8 Custom Tag

7/27/2019 8 Custom Tag

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JSP Custom Tags

Custom Tags are User-defined tag library

Provides the modularity

Code reusability at Presentation Layer

Makes JSP Development easier

Provides clear separation between

presentation design and code


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Basic Principles

Every custom tag is associated with a Javaclass called as tag handler, kept inside apackage.

The tag handler classes inherit the CustomTag API available injavax.servlet.jsp.tagext

Collection of Custom Tags called as TagLibrary is configured using XML files (*.tldand web.xml)


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Usage of Tags

Three kinds of usage of tags are

Simple Tag

Tag with Attribute

Tag with Body Content


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Tag in Detail

Start of Tag

Tag

Library

The Tag

End of Tag

Attribute

Value for

Attribute


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Simple Tag Life Cycle

Evaluate Body

Done

doTag()

Load class

SimpleTagHandler.class

Instantiate the class

No-arg constructor invokes

Call

setJspContext(JspContext)

If the tag is nested,CallsetParent(JspTag)

If the tag has attributes,

call setters

If the tag is not declared to have a

of empty, And the tag has a body,call

setJspBody(JspFragment)

Call doTag()

Container

Performed

tasks


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Life Cycle Methods (Simple)

setPageContext() links the tag with current pagecontext

setParent() Called only in the case of nested

custom tags doStartTag() - Called when Start of Tag is

encountered

doEndTag() - Called when End of Tag is encountered

release() - Called after End of Tag


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Page Context

home.jsp

user1

user2

user3

pageContext for user1



Custom

Tag

setPageContext()


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Making a Simple Tag handler

Write the class SimpleTest that extends SimpleTagSupport

Implementthe doTag() method

Create a TLD for the tag customtag.tld

Deploythe tag handler and TLD

Write aJSP that uses the tag - customtag.jsp


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Tag Library Descriptors

A tag library descriptor(TLD) is an XML

document that describes a tag library.

A TLD contains information about a library as a

whole and about each tag contained in the

library.

TLDs are used by a Web container to validate

the tags .


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Tag Element

Each tag in the library is described by giving its

name and the class of its tag handler, information

on the scripting variables created by the tag, and

information on the tag's attributes.

Each attribute declaration contains an indication

of whether the attribute is required, whether its

value can be determined by request-timeexpressions, and the type of the attribute


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Classic Tag API

JspTag

//no methods, only for polymorphism and organization

Tag

int doEndTag()

Tag getParent()

int doStartTag()

void setPageContext(PageContext)

void setParent(Tag)

void release()


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Classic tag handler Life Cycle-- TagSupport

doStartTag()

Evaluate Body

doAfterBody()

doEndTag()

Done

Evaluate Page

return EVAL_BODY_INCLUDE

return EVAL_BODY_AGAIN

return SKIP_BODY

return EVAL_PAGE

return SKIP_PAGE

return SKIP_BODY


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Load class -- SimpleTagHandler.class

Instantiate the class -- no-arg constructor invokes

Call setPageContext(PageContext)

If the tag is nested,Call setParent(Tag)

If the tag has attributes, call setters

If the tag is not declared to have an empty body,

And the tag is not invoked with an empty body,

And the doStartTag() returns EVAL_BODY_INCLUDE,

the body is evaluated. doInitBody() and

setBodyContent() is called exactly only once

doStartTag() returns EVAL_BODY_BUFFERED

Call doEndTag()

Call doStartTag()

If the body content was evaluated , Call doAfterBody()

Containers task


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Life Cycle Methods (Body Content)

setPageContext()

setParent()

doStartTag()

doInitBody()

Called when body content begins

doAfterBody() Called when body content ends

doEndTag()

release()


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Return Type constants

Four important Constant identifiers are defined in

Tag interface available through TagSupport class.

These constants should be used as return type to

control the flow of tag execution EVAL_BODY_INCLUDE

SKIP_BODY

EVAL_PAGE

SKIP_PAGE


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Steps to Develop a Custom Tag

A Tag Hanlder Class that extends either TagSupport

or BodyTagSupport

Override the lifecycle methods typically doStartTag()

and doEndTag() A Tag Library Descriptor (TLD) file (under WEB-INF

folder) that describes the tags in a tag library

An entry in web.xml that refers the TLD file

Using (invoking) the tag in JSP


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Programs

MyTagHandler.java customtag1.tld

customtag1.jsp


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Programs

CubeNumber.java

customtag2.tld

customtag2.jsp


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Programs

PowerNumber.java

customtag3.tld

customtag3.jsp


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Applications

8 Custom Tag

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