Introduction to JSTL

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Introduction to JSTL

(JSP Standard Tag Library)


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As J2EE programmers, we are familiar with Servlets , JSPand JavaBeans. Any JSP page should encapsulate thebusiness logic in a bean and invoke it by using tag.

Till recently, a combination of Servlets, JSP and beans wasthe standard practice. But, the JCP released an API forenabling programmers to create custom tags and use themin their JSP pages.

The difference between javabean and java custom tags wasthat, though both made use of java classes, tags can beused by non-programmers also without knowledge of Javaprogramming, just as they would use html tags.( From aprogrammer's perspective,however, a much moreimportant distinction is that tags are specific to the page inwhich they are created while javabeans are general. )


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Java, JSP, and JSTL


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// greeting.htm=============


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(relevant section of greeting.java servlet)// greeting.java ( code-snippet only)public void doPost(HttpServletRequest req,

HttpServletResponse resp)throws ServletException,IOException {

resp.setContentType("text/html");PrintWriter out = resp.getWriter();//-------------------------------String s = req.getParameter("text1");out.println("");

out.println("we welcome"+",
");out.println (s);out.println(" ");

}


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// greeting1.jsp


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----------------------------------------------// greeting2.jsp

we welcome


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// greeter.java//==============

package ourbeans;

public class greeter {

public greeter() { }public String greetme(String s) {

return "we welcome..."+s;}}


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/ greeting3.jsp


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// greeting4.jsp ( uses JSTL)

=========== We welcome


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There are five groups under which theJSTL tags have been organized.

They are as follows:

1) core

2) xml

3) sql

4) formatting 5) functions.


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JSTL and other frameworks

JSTL works with JSP versions 1.2 andhigher.

JSTL inherits all the benefits from JSP

JSTL will be useful to you whether you useStruts, JavaServer Faces, a differentframework, or nothing.


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JSTLA Template System

Web browsers dont care how web pages areproduced.

To a web browser, it makes no differencewhether the page its displaying is static(unchanging) or dynamic (produced by aprogramming language or template system)

Template systems like JSP and JSTL are similar toa word-processing feature called mail merge.

In a web template system, template text is mixed with anumber of placeholders.

These placeholders are filled in every time the pageneeds to respond to a web request.


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JSP Tags

An interesting subtlety about JSP tags and HTMLtags is worth highlighting:

You can use JSP tags inside HTML tags, because theseHTML tags are just arbitrary template text.

For instance, you can write:

If s purpose is to print a URL, then this tag mightbe replaced with and it works fine

However, JSP tags cannot appear inside another JSP

tags attributes. For instance, if and are both JSP

tags, then you cant write


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Standard JSP Tags

Some JSP tags are built into JSP; theyreeffectively hard-wired into the JSP standard.

These tags are often called standard tags,although the term is somewhat confusing.

This group of standard tags doesnt includeJSTL tags; instead, it includes core JSP tags thatpredate JSTL by several years.

JSTLs tags are also standard, but they fall intoa separate group of tags


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Standard JSP Tags


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Standard JSP Tags

Lets you include one JSP page from withinanother, including the static content

Always looks for files on the local server Ignores the htmls tag

Other related tags


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JSP Directives

Directives are pseudo-tags that havespecial meaning to the container;

they are not passed through to the browser

but, instead, are processed by the JSP engine directive

directive

directive


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directive

e.g.,

Different from

directive works by finding the

target file and inserting it into your JSP page, justas if you had cut and pasted it using a texteditor.

By contrast, locates the targetpage while your JSP page is executing.

When page A uses to includepage B, page Bs data is simply included in pageA every time it is compiled


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vs. If a file included with changes, its

changes will not be noticed until the page containing thedirective also changes page A must be changedand recompiled for any changes

in B to take effect notices changes immediately

cannot work with Servlets is more efficient than , but

it also uses much more disk space when large files areincluded.

With , the two pages involved are twoentirely separate pages They can use the same names for different variables, or they

can use different prefixes for the same tag library With , because page A and page B are

essentially merged before being compiled, there might beclashes between names within the two pages


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directive

, lets you modify someproperties of a JSP page

This directives goal is to provide meta-

information about how to process thepage


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The directive

Tag libraries use prefixes other than jsp, andthey must be explicitly imported into pagesbefore they can be used.

Thus, whereas the jsp: tags can be used in anyJSP page without fanfare or preparation, youneed to introduce others (including JSTLs) usinga special pseudo-tag known as a directive.

Think of JSP directives as being somewhat like

the HTML tag their function is not specifically to display anything in

the browser, but instead to describe some informationabout the page itself.


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The directive

Directives are one JSP feature that doesntstrictly follow an XML-like syntax.

Instead, a directive begins with .

One such directive, , is used toimport a tag library into a page.

Even though they begin with , directives are similar to XML tags in thatthey accept attributes.

The directive requires twoattributes: uri and prefix.


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Tag Libraries - URI and Prefix

Knowing a librarys URI or file path, you can usethe directive to register it and,at the same time, assign it an XML-likenamespace prefix for use within the page.

Imports the tag library identified by the URIhttp://www.acme.com/custom.tld into the page, usingthe prefix acme.

After this directive appears in a page, tags from thelibrary can be used with the acme prefix.


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Core Library

The core library includes tags for thefollowing uses:

Accessing and modifying data in memory

Making decisions in your pages Looping over data

Suggested Prefix: c

URI: http://java.sun.com/jstl/core

Example:


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XML processing library

The XML library includes tags for thefollowing purposes:

Parsing (that is, reading) XML documents

Printing parts of XML documents Making decisions in your page based on the

contents of an XML document

Suggested Prefix: x

URI: http://java.sun.com/jstl/jstl/xml

Example:


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Internationalization (i18n) & formatting

The formatting and internationalizationlibrary includes tags for these uses:

Reading and printing numbers

Reading and printing dates (with support fortime zones)

Helping your application work with more thanone language

Suggested Prefix: fmt URI: http://java.sun.com/jstl/jstl/fmt

Example:


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Database (SQL) access

The SQL library helps you read and writedata from databases.

Suggested Prefix: sql

URI: http://java.sun.com/jstl/jstl/sql Example:


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Using JSTL in your pages Before you can use a tag library, you need to import it.

For each page, you only need to import the libraries youactually use

Core

XML

Formatting

Database


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JSTL Tag Libraries

JSTL uses a simple language called anexpression language to make it easy foryou to access information

Before JSTL, you really had to know Javato produce an effective JSP page

JSTL makes writing pages easier.

Its expression language is much simplerthan Java; in fact, its even simpler thanJavaScript.


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Expressions and the tag

The most fundamental tag

used more often than any other tag in JSTL

Prints the result of an expression

tag is a little like JSPs and ASPs

prints the text, Hi there!

The tag becomes useful only when thevalue attribute contains an expression in JSTLsexpression language


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default attribute

Nobody

"/>


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Accessing Data using JSTL

The major goal of the JSTL expressionlanguage is to make data easy to access.

This data can fall into a number of

categories scoped variables,

request parameters


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Basic syntax to access scoped variables

${pageScope.username}

${requestScope.username}

${sessionScope.username}

${applicationScope.username}

JSTL tags also let you create and storescoped variables


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Basic syntax to access scoped variables

${sessionScope.shoppingCart} Such a variable refers to an entire collection of

objects, organized under a single name:shoppingCart.

${sessionScope.shoppingCart[0]} ${sessionScope.user.name}

${sessionScope.user["name"]} isequivalent to ${sessionScope.user.name}

R d h i


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Request parameters and the expression

language

One language you can read is


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Accessing other data with JSTL

Cookies

If youre told that a cookie called colorPreference isavailable, you can access it with an expression like${cookie.colorPreference}

Headers Web browsers send information about their make and

model to servers using a header called User-Agent

${header["User-Agent"]}

Initialization parameters

${initParam.headerUrl}.


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Comparisons You can use expression language to compare values

the expression ${2 == 2} results in true

Every comparison operator has a symbolic version (==)and a textual one (eq)


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Saving data with Many JSTL tags let you create scoped variables;

the most basic is value

The expression to compute Required - No Default - Use body

var The name of the scoped variable to save Required - Yes Default - None

scope The scope of the variable to save Required - No Default - page


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This tag creates a page-scoped variable named

eight and sets it to the string 8, which is theresult of the tag


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Deleting data with

var

The name of the scoped variable to delete

Required - Yes Default - None

scope

The scope of the variable to delete

Required - No

Default - Any


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JSTL Flow Control

JSTLs flow control comes in two forms:

Conditional logic, or conditions

Looping, or iteration


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JSTL Decisions

Yes-or-no conditions with

Mutually exclusive conditions with, , and


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Yes-or-no conditions with

test Condition to evaluate. If true, process the body; if false,

ignore the body

Required - Yes

Default - None

var Name of the attribute to expose a boolean value

Required - No

Default - None

scope Scope of the attribute to expose a boolean value

Required - No

Default - page


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test attribute The test attribute specifies a conditional

expression to evaluate.

--do something

if the test expression evaluates to false, the pageskips the body of the tag.

The body can contain any valid JSP code,including text, HTML tags, or other JSP tags

Dr.


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The var and scope attributes -- Saves variableA serious error has occurred.

[ large page body]

-- Uses variableSince a serious error occurred, your data has not been saved.


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Mutually exclusive conditions

Error 1 has occurred Error 2 has occurred Error 3 has occurred

Only one of these tags can succeed

Error 1 has occurred Error 2 has occurred Error 3 has occurred Everything is fine

Addition of to display the default message


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JSTL Looping

General-purpose looping with

Iterating over strings with

General purpose looping with


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General-purpose looping with

The basic function of is to considerevery item in the collection specified by its itemsattribute.

For each item in the collection, the body of the

tag will be processed once, the current item is exposed as a page-scoped variable

whose name is specified by s varattribute.



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A token is a single, discrete unit within alarger string.

The tag iterates over

tokens, which it parses from an inputstring



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Looping over numbers

Output: 1 2 3 4 5

Output: 2 4 6 8 10


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Accessing Database with JSTL

The tag does not support connectionpooling.

SELECT NAME, IQ FROM USERS WHERE IQ > 120


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Updating Database with JSTL

INSERT INTO PEOPLE(NAME, AGE, WEIGHT) VALUES(John"Fatso" Smith, 34, 540)

DELETE FROM CUSTOMERS WHERE AGE < 18

Our CUSTOMERS table had minors.They have all been removed. Close call; were lucky the Feds

didnt come after us.


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SELECT * FROM USERS WHERE BIRTHDAY < ?


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JSTL in Action

With this, you have covered approx. 8chapters of the book JSTL in Action(Manning)

For additional insight please refer the book Read Chapter 10 of the book regarding

formatting & internationalization itsinteresting

Documents

Introduction to JSTL