JSP Tag Libraries

Lec - 38

Last Lecture Example We incorporated JavaBeans in “Course

Outline” Example

But still have to write java code inside java.jsp & web.jsp

As discussed, JSPs are built for presentation purpose

Last Lecture Example What if we replace all code of scriptlet with one

single line (custom tag)

<% CourseDAO courseDAO = new CourseDAO(); …… for ( ……. ) { …….. } …… %>

<mytag:displaycourse pageName=“java” />

Contents What is a Custom Tag ?

Why build Custom Tag?

Types of Tags

Developing Custom Tags

Using Custom Tags

Examples

What is Custom Tag ? A user defined component to perform

certain action.

Provides mechanism for encapsulating complex functionality for use in JSPs.

We have already seen & used many built-in tags <jsp:useBean/> <jsp:include/> <jsp:forward/> etc

Advantages More cleaner separation of processing

logic and presentation, than Java Beans

Have access to all the JSP page objects

Can be customized using attributes

Types of Tags

Types of Tags

1. Simple Tag

2. Tag with Attributes

3. Tag with Body

Types of tags1. Simple Tags

Start and End of tag

No body within tag

No Attributes

For Example: < mytag:hello />

Tag library prefix

Tag Name

Types of tags2. Tags with attributes

Start and End of tag

Attributes within tag

No body enclosed

For Example :

< mytag:hello attribute=“value” />

Types of tags3. Tags with body

Start and End of tag

Body enclosed within tag

Attributes (optional)

For Example :

<mytag:hello optional_attributes …..>

some body </mytag:hello>

Building Custom Tags

Building Custom Tags We can build custom tag using either of

the following specs

JSP 1.2

JSP 2.0

We will use JSP 2.0

Building Custom Tags - Steps

1. Develop the Tag Handler class

2. Write the Tag Library Descriptor (tld) file

3. Deployment

1 - Tag Handler class Tag Handler is the java class

Implicitly called when the associated tag is encountered in the JSP

Must implement SimpleTag interface

Usually extend SimpleTagSupport class. For example

public class MyTagHandler extends SimpleTagSupport {

………….. }

1 - Tag Handler class cont.

doTag() method

By default does nothing

Need to implement / override to code tag’s functionality

Invoked when the end element of the tag encountered

1 - Tag Handler class cont. Implicit objects are available to tag handler

class through pageContext object

pageContext object can be obtained using getJspContext() method.

For example, to retrieve out object

PageContext pc = (PageContext)getJspContext();

JspWriter out = pc.getOut();

2 - Tag Library Descriptor (tld) XML based document

Specifies information required by the JSP container such as:

Tag library version JSP version Tag name Tag handler class name Attribute names etc.

3 - Deployment

Place Tag Handler class in myapp/WEB-INF/classes folder of web application

Place TLD file in myapp/WEB-INF/tlds folder

Using Custom Tags

Using Custom Tags Use “taglib” directive in JSP to refer to the tag library

<%@ taglib uri = “TLDFileName” prefix = “mytag” %>

Call the tag by its name as defined in TLD If tag name defined in TLD is hello then we write

< mytag:hello />

Behind the Scenes Container calls the appropriate Tag Handler

Tag Handler will write the appropriate response back to the page

Building Simple Tag

Building Simple Tag A simple tag that displays “Hello World”

Approach Extend Tag Handler class from

SimpleTagSupport class

• Override doTag() method

Build TLD file

Deploy

Example Code

Building Simple Tag(displays “Hello World” )

Building Tag with Attributes

Building Tags with Attributes E.g. <mytag:hello attribute=“value” />

To handle attributes, need to add Instance variables and Corresponding setter methods

Container call these setter methods Passed the custom tag attribute’s value as

arguments

Building Tag with Attribute (cont.)

Example:

We will modify our Course Outline Example to incorporate tags

Based on attribute value, tag will display the respective course outline

Example Code

Building Tag with Attribute(Modifying CourseOutline Example)