JSP Tag Lib by Kamalakar Dandu

8/2/2019 JSP Tag Lib by Kamalakar Dandu

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Objectives

Learn new features in custom tag libraries

in JSP1.2

Simple tag

Iteration tag Body tag

Tags Define Scripting Variables Learn new

features in JSP1.2

XML View of JSP syntax Bundling Listeners with Tags

TagLibrary Validator


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Tag Libraries

Tag library is a collection of related tagspackaged as a jar file containing:

A tag library descriptor: taglib.tld

Tag handler classes: .class files

Associated resource(s) Standard Tag Libraries: JSP 1.0/1.1 TL (JSR 052):

Actions: data sources and control of presentation

Examples: conditional logic, iterations and so on

Apache project @Jakarta: Jakarta Taglibs Useful TagLibs

Educational and cut-and-paste

Not Standard Tag Library


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Components of a Tag Library Tag library descriptor (TLD) file

XML format describes tag name, attributes and tag

handler class

Location specified in JSP file and web.xml

JSP file Imports a tag library which references descriptor file

Defines tag prefix and uses tags

Located in the same directory as the rest of JSP

Tag handler class Must implement javax.servlet.jsp.tagext.Tag Interface

Usually extends TagSupport or BodyTagSupport

Locates in the same directories as servlet class files


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Whats the Mechanism?


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Three Types of Tags

Simple Tag (Tag interface): For tags does not want to manipulate its body

The body of a tag is evaluated once

Iteration Tag (IterationTag interface): JSP1.2 only

Extends Tagby defining one additional methoddoAfterBody() that controls the reevaluation of its

body,but without manipulation of body content

Body Tag (BodyTag interface):

Extends IterationTagby defining additional methodsthat manipulate the content of evaluating its body

The body of a tag might be evaluated multiple times


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Tags


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Using Simple Tag in JSP

Some N-Digit Primes

20-digit:

40-digit:

80-digit:

Default (50-digit):


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Lifecycle of Simple Tag Handlers


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Lifecycle of IterationTag


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Need BodyTag

In order to manipulate the content of its

body and evaluate a Tags body multiple

times, need to use a BodyTag

BodyTag interface extends IterationTag

by defining additional methods

Examples:

Iterate rows, query, search results and etc.


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Using Body Tag in JSP

...


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How Is a BodyTag Invoked?


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BodyTag Method Semantics

doStartTag(): Same as Tag semantic, except:

EVAL_BODY_INCLUDE(JSP 1.2) returns: as in IterationTag

EVAL_BODY_BUFFERED (JSP 1.2) or EVAL_BODY_TAG (JPS1.1)returns:

manipulates the body by calling setBodyContent(), doBodyInit(),

doAfterBody, and doEndBody() DoBodyInit():

Prepare to process body

doAfterBody():

Handle processed body in BodyContent

Returns EVAL_BODY_BUFFERED (JSP 1.2) orEVAL_BODY_TAG (JSP 1.1) to reprocess

doEndTag():

Same as Tag Semantics


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Repeat Tag: Tags With BodyTagpublic class RepeatTag extends BodyTagSupport {

private int reps;public void setReps(String repeats) {

...

}

public int doAfterBody() {

if (reps-- >= 1) {

BodyContent body = getBodyContent();try {

JspWriter out = body.getEnclosingWriter();

// manipulate bodycontent.

out.println(body.getString());

body.clearBody(); // Clear for next evaluation

} catch(IOException ioe) {

System.out.println("Error in RepeatTag: " + ioe);

}

return(EVAL_BODY_BUFFERED);

} else {

return(SKIP_BODY);}


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Using Tag in JSP


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mytags

/WEB-INF/codecamp-taglib.tld


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Tag Library Descriptor

Used by translator to map custom tags toimplementation

An XML file

Describes a collection of custom Tags Defines:

Version dependencies

Tag(s) and their:

Implementation class Attributes

Tag content model

Authoring tool support (JSP 1.2)


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TLD File...

...

repeat

tags.RepeatTag

...

JSP

reps

true

true(r t express value)


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Tags Define Scripting Variables

Radio stations that rock:


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Specifying the variable TLD subelement

(JSP1.2 only):

member

String

true

NESTED


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JSP Pages as XML Documents

Direct Authoring

When content is XML

Manipulation Apply XSLT to generate JSP

Authoring

JSP pages from XML objects

Validation

TagLibraryValidator


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XML View of JSP syntax (From 1.2 Specification)

removed

jsp:root element is

annotated with namespace

information. expanded in place

...

...

...


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JSP Pages as XML Documents

Flowers available:

aFlower.kind


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New DOCTYPE Declaration

DOCTYPE decalration in JSP1.1

DOCTYPE declaration in JSP1.2


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Tag LibraryDescriptor Format


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Bundling Listeners with Tags

Define listeners with tags in web.xml? Tag library maintenance will be difficult

Much more work to define in web.xml

Define listener declarations in tag library

descriptor (TLD)

Always put TLD files in the WEB-INFdirectory or a subdirectory thereof

somelistener_class

...


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Session Counter Sample Code (Listener)public class ActiveSessionCounter

implements HttpSessionListener {private static int sessionCount, sessionLimit;

// Each time a session is created, increment the

running count. If the count exceeds the limit,

print a warning in the log file.

public void sessionCreated(HttpSessionEvent event) {sessionCount++; ...

String warning = getSessionCountWarning(); ...}

public void sessionDestroyed(HttpSessionEvent event) {

...}

public static String getSessionCountWarning() {

String warning = null;

if (sessionCount > sessionLimit) {

warning = ...

return(warning);}

..}


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Session Counter Sample Code

//If the number of active sessions is above the limit,

this prints a warning. Otherwise, it does nothing.public class SessionCountWarningTag extends TagSupport {

public int doStartTag() {

try {

String warning =

ActiveSessionCounter.getSessionCountWarning();if (warning != null) {

JspWriter out = pageContext.getOut();

out.println(...); ...

}

} catch(IOException ioe) {

System.out.println("Error printing session warning.");

}

return(SKIP_BODY);

}

}


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Session Counter Configuration:

TLD

moreservlets.listeners.ActiveSessionCounter

sessionCount

moreservlets.tags.SessionCountTag

empty

The number of active

sessions.


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Session Counter Sample Code

(JSP)

...

Number of sessions in memory:


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Session Counter Configuration:

web.xml

/session-count-taglib.tld

/WEB-INF/session-count-taglib-0.9-beta.tld


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Checking Syntax with TagLibrary

Validator Need sufficient validation mechanism for

complicated situation

e.g. Checking logic in optional attributes usage

Handling error checking in tags may slow down

execution New class TagLibararyValidator performs arbitrary

page translation time syntax checking

Tag library validators are only triggered when

associated JSP pages are translated into servlets

Modify a validator, make sure also modify andredeploy JSP


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Checking Syntax with

TagLibraryValidator Basic Steps

Create a subclass of TagLibraryValidator

Override the validate() method

ValidationMessage[] validate(String prefix, String uri,

PageData page) Return null if syntax is correct

Return ValidationMessage[] if there are errors

Optionally override other methods: initialization

Declare the validator in the TLD file

Try JSP pages that use the tag library If validate() returns null, no action is taken

If validate() returns a non empty array, the servermakes the error messages available


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Checking Syntax with TagLibrary

Validator TLD

someValidator


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Session Counter Demo


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Lab

Understand Event Listener in Tag

Libraries

Run Event Listener in Tag Libraries


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Summary

Develop custom tag libraries with new

capabilities in JSP1.2

Take advantage of JSP 1.2 new features

JSP XML syntax

Bundle event listeners with tag libraries

Validate your JSP syntax