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Custom Tags in JSP by R4R Team

Custom Tags in JSP:
Custom tags are user-defined tags. They eliminates the possibility of scriptlet tag and separates the business logic from the JSP page.

The same business logic can be used many times by the use of costom tag.

Advantages of Custom Tags:

The key advantages of Custom tags are as follows:

Eliminates the need of srciptlet tag
The custom tags eliminates the need of scriptlet tag which is considered bad programming approach in JSP.
Separation of business logic from JSP The custom tags separate the the business logic from the JSP page so that it may be easy to maintain.
Reusability The custom tags makes the possibility to reuse the same business logic again and again.
Syntax to use custom tag

There are two ways to use the custom tag. They are given below:

<prefix:tagname attr1=value1....attrn=valuen />

<prefix:tagname attr1=value1....attrn=valuen >

body code

</prefix:tagname>

JSP Custom Tag API

The javax.servlet.jsp.tagext package contains classes and interfaces for JSP custom tag API. The JspTag is the root interface in the Custom Tag hierarchy.

JspTag interface

The JspTag is the root interface for all the interfaces and classes used in custom tag. It is a marker interface.

Tag interface

The Tag interface is the sub interface of JspTag interface. It provides methods to perform action at the start and end of the tag.

Fields of Tag interface

There are four fields defined in the Tag interface. They are:
 

Field Name Description
public static int EVAL_BODY_INCLUDE  it evaluates the body content.
public static int EVAL_PAGE  it evaluates the JSP page content after the custom tag.
public static int SKIP_BODY it skips the body content of the tag.
public static int SKIP_PAGE it skips the JSP page content after the custom tag.
Methods of Tag interface

The methods of the Tag interface are as follows: 

Method Name Description
public void setPageContext(PageContext pc) it sets the given PageContext object.
 
public void setParent(Tag t) it sets the parent of the tag handler.
 
public Tag getParent() it returns the parent tag handler object.
 
public int doStartTag()throws JspException it is invoked by the JSP page implementation object. The JSP programmer should override this method and define the business logic to be performed at the start of the tag.
 
public int doEndTag()throws JspException it is invoked by the JSP page implementation object. The JSP programmer should override this method and define the business logic to be performed at the end of the tag.
 
public void release() it is invoked by the JSP page implementation object to release the state.

IterationTag interface
The IterationTag interface is the sub interface of the Tag interface. It provides an additional method to reevaluate the body.

Field of IterationTag interface
There is only one field defined in the IterationTag interface.
public static int EVAL_BODY_AGAIN it reevaluates the body content.
Method of Tag interface

There is only one method defined in the IterationTag interface.

public int doAfterBody()throws JspException it is invoked by the JSP page implementation object after the evaluation of the body. If this method returns EVAL_BODY_INCLUDE, body content will be reevaluated, if it returns SKIP_BODY, no more body cotent will be evaluated.

TagSupport class

The TagSupport class implements the IterationTag interface. It acts as the base class for new Tag Handlers. It provides some additional methods also.

Understanding Flow and Example of JSP Custom Tag
There is given two simple examples of JSP custom tag. One example of JSP custom tag, performs action at the start of the tag and second example performs action at the start and end of the tag.

Attributes in Custom Tag

Here, we will learn how we can define attributes for the custom tag.

Iteration using Custom Tag
In this example, we are iterating the body content of the custom tag.

Custom URI in Custom Tag
We may also refer the TLD file by using the URI. Here we will learn how can we use custom URI. 

Example:JSP Custom Tag:

In this example, we are going to create a custom tag that prints the current date and time. We are performing action at the start of tag.

For creating any custom tag, we need to follow following steps:

Create the Tag handler class and perform action at the start or at the end of the tag.
Create the Tag Library Descriptor (TLD) file and define tags
Create the JSP file that uses the Custom tag defined in the TLD file

To create the Tag Handler
, we are inheriting the TagSupport class and overriding its method doStartTag().To write data for the jsp, we need to use the JspWriter class.

The PageContext class provides getOut() method that returns the instance of JspWriter class. TagSupport class provides instance of pageContext bydefault.

File: MyTagHandler.java:

package ycd.com;

import java.util.Calendar;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.JspWriter;

import javax.servlet.jsp.tagext.TagSupport;

public class MyTagHandler extends TagSupport{

public int doStartTag() throws JspException {

JspWriter out=pageContext.getOut();//returns the instance of JspWriter

try{

out.print(Calendar.getInstance().getTime());//printing date and time using JspWriter

}catch(Exception e){System.out.println(e);}

return SKIP_BODY;//will not evaluate the body content of the tag

}

}

 

2) Create the TLD file:
Tag Library Descriptor (TLD) file contains information of tag and Tag Hander classes. It must be contained inside the WEB-INF directory.

File: mytags.tld 

<?xml version="1.0" encoding="ISO-8859-1" ?>

<!DOCTYPE taglib

PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"

"http://java.sun.com/j2ee/dtd/web-jsptaglibrary_1_2.dtd">

<taglib>

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

<jsp-version>1.2</jsp-version>

<short-name>simple</short-name>

<uri>http://tomcat.apache.org/example-taglib</uri>

<tag>

<name>today</name>

<tag-class>ycd.com.MyTagHandler</tag-class>

</tag>

</taglib>

3) Create the JSP file:

Let's use the tag in our jsp file. Here, we are specifying the path of tld file directly. But it is recommended to use the uri name instead of full path of tld file. We will learn about uri later.

It uses taglib directive to use the tags defined in the tld file.

File: index.jsp

<%@ taglib uri="WEB-INF/mytags.tld" prefix="m" %>

Current Date and Time is: <m:today/>

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