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JSP Custom Tags

JSP Custom Tags. 4.1.0.3. Unit objectives. After completing this unit, you should be able to: Describe the advantages of using JSP custom tags List the major steps in developing and using JSP custom tags Develop basic tag handler classes to implement JSP custom tags

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JSP Custom Tags

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  1. JSP Custom Tags 4.1.0.3

  2. Unit objectives After completing this unit, you should be able to: • Describe the advantages of using JSP custom tags • List the major steps in developing and using JSP custom tags • Develop basic tag handler classes to implement JSP custom tags • Create and modify taglib descriptor files • Package JSP taglib implementation classes and taglib descriptor files • Understand the uses of the JSTL • Name some of the tags included in the JSTL and their purposes

  3. JSP Custom Tags Overview • Nine standard actions must be provided by any compliant JSP implementation: • useBean, setProperty, getProperty • include, forward • plug-in, params, param, fallback • Custom tags allow developers to create additional actions beyond the standard set • Custom actions are invoked via custom tags in a JSP page • Tag libraries are collections of custom tags • Support for JSP custom tags is required by the JSP specification <jsp:useBean id="customer" class="com.ibm.model.customer" /> <jsp:setProperty name="customer" property="id" value="0" /> <jsp:include page=“banner.jsp” />

  4. Why Use JSP Custom Tags? • Role-based development • Model classes (business objects and data storage layers) are developed by Java and EJB developers • Controller classes (servlets) are developed by Java developers • View-based JSP pages are developed by HTML developers • Different roles: • Use different tools • Have different skills • Best Practice • MVC design is well established • Use the right tools for the right jobs

  5. Steps to Create and Use a Custom Tag Library • To develop a tag, you need to: • Design your tags and attributes • Declare the tag in a tag library descriptor (TLD) • Develop a tag handler class • Develop helper classes for the tag (if needed) • To use a custom tag, the JSP needs to: • Include the tag library using the taglib directive • Code the custom tag with any needed attributes • Test your tags class 1 helper class JSP Page TLD class 2

  6. Tag Usage Example <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 transitional//EN"> <HTML><HEAD> <%@ taglib uri="/WEB-INF/tld/taglib.tld" prefix=“tl" %> <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1“ %> <TITLE>Date Demo</TITLE> </HEAD> <BODY> <h1>Date Demo</h1> <P>Fully formatted date: <tl:date format=”full”/> </P> </BODY></HTML>

  7. JSP Page Without Custom Tags <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML><HEAD> <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%> <TITLE>Date Demo</TITLE> </HEAD><BODY> <h1>Date Demo</h1> <P>Fully formatted date: <% java.util.Locale locale = pageContext.getRequest().getLocale(); java.text.DateFormat fmt = java.text.DateFormat.getDateInstance (java.text.DateFormat.FULL,locale); String date = fmt.format(new java.util.Date()); %> <%= date %> </P> </BODY></HTML>

  8. 1 2 Using Custom Tags with Application Developer • Page Designer has different ways of selecting a tag for inclusion with JSP • Select JSP->Insert Custom • Drag Custom from JSP Tags drawer in Palette • Select desired tag from Insert Custom Tag dialog

  9. JSP Standard Tag Library (JSTL) • Encapsulates as tags core functionality of many Web applications • Supports tasks such as: • Flow (iteration and conditionals) • Manipulation of XML documents • Internationalization tags • SQL tags • J2EE 1.4 includes both JSP and JSTL • JSTL taglibs included with Rational Application Developer

  10. Sample JSTL Tags • Set a variable in a specific scope to a value • <c:set var="name" scope="scope" value="expression"/> • Display a value, or an alternative if the first value is null • <c:out value="expr" default="expr" escapeXml="boolean"/> • Example: Hello <c:out value="${user.name}" default="Guest"/>! • Conditional execution • <c:choose>, <c:when> and <c:otherwise> <c:choose> <c:when test="${user.role == 'member'}"> <p>Welcome, member!</p> </c:when> <c:otherwise> <p>Welcome, guest!</p> </c:otherwise></c:choose>

  11. forEach Tag • Provides flexible iteration through a set of items • Targets include: • Collections, Maps, Iterators, Enumerations • Arrays • Comma-separated values • SQL ResultSets • Example: <table> <c:forEach items="${customers}" var=“cust"> <tr> <td>${cust.name}</td> <td>${cust.addr}</td> </tr> </c:forEach> </table>

  12. Anatomy of a Tag Element Attribute (optional) Start tag <tl:asis tab="5">Instructions for logging in to the system: (1)Enter your Patron identifier in ID field (2)Enter assigned password in PW field (3)Click on LOGIN button </tl:asis> Body (optional) End tag

  13. Tag Examples • Basic<tl:fullText /> • With attributes<tl:code language="java“/> • With attributes and a body<tl:iterator times=10> <p>"Hello world."</p></tl:iterator> • Defining scripting variables<tl:iterator name="list" id="customer” type="domain.Customer"> <jsp:getProperty name="customer" property="name" /></tl:iterator>

  14. Describing Tags to the JSP Container • Done with the taglib descriptor (TLD) • XML file • Describes the tag library • Files use the .tld extension • Defines the syntax of the tags (actions) • Defines the attributes (if any) for the tags • Specifies if the attribute is optional or required • Specifies the Java class that implements the tag • Specifies if the tag allows or uses a body • Used by the JSP container to validate the JSP at compile time

  15. General Format of the TLD (1 of 2) <?xml version="1.0" encoding="UTF-8"?> <taglib version="2.0" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web- jsptaglibrary_2_0.xsd"> Required <description>Tag Library from IBM Library System </description> <tlib-version>1.0</tlib-version> <short-name>ilib</short-name> Info about TLD <tag> <name>date</name> <tag-class>com.ibm.library.tag.FormattedDate</tag-class> <body-content>empty</body-content> <description>Display current date</description> </tag> Tag Info Defines the date tag

  16. General Format of the TLD (2 of 2) <tag> <name>date2</name> <tag-class>com.ibm.library.tag.FormatDate2</tag-class> <body-content>JSP</body-content> </tag> </taglib> Action name Tag handler class implementation How to process the body <attribute> <name>format</name> <required>true</required> </attribute> Attribute name (multiple allowed) Optional (false) or mandatory (true)

  17. Location of TLD File • Resides in the META-INF directory or subdirectory when deployed inside a JAR file • Resides in the WEB-INF directory or some subdirectory when deployed directly into a Web application • XML Schema is located at URL:http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd

  18. JSP Taglib Directive • Taglib directive tells your JSP the prefix to be used for a specific JSP tag library Taglib directive Location of TLD <!DOCTYPE … > <HTML> <HEAD> <%@ taglib uri="/WEB-INF/tld/taglib.tld" prefix="tl" %> … <TITLE>date test</TITLE> </HEAD> <BODY> … <tl:date format="full"/> … </BODY> </HTML> Prefix for this JSP Taglib usage

  19. Tag Handler Base Classes • Tag handlers must implement specific interfaces or extend specific classes, and must override key methods • These classes all reside in javax.servlet.jsp.tagext • JSP 2.0 introduced SimpleTag “classic” tags

  20. Example Tag • The <transform> tag allows page developers to transform the contained text in two ways: • Convert it to upper case • Hide it • The tag has a required attribute mode with the following values: • upper • hide • The value of the attribute can be taken from a runtime expression <m:transform mode="upper"> <P>This is text to be transformed.</P> </m:transform>

  21. Processing Tags with Attributes: How It Works • Initialize and set attributes (setMode()) 2) Call doTag() method <m:transform mode="upper"> <P>This is text to be transformed.</P> </m:transform>

  22. What Needs to Be Done? • Create the TransformTag class • Update the TLD for the new date tag • Use the new <transform> tag in your JSPs JSP Page TLD handler class

  23. The TransformTag Class package com.ibm.library.tag; import java.io.IOException; import java.io.StringWriter; import javax.servlet.jsp.JspException; import javax.servlet.jsp.JspWriter; import javax.servlet.jsp.tagext.JspFragment; import javax.servlet.jsp.tagext.SimpleTagSupport; public class TransformTag extends SimpleTagSupport { String mode = ""; public void setMode(String mode) { this.mode = mode.toUpperCase(); } // class continues on next page

  24. The doTag() Method public void doTag() throws JspException, IOException { JspFragment body = getJspBody(); StringWriter oldbody = new StringWriter(); String newbody = null; body.invoke(oldbody); if (mode.equals("UPPER")) { newbody = oldbody.toString().toUpperCase(); } else if (mode.equals("HIDE")) { newbody = ""; } else { newbody = oldbody.toString(); } JspWriter out = getJspContext().getOut(); out.write(newbody); }

  25. The Taglib Descriptor <?xml version="1.0" encoding="UTF-8"?> <taglib version="2.0" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd "> <description>Tag Library for Library System</description> <tlib-version>1.0</tlib-version> <short-name>ilib</short-name> <tag> <name>transform</name> <tag-class>com.ibm.library.tag.TransformTag</tag-class> <body-content>scriptless</body-content> <attribute> <name>mode</name> <required>true</required> <rtexprvalue>true</rtexprvalue> </attribute> </tag> </taglib>

  26. Using the <transform> Tag <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML> <HEAD> <%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%> <%@ taglib uri="WEB-INF/tld/mytags.tld" prefix="m" %> <TITLE>transformDemo.jsp</TITLE> </HEAD> <BODY> <H1>Demonstrate &lt;transform&gt; tag</H1> <m:transform mode="UPPER"> <P>This is text to be transformed</P> </m:transform> <P>This text is not to be transformed</P> </BODY> </HTML>

  27. Packaging • To facilitate reuse, the tag handler classes can be packaged together • Place the class files in a JAR • Import the TLD into /WEB-INF/tld • Import the JAR into /WEB-INF/lib • An additional option is to package the TLD with the class files JAR • Application Developer providessupport for JSP tag library resource references • Web Deployment Descriptor editor • Variables tab • Allows URI to be specified to reference the TLD

  28. Checkpoint • What are some of the advantages of JSP custom tags? • What are the major steps that must be performed during JSP custom tag development? • How are attributes’ values processed in a tag handler class? • What method of the SimpleTag interface does the main work of processing a tag? • What is the purpose of the JSP taglib directive?

  29. Checkpoint solutions • Advantages of custom tags include: • Make JSPs easier to develop, test, and maintain • Web developer can focus on presentation (role-based developmental • Presentation logic is reusable • The major steps in JSP custom tag development are: • Design tags and attributes • Write tag handler class • Construct or modify TLD • Test in a JSP • Attribute values are processed in a tag handler class through JavaBean-like setter methods. • doTag() • The taglib directive describes the location of the TLD and designates the tag prefix.

  30. Unit summary Having completed this unit, you should be able to: • Describe the advantages of using JSP custom tags • List the major steps in developing and using JSP custom tags • Develop basic tag handler classes to implement JSP custom tags • Create and modify taglib descriptor files • Package JSP taglib implementation classes and taglib descriptor files • Understand the uses of the JSTL • Name some of the tags included in the JSTL and their purposes

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