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

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