jstl: the javaserver pages standard tag library mark a. kolb security broadband, austin, tx...

JSTL: The JavaServer Pages Standard Tag Library

Mark A. KolbSecurity Broadband, Austin, [email protected]

Prerequisites Servlet API JavaServer Pages

Basic syntaxImplementation via Servlet API

Using resource bundles for I18N XML

Related Sessions Kimberly Bobrow’s “Introduction to

JavaServer Pages (JSP)” Bryan Basham’s “Web Application

Development, A Case Study” Noel Bergman’s “A Visitor’s Guide to

Jakarta” Mark Kolb’s “Authoring JSP Custom

Tags”

What Is the JSTL? A poor acronym

J = JSP?There are actually four tag libraries in the

JSP Standard Tag Library Based on JSP 1.2/Servlet 2.3

What Is the JSTL? A collection of tag libraries

implementing common JSP functionalityCore functions

• Variables, I/O, conditionalization, iterationFormatting/I18n

• Message bundles, numbers, datesXML operations

• parsing, transformationsDatabase operations

• SQL

What Is the JSTL? An expression language for specifying

custom tag attribute valuesBased on ECMAScript and Xpath

Expression language is optionalEach of the four JSTL libraries has an EL

version and an RT version.• EL = JSTL expression language• RT = JSP request-time attribute values

What Is the JSTL? A set of Java classes

Interfaces and support classes for custom tag implementors• Provides for interoperability with JSTL tags

General-purpose tag library validators• JSTL 1.0 validators focus on enforcing coding

standards•ScriptFreeTLV - prohibit scripting elements•PermittedTaglibsTLV - restrict tag library

usage

Why JSTL? JSTL tags provide a standard

implementation for typical application functionalityReusabilityAvoid reinventing the wheel

Another mechanism for avoiding the use of JSP scripting elementsEL considerably simpler than JavaStrong emphasis on “variables”Two steps forward, one step back?

JSTL Expression Language Delimiters are “${” and “}” The EL can only be used for specifying

attribute values in JSTL tags<c:out value=”${user.firstName}”/>

Multiple expressions can be combined and mixed with static text (i.e., implicit string concatenation)<c:out value=”Hello ${user.firstName} ${user.lastName}!”/>

JSTL Expression Language Individual expressions are

combinations of identifiers, accessors, and operators

There are also identifiers for a set of JSTL implicit objectsNot the same as the JSP implicit objects

(with one exception)

EL Identifiers Identifiers are resolved against the four JSP

scopesUsing PageContext.findAttribute(name)Scopes are searched sequentially: page, request, session, application

Reserved identifiers for the 11 JSTL implicit objects:pageContext, pageScope, requestScope, sessionScope, applicationScope

param, paramValues, header, headerValuesinitParam, cookie

EL Accessors Properties of objects are accessed via

the “.” operatoruser.firstName represents the value of

the “firstName” property of the object referenced by the “user” identifier

EL Accessors Elements of a Map, List, or array are

accessed via the “[]” operatorFor a map associated with the identifier users, users[”frodo”] represents the value mapped to the ”frodo” key

For a list or array associated with the identifier track, track[4] represents the fifth element of the sequence

Elements can be referenced via identifiers, as in users[username] or track[index]

EL Accessors Actually, the “.” and “[]” operators are

interchangeableuser.firstName and user[”firstName”]

are equivalent expressions

EL Implicit Objects pageContext is identical to the JSP implicit object of

the same name Provides access to the other JSP implicit objects and their

properties${pageContext.request.queryString}

The “scope” implicit objects are maps for looking up scoped attributes pageScope, requestScope, sessionScope, and applicationScope

For example, ${sessionScope[”userProfile”]} retrieves the attribute named ”userProfile” from the user’s session, equivalent to

<%= session.getAttribute(”userProfile”)%>

EL Implicit Objects The param implicit object is a map for

looking up the value of a request parameter${param[”keyword”]} is equivalent to

<%= request.getParameter(”keyword”)%>

paramValues is also a map, which returns an array of strings containing all of the values associated with a request parameter${paramValues[”keyword”]} is equivalent to<%= request.getParameterValues(”keyword”)%>

EL Implicit Objects The header implicit object is a map for

looking up the value of a request header${header[”User-Agent”]} is equivalent to

<%= request.getHeader(”User-Agent”)%>

headerValues is also a map, which returns an array of strings containing all of the values associated with a request header${headerValues[”Accept”]} is equivalent to

<%= request.getHeaders(”Accept”)%>

EL Implicit Objects The initParam implicit object is a map

for looking up the value of a context initialization parameter

The cookie implicit object is a map for looking up a cookie from its name

EL Operators Arithmetic operators

+, -, *, / (or div), % (or mod)

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

(or ge) Can be applied to both numeric and string values

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

Empty operator empty expr indicates whether expr is null or an empty String, Map, List, or array.

EL Operators Logical operators will short-circuit

evaluationFor example, in ${expr1 && expr2}, expr2 will not be evaluated if expr1 has a value of false.

Parentheses can be used for grouping and will override operator precedence rulesFor example, ${a * (b + c)} overrides

the normal precedence of multiplication over addition

Core Library EL library

Dynamic attribute values specified using the JSTL expression language (i.e., ${ expr }) <%@ taglib uri=”http://java.sun.com/jstl/core”

prefix=”c”>

RT libraryDynamic attribute values specified using

the JSP expressions (i.e., <%= expr %>) <%@ taglib uri=”http://java.sun.com/jstl/core_rt”

prefix=”c_rt”>

Core Library Tags General-purpose actions

<c:out><c:set><c:remove><c:catch>

Conditional actions<c:if><c:choose>, <c:when>, <c:otherwise>

Core Library Tags Iteration actions

<c:forEach><c:forTokens>

URL actions<c:url>, <c:param><c:redirect>, <c:param><c:import>, <c:param>

<c:out> Tag <c:out value=”value”

default=”defaultValue”escapeXml=”bool”/>

Evaluates the value attribute and outputs the result as a string

Provides equivalent functionality to JSP expressions and the <jsp:getProperty> action

Prints the value of the default attribute if the value attribute evaluates to nullDefault result can also be specified via body

content

<c:out> Tag The escapeXml attribute determines

whether or not characters are converted to XML entities (defaults to true)

< <

> >

& &

’ '

” "

<c:set> Tag <c:set var=”varName”

value=”value”scope=”varScope”/>

Evaluates the value attribute and assigns the result to a scoped variable

Variable scope is either page (the default), request, session, or application

Variable value can also be specified via body content

<c:set> Tag <c:set target=”beanOrMap”

property=”propertyOrKey”value=”value”/>

Evaluates the value attribute and assigns the result to the specified JavaBeans property or map key In the former role, provides equivalent

functionality to the <jsp:setProperty> action

Property value can also be specified via body content

<c:remove> Tag<c:remove var=”varName”

scope=”varScope”/> Removes the named variable from the

indicated scope Variable scope is either page (the

default), request, session, or application

<c:catch> Tag <c:catch var=”varName”>

nested actions </c:catch>

Catches any exception thrown by the nested actionsCaught exception is assigned to the

(optional) named variable (with page scope)

If no exception is thrown, the variable is removed from page scope

<c:if> Tag<c:if test=”condition”

var=”varName”scope=”varScope”>

body content</c:if>

Conditionally processes the body content Body content can be omitted to just

perform the variable assignment, in which case the var attribute is no longer optional

<c:choose> Tag<c:choose>

<c:when test=”condition”>body content

</c:when>…<c:otherwise>

body content</c:otherwise>

</c:choose> Enables mutually exclusive conditionalization

<c:choose> Tag There must be at least one <c:when> action Only the first <c:when> whose test condition

evaluates to true will have its body content processed

There can be at most one <c:otherwise> action, and it must be the last action within the <c:choose> body

The <c:choose> body can contain only whitespace, <c:when> actions, and <c:choose> actions

ConditionalTagSupport Class

JSTL provides base class for custom tag implementors javax.servlet.jsp.jstl.core.ConditionalTagSupport

Utility class for implementing custom tags for conditionalizing contentCustom tag works like <c:if>, conditionalizing

body content and optionally exposing a scoped variable

This variable can then be referenced by the test attribute of subsequent <c:if> and <c:when> actions

<c:forEach> Tag <c:forEach var=”varName”

varStatus=”varStatusName”begin=”begin” end=”end”step=”step”>

body content</c:forEach>

Iteratively processes the body content for a fixed number of times, like a for statementFrom begin to end, by an optional step

<c:forEach> Tag <c:forEach var=”varName”

items=”collection” varStatus=”varStatusName”begin=”begin” end=”end”step=”step”>


Iteratively processes the body content for all of the items in a collectionOptionally bound by begin, end, and step

<c:forEach> Tag The items attribute supports all

standard J2SE collection typesjava.util.Collection, java.util.Mapjava.util.Iterator, java.util.EnumerationArrays, including arrays of primitivesString objects which use embedded comma

delimiters

<c:forEach> Tag The variable named by the var

attribute references the current item of the iterationPrimitives (from an array) are wrappedFor maps, the variable references an

instance of java.util.Map.Entry (inner class of Map)•Entry has two properties, key and value

This variable has nested visibility

<c:forEach> Tag The variable named by the varStatus

attribute references an instance of the LoopTagStatus class from the javax.servlet.jsp.jstl.core package Properties indicate the current iteration

status This variable has nested visibility

LoopTagStatus Class The LoopTagStatus properties are

defined by eight getter methodsgetCurrent() returns the current itemgetIndex() returns the current index

(0-based, initialized via the begin attribute)getCount() returns the current count

(1-based, independent of the begin attribute)isFirst(), isLast()getBegin(), getEnd(), getStep()

LoopTag Interface JSTL provides an interface and a

corresponding base class for custom tag implementors javax.servlet.jsp.jstl.core.LoopTag interface javax.servlet.jsp.jstl.core.LoopTagSupport class

Allows developers to leverage the functionality of <c:forEach> in their own custom tags

<c:forTokens> Tag <c:forTokens var=”varName”

items=”stringOfTokens”delims=”delimiters” varStatus=”varStatusName”begin=”begin” end=”end”step=”step”>


JSTL version of java.util.StringTokenizer Iteratively processes the body content for all of the

tokens in a String String is tokenized using the specified delimiters

<c:url> Tag <c:url value=”baseURL”

context=”context”var=”varName”scope=”varScope”/>

URL re-writingAppends session id (if appropriate)Prepends context to relative URLs (defaulting to

current context)

Request parameters can be specified via body content

<c:param> Tag <c:param name=”paramName”

value=”paramValue”/> Used to add request parameters to a URL Nested in the body content of a <c:url>, <c:redirect>, or <c:import> tag

The values of the name and value will be URL encoded when added to the URL

<c:redirect> Tag <c:redirect url=”baseURL”

context=”context”/> Sends an HTTP redirect response

Aborts processing of remainder of page Request parameters can be specified via body content

<c:import> Tag <c:import url=”baseURL”

context=”context”charEncoding=”encoding” var=”varName”scope=”varScope”/>

Fetches the content of a URL If a variable is specified, content is assigned to variable as a String

If no variable is specified, content is inserted into current page (like <jsp:include> action)

Request parameters can be specified via body content

<c:import> Tag <c:import url=”baseURL”

context=”context”charEncoding=”encoding” varReader=”readerName”>

body content </c:import>

Fetches the content of a URL URL content is exposed via a variable referencing a java.io.Reader

The varReader variable has nested scope

Request parameters cannot be specified via body content Can use <c:url> with var attribute to build URL with

request parameters, then pass variable to <c:import>

Formatting Library EL library

Dynamic attribute values specified using the JSTL expression language (i.e., ${ expr }) <%@ taglib uri=”http://java.sun.com/jstl/fmt”

prefix=”fmt”>


the JSP expressions (i.e., <%= expr %>) <%@ taglib uri=”http://java.sun.com/jstl/fmt_rt”

prefix=”fmt_rt”>

Formatting Library Tags Internationalization actions

<fmt:message>, <fmt:param><fmt:bundle><fmt:setBundle><fmt:setLocale><fmt:requestEncoding>

Formatting Library Tags Data formatting actions

<fmt:formatNumber><fmt:parseNumber><fmt:formatDate><fmt:parseDate><fmt:timeZone><fmt:setTimeZone>

<fmt:message> Tag <fmt:message key=”messageKey”

bundle=”context”var=”varName”scope=”varScope”/>

Displays a message fetched from a resource bundle

The bundle attribute specifies an instance of javax.servlet.jsp.jstl.fmt.LocalizationContext

Message key can also be specified in body content

Message parameters can be specified via body content (after the key value, if present)

<fmt:param> Tag<fmt:param value=”messageParam”/>

Supplies a single value for parametric replacement within a message

Each parameter in a message requires a corresponding <fmt:param> tag

Nested within a <fmt:message> action

<fmt:bundle> Tag <fmt:bundle basename=”basename”

prefix=”prefix”> body content </fmt:bundle>

Specifies a localization context for nested formatting actions (e.g., <fmt:message>)

Required basename attribute identifies the resource bundle (subject to localization)

Optional prefix attribute specifies a prefix to be prepended to all message keys appearing in the body content

Other I18N Tags <fmt:setBundle> assigns a localization

context to a variable, or assigns the default localization context for a scope

<fmt:setLocale> sets the current locale across a JSP scope (overrides browser-based locale) JSTL also provides a configuration variable for

specifying a fallback locale <fmt:requestEncoding> set the character

encoding for a request so that request parameter values can be correctly decodedCompensates for browser misbehavior with

respect to the Content-Type header

<fmt:formatNumber> Tag <fmt:formatNumber value=”numericValue”

type=”type” pattern=”pattern”currencyCode=”code” currencySymbol=”symbol” groupingUsed=”bool”maxIntegerDigits=”maxIntDigits”

minIntegerDigits=”minIntDigits” maxFractionDigits=”maxFracDigits”

minFractionDigits=”minFracDigits” var=”varName” scope=”varScope”/>

Displays a formatted number, or assigns the formatted result to a variable

<fmt:formatNumber> Tag The value to be formatted can also be

specified via body content. Formatting type is either number, currency, or percentage

The pattern attribute takes precedence over the type attribute, and must follow the pattern conventions of the java.text.DecimalFormat class

Formatting is influenced by localization context

<fmt:formatDate> Tag <fmt:formatDate value=”numericValue”

type=”type” pattern=”pattern”dateStyle=”dateStyle” timeStyle=”timeStyle” timeZone=”timeZone”

var=”varName”scope=”varScope”/>

Displays a formatted date and/or time, or assigns the formatted result to a variable

<fmt:formatDate> Tag Formatting type is either time, date, or both Permitted values for the dateStyle and timeStyle attributes are default, short, medium, long, and full Values follow java.text.DateFormat conventions

The pattern attribute takes precedence over the type, dateStyle, and timeStyle attributes, and must follow the pattern conventions of the java.text.SimpleDateFormat class

Formatting is influenced by localization context

Parsing Tags <fmt:parseNumber> parses a String

into a numeric valueResulting value can be assigned to a

variable or output to page <fmt:parseDate> parses a String into

a numeric valueResulting value can be assigned to a

variable or output to page Both parsing tags are locale-sensitive

<fmt:timeZone> Tag<fmt:timeZone value=”timeZone”>

body content</fmt:timeZone>

Specifies the timezone in which to format or parse any nested <fmt:formatDate> or <fmt:parseDate> tags

The timezone value can be either a String identifying a timezone or an instance of java.util.TimeZone

<fmt:setTimeZone> assigns a time zone to a variable, or assigns the default time zone

<fmt:setTimeZone> Tag<fmt:setTimeZone value=”timeZone”

var=”varName”scope=”varScope”/>

Assigns a time zone to a variable, or assigns the default time zone for a scope

XML Library EL library

Dynamic attribute values specified using the JSTL expression language (i.e., ${ expr }) <%@ taglib uri=”http://java.sun.com/jstl/xml”

prefix=”x”>


the JSP expressions (i.e., <%= expr %>) <%@ taglib uri=”http://java.sun.com/jstl/xml_rt”

prefix=”x_rt”>

XML Library Tags Core actions

<x:parse><x:out><x:set>

Flow control actions<x:if><x:choose>, <x:when>, <x:otherwise><x:forEach>

XML Library Tags Transform actions

<x:transform>, <x:param>

<x:parse> Tag <x:parse xml=”document”

systemId=”systemId” filter=”filter”

var=”varName”scope=”varScope”

varDom=”varDomName”scopeDom=”varDomScope”/>

Parses an XML document via a String or Reader specified via the xml attributeInteroperable with <c:import> action

<x:parse> Tag <x:parse systemId=”systemId”

filter=”filter” var=”varName”

scope=”varScope” varDom=”varDomName”

scopeDom=”varDomScope”> body content

</x:parse> Parses an XML document provided as

body content

<x:parse> Tag Result is stored in a variable

Variable specified by either var and optional scope, or by varDom and optional scopeDom

When var is used, the type of the result is implementation-specific

When varDom is used, the result will implement the org.w3c.dom.Document interface

The filter attribute can specify an instance of org.xml.sax.XMLFilter for filtering the XML document during parsing

<x:out> Tag <x:out select=”XPathExpression”

escapeXml=”bool”/>

Displays data from a parsed XML document

The select attribute employs a syntax based on XPath to identify the data to be displayedReferences variable created by <x:parse>

Analogous to <c:out> action

<x:set> Tag <x:set var=”varName”

select=”XPathExpression”scope=”varScope”/>

Assigns data from a parsed XML document

The select attribute employs a syntax based on XPath to identify the data to be displayedReferences variable created by <x:parse>

Analogous to <c:set> action

XML Flow Control Tags Three sets of flow control actions

<x:if><x:choose>, <x:when>, <x:otherwise><x:forEach>

Analogous to like-named JSTL core actionsConditionalization or iteration driven by

XPath expression values (select attribute), rather than EL or RT values

<x:transform> Tag <x:transform xslt=”stylesheet”

xmlSystemId=”systemId” xsltSystemId=”systemId”

result=”resultObject” var=”varName”

scope=”varScope”/> Transforms an XML document specified via

the xml attributeString, Reader, javax.xml.transform.Source, org.w3c.dom.Document, or implementation-specific class

Transformation parameters can be supplied via nested <x:param> actions

<x:transform> Tag <x:transform xml=”xmlDocument”

xslt=”stylesheet”xmlSystemId=”systemId”

xsltSystemId=”systemId” result=”resultObject”

var=”varName”scope=”varScope”>

body content</x:transform>

Transforms an XML document provided as body content, via a nested document, <c:import> , or <x:transform>

XML document can be followed by <x:param> actions for setting transformation parameters

<x:transform> Tag Stylesheet specified via the xslt attribute

String, Reader, or javax.xml.transform.Source

The result attribute specifies an instance of javax.xml.transform.Result for capturing the transformation result

If the var attribute is specified, the named variable will be assigned an instance of org.w3c.dom.Document representing the transformation result

If neither var nor result is specified, the transformation result is written to the JSP page.

<x:param> Tag <x:param name=”name” value=”value”/>

Supplies a value for the named transformation parameter

Parameter value can also be specified via body content

SQL Library EL library

Dynamic attribute values specified using the JSTL expression language (i.e., ${ expr }) <%@ taglib uri=”http://java.sun.com/jstl/sql”

prefix=”sql”>


the JSP expressions (i.e., <%= expr %>) <%@ taglib uri=”http://java.sun.com/jstl/sql_rt”

prefix=”sql_rt”>

SQL Library Tags Database actions

<sql:query>, <sql:param>, <sql:dateParam>

<sql:update>, <sql:param>, <sql:dateParam>

<sql:transaction><sql:setDataSource>

Useful for debugging and quick one-offsHeinous violation of MVC design pattern:

DB code (i.e., raw SQL) doesn’t belong in the presentation layer!

JSTL Resources URLs

JSP home pagehttp://java.sun.com/products/jsp

Reference implementationhttp://jakarta.apache.org/taglibs/index.html

Books“JSTL in Action” by Shawn Bayern“Core JSTL” by David Geary“JSTL: JSP Standard Tag Library Kick Start”

by Jeff Heaton

jstl: the javaserver pages standard tag library mark a. kolb security broadband, austin, tx...

Documents