Creating Custom Tags Objectives In this lesson, you will learn to: Use JSP custom tags Create a JSP page by using custom tags
Need for Custom Tag Library To segregate presentation and business logic and detaching designers from the intricacies of programming constructs Provide a mechanism to encapsulate complex recurring codes Reuse the codes or tasks in a
The Tag Library ☛ Consists of a collection of functionally related, userdefined tags called custom tags ✓ Custom tags are user-defined and explicitly render information about the type of data ✓ Are very similar to XML tags ✓ Allows interactions between the user and the browser ✓ Facilitates reuse of existing codes to improve work productivity
Comparing the HTML and XML code outputs ☛ The HTML code:
| Ms Anne Brown | 9,Sunley House,GunthorpeStreet | London E1-7RW |
The HTML code (Contd.) ☛ The output:
✓ Is presentation-centric ✓ Contains no additional information about the type and importance of each section of data input ✓ Use of predefined HTML tags results in loss of information about the type of data
The XML code ☛ The XML code: <TITLE>Ms Anne Brown
The XML code (Contd.) <APTNAME rel="nofollow">9, Sunley House <STREETNAME>Gunthorpe Street London E1-7RW
The XML code (Contd.) ☛ The output:
The XML code (Contd.) ✓ Is more data- centric ✓ Data is presented in a format with emphasis on its type ✓ Specific custom tags such as firstname, lastname, street, and zip are used to differentiate various sections of the data
Advantages of XML ☛ Easy to code ☛ Use of explicit custom tags allows exchange of data at different levels without having to decode or interpret its structure ☛ Exchange of data between organizations without the need to understand the intricacies of business ☛ The detailed rules and specifications followed in the XML code are written in Document Type Definition (DTD)
Custom Tags ☛ Enable segregation of business complexities from the content presentation ☛ Incorporate features such as: ✓ Communication ✓ Object accessibility ✓ Nesting
Structure of a Tag ☛ Contain the start/end tags and a body ☛ Can be categorized as body tags or empty tags ☛ Can also be nested to contain tags of various levels ☛ Can include attributes used as parameters to customize the tag behavior
The tag library consists of:
Tag handler
Contain definitions of the classes and methods Define the functionality of the tag
Tag Library Descriptor file (TLD)
Is an XML file Describes the tag library
The tag handler:
Is used to define the working of custom tags Derives its methods from the javax.servlet.tagext package Implements the TagSupport interface for tags with an empty body Implements the BodyTagSupport interface for tags that use a body Also includes classes from other packages, such as javax.servlet.jsp and java.io
Can be structured for a basic tag, a tag with attributes, and a tag with a body Based on its structure consists of the following methods: Structure of the Tag Methods to be implemented
handler Simple tag with no body and no attributes
doStartTag, doEndtag, and release
Tag with attributes
doStartTag, doEndtag, and the respective set() and get() methods for each of the tags defined
Can define its functionality using the methods from the abstract class Tag such as:
doStartTag() doEndTag() release() doAfterBody() doBeforeBody()
The doStartTag() method returns the following Tag objects :
SKIP_BODY EVAL_BODY_INCLUDE EVAL_BODY_TAG
The doEndTag()method returns the following Tag objects :
SKIP_PAGE EVAL_PAGE
The tag handler also includes the following classes and methods :
The JspWriter() method to write the output to a JSP page The getAttribute() and setAttribute() methods retrieve variable values from scriplets Methods of the TagExtraInfo class such getVariableInfo(), TagExtraInfo(), setTagInfo(),and getTagInfo()
The diagrammatic representation of the execution of a JSP file containing custom tags:
The sequence of the execution cycle of a JSP file containing custom tags: ✓
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The JSP engine identifies the taglib directive in the JSP page The specified tag handler is initialized by using the uri and prefix as reference The get() and the set() methods for each tag is then executed The doStartTag() method is invoked
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The tag body is evaluated next, but is skipped if the SKIP_BODY field constant is specified The setBodyContent() method is invoked to store the output of the tag into a special PrintWriter called JspWriter Next, the doAfter() method is invoked to process the content generated after the evaluation of the tag body The doEndTag() method is invoked and all connections created earlier are closed to direct the output to the browser
The TLD file:
Is an XML file that contains the tag library description Contains a list and description of all the custom tags in the library Can be classified into two groups:
The first group comprises the sub-elements of the root tag of the TLD or the tag The second group within the taglib tag comprises of the element tag
The sub-elements of the TLD file at the taglib level are: Component
Specifies
The version of the tag library such as 1.0
<JSPVERSION>
The version of JSP that the tag library depends on such as <JSPVERSION>1.2
<SHORTNAME>
The name for the tag library
Component
Specifies
The universal resource identifier, an optional component that is a unique id for the tag library
The detailed information about the tag library
Defines a name for the tag
Specifies the tag handler class
The elements of the TLD file at the tag level are listed in the table below: Component Description
Provides additional information about the tag and its functionality
Specifies the attribute name and requirement specification for the tag
Contains the definition for the body of the tag
The steps to create the TLD file are: ✓
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In the Notepad, include the definitions for the document type and its definition (DTD) as the header Add the tlibversion, jspversion, uri, and info tags, along with their relevant information within the tag Add the tag definitions separating each element within the and tags Add to end the taglib tag.
The errors in JSP can be categorized as:
Translation time errors that are handled by the JSP engine Request time errors that are runtime errors and throw exceptions
Errors are trapped and handled in the calling JSP page . Errors not trapped are forwarded to the error page
The error page:
Is a separate JSP file Uses the following attributes of the page directive:
errorPage: To specify the name of an error page in the basic JSP page
<%@ page errorPage=”errorpage.jsp” %>
isErrorPage: To specify a JSP page as an error page
<%@ page isErrorPage=”true” %>
A JSP page using custom tags specifies:
The tag usage with the taglib directive <% @taglib uri=”taglib-examples” prefix=”example” %>
uri is used to specify a unique identifier for the particular tag library prefix is used to specify a reference name for the particular tag library
The inclusion of a new tag named first for the tag library example, the tag is written as: <example:first>
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