tags (can be well-formed XML)

Invoke Java logic behind the scenes.

May access body, e.g., for iteration, conditional exclusion—or just as arbitrary parameter

May access PageContext

Libraries and prefixes

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.

Taglibrary

Tags: Abstracting logic

JSPpage

Back-endJavacode

Database

Directory

Taglibrary

What is a JSP tag?

“Standard actions” – e.g.,<jsp:useBean> 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)

Tag handler

Tag handler

doStartTag() doEndTag()

doCatch() doFinally()

Tagattributes

Tagbody

Where do tag libraries come from? Java developers like us

To support ourselves, our page authors Vendors—e.g., Allaire, BEA, iPlanet Standard tag library

Java Community Process (JSR-052) Input from familiar vendors, motivated

individuals Under Sun’s leadership Implementation distributed through Apache

JSTL design principles

JSTL 1.0: Keep it simple! Targeted tags

Could have a single <go> tag:• <go action=“forEach”

argument1=“blargh”>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.

The parable of Mike and Phillipe

Mike Phillipe

Credit: Pierre Delisle (spec lead)

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

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

JSTL 1.0 libraries

Library features Recommended prefix

Core (control flow, URLs, variable access)

c

Text formatting fmt

XML manipulation x

Database access sql

JSTL features:expression language

JSTL’s major structural innovation Replaces “rtexprvalues”

<%= pageContext.getAttribute(“duck”) %>

Allows simple retrieval of scoped attributes… $session:duck

… and other data $cookie:crumb $param:email

JSP history: progress

Scriptlets<%= findName(request.getParameter(“netid”)) %>

Beans and standard actions<jsp:useBean id=“netid” class=“edu.yale.its.Netid”/>

<jsp:setProperty name=“netid” property=“netid”/>

<jsp:getProperty name=“netid” property=“name”/>

Traditional tag libraries<yale:fullName netid=‘<%= request.getParameter(“netid”)) %>’/>

Expression-language capable library<yale:fullName netid=“$netid”/>

rtexprvalue

State of expression language

Syntax currently being debated EA2 supports “pluggable” languages

for experimentation. Candidates include:SPEL (straw man)ECMAScriptJXPath

Comments?

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

JSTL features:core tags (1) Iteration

<c:forEach items=“$list”begin=“5” end=“20” step=“4”var=“item”>

<c:expr value=“$item”/></c:forEach>

“var” convention and variable access “paging”

JSTL features:core tags (2) Conditional evaluation

<c:if test=“$a = b”> a equals b</c:if>

Mutually exclusive conditionals

<c:choose> <c:when test=“$a = b”> a equals b </c:when> <c:when test=“$a = c”> a equals c </c:when> <c:otherwise> I don’t know what

‘a’equals. </c:otherwise></c:choose>

JSTL features:core tags (3)

URL retrieval

<c:import var=“cnn”

url=“http://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

JSTL features:text 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}.”

JSTL features:XML manipulation

Use of XSLT, XPath to access, display pieces of XML documents

<c:import url=“http://www.cnn.com/cnn.rss” var=“cnn”/>

<x:parse source=“$cnn” var=“dom”>

<x:expr value=“$dom//item[1]/title”/>

Example later

JSTL features:database manipulation Tags for

Queries• ResultSet caching

Updates / insertsTransactionsParametric (PreparedStatement)

argument substitutionDataSource-based connection

management

SQL tags: the debate

Taglibrary

JSPpage

Back-endJavacode

DatabaseTag

library

JSTL programmer support

JSTL also supports Java developersSimplifies tag developmentIteratorTagSupport,

ConditionalTagSupportInstead of writing whole tag handler

(doStartTag(), doEndTag()), simply override a few methods:

• protected boolean condition()• protected Object next()

JSTL schedule

Currently in Early Access Release 2No 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 featuresLater versions fill in other convenient

but less general functionality.• Perhaps more J2EE-specific support.

Applies tothis presentation too!

JSP/JSTL future

Finalize JSTL JSP 1.3

Universalize “expression language”Support creation of tags in JSP itself

• Works nicely with JSTL’s focused tags

Books, educational materials on JSTL to assist page authors (and us developers vicariously).

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


HTML, JSP code produced and maintained by nonprogrammers

Application uses early-access “standard” – plus locally written – tags.


Example codeRetrieve full name from database

<reg:fullName

var="name“ netid=“$netid"/>

Conduct CAS authentication<cas:auth var="netid" scope="session"/>

Discover Ethernet address<reg:eha var="eha" scope="session" onError="discoveryFailed.jsp"/>


Custom “registration” tags documented, available to page authors.

Via attributes, the data these tags expose is accessible to JSTL tags.

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.

Yale example: XML/XPath

<c:import var="ydn" url="http://www.yaledailynews.com/"/>

<tidy:parse input=“$ydn” output=“ydn”/><x:parse var="dom" source="$ydn"/>

<%-- Print the lead headline --%><x:expr select="$dom//a[@$class='leadheadline’]”/>

Sample XML:<table> … <a class=“leadheadline” href=“article.asp”>Headline</a> …</table>

Advantages of JSTL XML/XPath support

Why not always use XSLT?JSTL provides XPath support but

doesn’t 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.

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

URLs

Jakarta Taglibshttp://jakarta.apache.org/taglibs/

JSTL JSRhttp://www.jcp.org/jsr/detail/052.jsr

JSP 1.3 JSRhttp://www.jcp.org/jsr/detail/152.jsr

Official email address for JSTL commentsmailto:[email protected]

My upcoming JSTL book’s sitehttp://www.jstlbook.com/

My email addressmailto:[email protected]

Documents

JSTL and Web Development at Universities Shawn Bayern Research programmer, Yale University JSTL reference-implementation lead Author, Web Development with