SE 452 Lecture Notes

• Any question on the demo option for homework grading?
• Additional extra credit option
  • We discussed the initial extra credit option last week
  • Or, you can give a 15 minute presentation on a J2EE pattern during week 10 (the optional week where there will be no quiz)
    • You need to explain what it is
    • Why you would do it in a certain situation
    • And how your code illustrates it
  • A presentation will be worth 120 points, but you will not be allowed to do the in person extra credits
  • No duplicate J2EE patterns will be presented
    • First come first serve
    • Send me an e-mail with your request
    • Your best bet is to send an e-mail with a few you are willing to cover
    • Once you commit to do a pattern you are required to do so
    • If you commit and change your mind there will be a score deduction (more of an empty threat, but just don't do it because you mess up other people)
  • In addittion to the Sun list; you can also demostrate the use of a GoF (Gang of Four) design pattern in a JSP/Servlet context
    • If you choose the GoF route you need to show how you combine multiple GoF patterns to be effective
    • Overlapping GoF patterns will be allowed as long as each presentation has one unique pattern
    • E-mail me with your requests
  • DL students who would like to choose this option should contact me about taking the final at a different time
• What's an example where JavaBeans would have an advantage over scriplets?
• What's the point of JavaBeans?
• Let's go over the homework 4/5
• Any questions before the quiz?

• Allows introduction of new custom actions into a JSP that we define
• A Tag Library can be used to define a specialized sub language of JSP that allows the users of that library to use it naturally with JSP
• Allows us to create a Tag Library
  • A set of Tags
  • Portable
    • Usable in any JSP container
  • Simple and easy to use
    • Hides the implementation
    • Allows non programmers to develop more complex pages without using complex scripting elements

• Accomplish many of the same goals
  • Encapsulate complex behaviors
  • Allows for code reuse
  • Object Oriented
• Some improvements with custom tags
  • Tag can manipulate JSP content
  • Support for complex operations such as iterations
  • does take a little more work to set up

• Three parts
  • Tag Handler
    • A Java class that implements javax.servlet.jsp.taxext.Tag interface
    • Usually indirectly by extending TagSupport or BodyTagSupport
  • Tag Library Descriptor(TLD)
    • An XML configuration file
    • Loaded by the servlet engine at startup
  • A JSP file
    • Use a taglib directive
    • Use your custom JSPs

• The Tag interface defines the basic methods for all tag handlers
  • doStartTag()
    • Processes the start tag of this action (before the body)
    • pageContext is available
    • return constant EVAL_BODY_INCLUDE to evaluate the tag body
    • return constant SKIP_BODY to not evaluate the body
      • Required if the TLD has a body content of EMPTY
  • doEndTag()
    • Processes the end tag of this action (after the body)
    • If returns EVAL_PAGE, the rest of the page continues to be evaluated
    • If returns SKIP_PAGE, the rest of the page is skipped
  • TagSupport implements Tag
    • Extend this class for empty tags
• The BodyTag interface extends Tag interface provides two additional methods for handling tag bodies
  • doInitBody()
    • Evaluated prior to doAfterBody
    • Oncly called once
    • Can view BodyContent
  • doAfterBody()
    • Used for tags that repeat data
    • Called to determine if tag should be evaluated again
    • return EVAL_BODY_AGAIN or SKIP_BODY
  • BodyTagSupport implements BodyTag
    • Enxtend this one for non empty tags
  • More on this one in a bit
• Tags also contain writeable attributes
  • provide setters for the properties
  • setters are called by the JSP container prior to any other methods

You can overwrite as many of the below methods as you need to. So you should know what happens when.

• XML file that informs the server about the handler
• name: unique name used to access the tag
• tagclass: the fully qualified name of the handler
• bodycontent: indicates if our tag has a body to execute
• attribute: required if our handler has attributes to set
  • name: attribute's name
  • required: indicates if the attribute must always be supplied by the jsp
  • rtexprvalue: can the attribute value be a JSP expression

• Use taglib directive
  • <%@ taglib uri="se452-taglib.tld" prefix="se452" %>
  • uri: absolute or relative url
• Use tag in the JSP
  • <se452:tag1>
  • <se452:tag1>Body (any JSP/ html/ Text)</se452:tag1>

• C:\jakarta-tomcat-4.0.6\webapps\jjungman\WEB-INF\classes\edu\depaul\se452\HelloWorldTag.class
• C:\jakarta-tomcat-4.0.6\webapps\jjungman\WEB-INF\se452-taglib.tld
• C:\jakarta-tomcat-4.0.6\webapps\jjungman\myFirstTag.jsp

• To include the tag body have your doStartTag method return EVAL_BODY_INCLUDE
  • Any closing action can be placed in doEndTag (executed after the body)
• extending BodyTagSupport gives us two new important methods
  • doAfterBody
    • return SKIP_BODY once you no longer need to interpret the body
    • return EVAL_BODY_TAG(deprecated for JSP1.1) or EVAL_BODY_AGAIN(same int) to view the body again
  • getBodyContent returns a BodyContent object encapsulating the contents of the body and provides three important methods
    • getEnclosingWriter the writer used in doStartTag and doEndTag
    • getReader a reader that can read the tag's body
    • getString the body content as a String