EBIZ 5535 Lecture 7

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EBIZ 5535Lecture 7

• Custom JSP Tags

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Custom JSP Tags

• Permits JAVA developers to boil down complex
  server-side behavior into easy-to-use elements
  that content developers can use
• Accomplish the same goals as beans
• Encapsulating complex behaviors into a simple
  accessible form

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Differences Between Beans and Custom Tags

• Beans cannot manipulate JSP content custom tags
  can
• Custom tags are more powerful than beans
• Custom tags require more work to set up

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Components of a Tag Library

• The tag handler class
• Java code that says how to actually translate tag
  into code
• Must implement javax.servlet.jsp.tagext.Tag
  interface
• Usually extends TagSupport or BodyTagSupport
• Goes in the same directories as servlet class
  files and beans

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Components Contd

• The tag library descriptor file
• XML file describing tag name, attributes and
  implementing tag handler class
• Goes with JSP file or at arbitrary URL
• The JSP file
• Imports a tag library
• Defines tag prefix
• Uses the tags

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Defining a Simple Tag Handler Class

• Extend the TagSupport class
• Imports needed packages
• import javax.servlet.jsp.
• import javax.servlet.jsp.tagext.
• import java.io.
• Override doStartTag
• Obtain the JspWriter by means of
  pageContext.getOut()
• Use the JspWriter

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Simple Tag Handler Class Contd

• Return SKIP_BODY
• Translated into servlet code at page-translation
  time
• Code is called at request time

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Defining a Simple Tag Library Descriptor

• Start with XML header and DOCTYPE
• Top-level element is taglib
• Each tag is defined by tag element with
• name defines the base tag name
• ltnamegtsimplePrimelt/namegt
• tagclass gives the fully qualified class name of
  the tag handler
• lttagclassgtprime.SimplePrimeTaglt/tagclassgt

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Tag Library Descriptor Contd

• bodycontent gives hints to development
  environments
• info gives a short description
• example-taglib.tld

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Accessing Custom Tags From JSP Files

• Import the tag library
• Specify location of TLD file
• lt_at_taglib urimytaglib.tld prefixmyprefix gt
• Use the tags
• ltmyprefixtagName /gt
• Tag name comes from TLD file
• Prefix comes from taglib directive

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Simple Examples

• SimpleExample.jsp
• SimplePrimesExample.jsp

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Using tags with attributes

• ltprefixtagname
• attrib1value1
• attrib2value2 /gt
• Still no tag body
• Tag handling class needs getter and setter
  methods

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Example of tag handler class with tag attributes

• To support
• ltprefixtagname attrib1value1/gt
• Place the following method in the tag handler
  class for tagname
• public void setAttrib1(String value)
• doSomethingWith(value)
• PrimeExample.jsp
• How does the taglib change?

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Including a tag body

• Simple tags
• ltprefixname/gt
• Tags with attributes
• ltprefixname attrib1value1 attrib2value2/gt
• Now, tags with bodies
• ltprefixname attrib3value3gt
• JSP content
• lt/prefixnamegt

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Handler classes for tags with bodies

• The doStartTag method
• Instead of returning SKIP_BODY, return
  EVAL_BODY_INCLUDE
• The doEndTag method
• This method is only necessary if you want to take
  some action after handling the body
• Return eval_page
• HeadingExample.jsp
• How does the taglib entry differ?

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Optional tag bodies

• First examples had no tag bodies
• doStartTag returned SKIP_BODY
• Then examples that include tag bodies
• doStartTag returned EVAL_BODY_INCLUDE
• We can decide whether or not to include the tag
  body at request time
• Either return SKIP_BODY or EVAL_BODY_INCLUDE

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Example of optionally including tagbody

• DebugTag.java