Organization of the platformYour application

Java language

Java Servlet API

JavaServer Pages (JSP)

JSTL

Yourweb pages

What’s irritating about Custom Tags?The tag-extension protocol is too complicated

Tag handler

doStartTag() doEndTag()

doCatch() doFinally()

doInitBody() doAfterBody()

release()

Too hard for

Gosling,

even?

What’s irritating about JSP? Also, tags don’t support certain kinds of code

reuse.

<font color=”<%=statusColor%>”>

<% for (…) { %>

<%= customer %>: <%= hatSize %>

<% } %>

</font>out.println(…);

for(…) {

out.println(…);

…

}

What’s bad about JSP? ()

The general consensus says…

Scriplets

They complicate abstraction and code reuse.They make it harder for nonprogrammers to

maintain pages

JSP 2.0Sun introduced the solution to these

problems in JSP 2.0.It addresses theses issues by introducing

Expression languageTag filesSimplified Tag API (SimpleTag versus Tag)Improved XML syntaxJSTL, although it is not part of JSP specificiation.

JSP Expression LanguageJSP became beautiful after this The motivation is to lose scriplets whatsoever

.Also non java programmers should be able to

use it easily, because it is just an expression language.

It was inspired by xpath and java script except it is much easier than any of them

EL syntaxExpressions appear between ${ and }.

Note that ${ and } may contain whole expressions, not just variable names

E.g., ${myExpression + 2}

Access JavaBean properties using . (dot) ${book.title} translates to book.getTitle()

Access to Array and Collection elements using collection[]Access to a Map value by key using map[“key”]

EL syntaxEL searches each scope area if a

scope isn’t specifiedSpecific scope areas can be used:

${pageScope.title}${requestScope.title}${sessionScope.title}${applicationScope.title}

${duck.beakColor} can resolve to ((Duck) pageContext.getAttribute(”duck”)).getBeakColor()

Automatic Casting, wow

EL OperatorsArithmetic

+, -, *, / (or div), % (or mod) – note on XPath Relational

== (or eq), != (or ne), < (or lt) > (or gt), <= (or le), >= (or ge)

Logical&& (or and), || (or or), ! (or not)

Validationempty

null values Collections or Arrays that are empty Strings that evaluate to “”

EL Operators (Examples)Arithmetic ${age + 3}Comparisons ${age > 21}Equality checks ${age = 55}Logical operations ${young or beautiful}Emptiness detection ${empty a}

‘a’ is empty String (“”), empty List, null, etc. Useful for ${empty param.x}

Implicit objects in ELParam and paramvalues

Access HTTP request parametersExample: ${param.password}

Header and headervaluesRequest header information

Example: ${header[“User-Agent”]}Initparam (for the context init parameters of

the webapp)Example: $

{pageContext.request.remoteUser}Cookies

Example: ${cookie.crumb}

EL: Uses JSTL 1.0 introduced the EL, but it could be

used only within tags.In JSP 2.0, it can be used almost anywhere

<font color=”${color}”> Hi, ${user}. You are <user:age style=”${style}”/> years old.

</font>

JSTLJSP Standard Tag LibrarySet of tags that should be available in all

compliant JSP containersPromotes rapid application development at

the web page layer

Why JSTL?A big reason is that writing your own

custom actions, or tags, can be a painThere was lots of reinventing the

wheel going on, for tasks like displaying a date in a special format

Many open source libraries sprung up, but there was still no standard

This fueled the fire behind JSTL

JSTL: FeaturesControl flow

Iteration, conditionsURL management

Retrieve data, add session IDsText formatting and internationalization

Dates and numbersLocalized messages

XML manipulationXPath, XSLT

Database accessQueries, updates

(you do not need to write java any more )

JSTL: Libraries

Library features Recommended prefix

Core (control flow, URLs, variable access)

c

Text formatting fmt

XML manipulation x

Database access sql

Managing variablesOutputting values with EL

<c:out value=”${user.IQ}” />

Storing data<c:set var=”user” scope=”session”> // arbitrary text</c:set>

Note the use of “var” and “scope”: a JSTL convention

IterationA loop with a start, end, and step (typical for statement)

<c:forEach var="name" varStatus="name" begin="expression" end="expression" step="expression"> body content</c:forEach>

A loop using an Iterator for a collection

<c:forEach var="name" items="expression" varStatus="name" begin="expression" end="expression" step="expression"> body content</c:forEach>

Example<table> <tr><th>Value</th><th>Square</th></tr> <c:forEach var="x" begin="0" end="10" step="2"> <tr> <td>${x}</td> <td>${x * x}</td> </tr> </c:forEach></table>

Conditional logicConditional 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>

Example<c:choose> <c:when test="${pageContext.request.scheme eq 'http'}"> This is an insecure Web session. </c:when> <c:when test="${pageContext.request.scheme eq 'https'}"> This is a secure Web session. </c:when> <c:otherwise> You are using an unrecognized Web protocol. How did this happen?!

</c:otherwise></c:choose>

URL managementRetrieving data<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

Printing URLs<c:url value=”/foo.jsp”>

Redirection<c:redirect url=”/foo.jsp”>

Text formattingLocale-sensitive formatting and parsing

NumbersDates

InternationalizationMessage bundles

Message argument substitution“Hi {0}. I would like to {1} your money today.

I will use it to buy myself a big {2}.”

<fmt:formatNumber type=“currency” value=“$

{salary}” />

<fmt:message key=“welcome” />

Example<table><fmt:timeZone value="US/Eastern"><c:forEach items="${entryList}" var="blogEntry" varStatus="status"> <c:if test="${status.first}"> <tr> <td align="left" class="blogDate"> <fmt:formatDate value="${blogEntry.created}" dateStyle="full"/> </td> </tr> </c:if> <tr> <td align="left" class="blogTitle">${blogEntry.title}</td> </tr> <tr> <td align="left" class="blogText">${blogEntry.text} <font class="blogPosted"> [Posted <fmt:formatDate value="${blogEntry.created}" pattern="h:mm a zz"/>] </font> </td> </tr></c:forEach></fmt:timeZone></table>

database manipulationQueries (and ResultSet caching)

Updates / insertsTransactions (<sql:transaction>)Parametric (PreparedStatement) argument

substitution (<sql:param>)DataSource-based connection management

<sql:query sql=“SELECT * FROM USERS” var=“result” />

<c:forEach items=“${result.rows}”> … </c:forEach>

XML manipulationUse of XPath to access, display pieces of XML

documents

<c:import url=”http://www.cnn.com/cnn.rss” var=”cnn”/><x:parse xml=”${cnn}” var=“dom”><x:out value=”$dom//item[1]/title”/>

Chaining XSLT transformations

<x:transform xslt=”${xsl2}” /> <x:transform xml=”${xml}” xslt=”${xsl}” /></x:transform>

Why Not XSLTJSTL integrates XPath with 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.Functional versus imperative programming

Adding New TagThere are two ways to add news

tagsExtend the JSTL and override methods

Define Tag files (This is much easier, so we gone study this )

Tag FilesSolve difficulty of reusing

text/HTML within a tag.And makes it much easier to write simple tags, since you can do so in JSP instead of Java.

Stand-alone file with <%@ tag %> directive instead of traditional <%@ page %> directive.

Tag Files<%@ tag name=”tableTag” %><%@ attribute name=”items” %>

<table width=”…” bgcolor=”…”> <th> <td>Name</td> <td>IQ</td> </th> <c:forEach var=”i” items=”${items}”> <tr> <td>${i.fullName}</td> <td>${i.IQ}</td> </tr> </c:forEach></table>

Using new Tag....Your shopping cart:

<my:tableTag items=”${cart}” />

Your wish list:

<my:tableTag items=”${wishList}” />

Things we want you to buy:

<my:tableTag items=”${pressuredSales}” />