JSP : Creating Custom Tag

CUSTOM TAG LIBRARY

Prof. AshishSingh Bhatia

1Prof. AshishSingh Bhatia

Two Approach

Java Based Custom Tag

No Version Restriction

Uses Java file for Tag Handling

Preferred when lot of java code is required for getting output

JSP Based Custom Tag

Only from JSP 2.0

Uses JSP file for Tag Handling

Preferred when lot of html code is required for getting output.

Prof. AshishSingh Bhatia 2

Tag Library Components

Tag handler class that defines the tag behavior.

The TLD file that maps the XML element names to the tag implementation.

The JSP file that uses the tag library.


The Tag Handler Class

Class that tells what to do when system see the tag.

Class must implement SimpleTag interface.

In practice, extends SimpleTagSupport which implements SimpleTag.

javax.servlet.jsp.tagext package.

Every Tag Handler class must have 0 argument constructor.

doTag() is the main method for tag handling.

We need JspWriter [ getJspContext().getOut() ]

New instance is created for every tag occurrence on the page.


Directory Structure

hellowordtag

index.jsp

WEB-INF

tlds tag.tld

classes mytag HelloWorldTag.java


Tag Handler Class

package mytag;

import javax.servlet.jsp.*;

import javax.servlet.jsp.tagext.*;

import java.io.*;

public class HelloWorldTag extends SimpleTagSupport {

public void doTag() throws JspException, IOException {

JspWriter out = getJspContext().getOut();

out.print("Hello World");

}

}

TLD : Tag Library Descriptor

<?xml version="1.0" ?>

<taglib version="2.0">

<tlib-version>1.0</tlib-version>

<short-name>mytag</short-name>

<tag>

<description>HelloWorld Tag</description>

<name>helloworld</name>

<tag-class>mytag.HelloWorldTag</tag-class>

<body-content>empty</body-content>

</tag>

</taglib>


Optional

Required Element

empty, scriptless, tagdependent, JSP

JSP File

<html>

<head>

<title>Tag Example</title>

</head>

<body>

<%@ taglib uri="/WEB-INF/tlds/tag.tld" prefix="ashish" %>

<ashish:helloworld/>

</body>

</html>

Assigning Attributes to Tags

For every attribute we need a set method in tag handler class

<prefix:tag attribute1=“value1” attribute2=“value2” … />


setAttribute1(String value) setAttribute2(String value)

TLD File

<attribute>

<description>…. </description>

<name> X </name>

<required> true/false </required>

</attribute>


X = Must match with the variable in class

Example : Custom tag to reverse a String

public class StringReverseTag extends SimpleTagSupport {

private String data;

public void setData(String data) {

this.data=data;

}

public void doTag() throws JspException, IOException {

JspWriter out = getJspContext().getOut();

StringBuffer sb = new StringBuffer(data);

sb.reverse();

out.print(sb);

}

}


TLD File

<tag>

<description>StringReverse Tag</description>

<name>string</name>

<tag-class>mytag.StringReverseTag</tag-class>

<body-content>empty</body-content>

<attribute>

<name>data</name>

<required>true</required>

</attribute>

</tag>

JSP FILE

<html>

<head>

<title>Tag Example</title>

</head>

<body>

<%@ taglib uri="/WEB-INF/tlds/tag.tld" prefix="ashish" %>

<ashish:string data="EARTH"/>

</body>

</html>

Including Tag body in Tag Output

getJspBody().invoke(null)

null means the resulting output of that JSP content is passed verbatim to the client.

doTag() has no way to access the tag body output.

Example In JSP File : <ashish:tag> This is the test </ashish:tag>

In Java File

out.print(“Hello World ”);

getJSPBody().invoke(null);

out.print(“This is my tag”);

In Tag File : <body-content>scriptless</body-content>

Using Tag Files

Java Based Custom Tag What we have seen is Java Based Custom Tag

Tag handler class is Java File

JSP Based Custom Tag [ Tag Files ] Tag Handler class is JSP file

When to use which ?

Simple Rule : Use Java Based Custom Tag when lot of java code is involved. IF more is of formatting use JSP Based Custom Tag.

Remember Tag files run only on JSP 2.0.

Java Base Tag have no such restriction.


JSP Based Custom Tag

Create a JSP Base tag file

Create a JSP page that uses the tag file


Structure

tagdemo

index.jsp

WEB-INF tags helloworld.tag


helloworld.tag and index.jsp

Tag File [ helloworld.tag ]

Hello World

JSP File

<html>

<head><title>Tag Example</title></head>

<body>

<%@ taglib tagdir="/WEB-INF/tags" prefix="ashish" %>

<ashish:helloworld/>

</body>

</html>

String Reverse [ Using Attribute and Tag File ]

Tag File [ reverse.tag ]

<%@ attribute name="data" required="true" %>

<%

StringBuffer sb = new StringBuffer(data);

sb.reverse();

%>

<%= sb %>

JSP File

<%@ taglib tagdir="/WEB-INF/tags" prefix="ashish" %>

<ashish:reverse data="EARTH"/>

Using Body part of the tag using Tag File

Use <jsp:Body/> to get the out put of body

Example In JSP File : <ashish:test> This is the test </ashish:test>

In Tag File [ test.tag ] :

Hello World 

<jsp:doBody/>

This is my tag

END OF SESSION


JSP : Creating Custom Tag

Education

Transcript of JSP : Creating Custom Tag