MIT AITI 2004 JSP Lecture 3

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MIT AITI 2004JSP Lecture 3

• Including and Forwarding

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Shared Code

• Usually every page of a site requires a common
  header, footer, and structure.
• Suppose each page of a site is written as
• lt_at_page import"java.util.Date"gt
• ltHTMLgt
• ltHEADgtltTITLEgtMy Pagelt/TITLEgtlt/HEADgt
• ltBODY BGCOLOR"blue"gt
• Todays date ltnew Date()gt
• . . .
• lt/BODYgt
• lt/HTMLgt
• Lots of duplicate HTML and JSP!

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Static Include

• Put common code into separate files
• lt_at_page import"java.util.Date"gt
• ltHTMLgt
• ltHEADgtltTITLEgtMy Pagelt/TITLEgtlt/HEADgt
• ltBODY BGCOLOR"blue"gt
• Todays date ltnew Date()gt
• . . .
• lt/BODYgt
• lt/HTMLgt

header.jsp
footer.html

• becomes . . .
• lt_at_include file"header.jsp" gt
• . . .
• lt_at_include file"footer.html" gt

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Include Directive

• lt_at_include file"filename"gt
• Pastes text of file directly into the page before
  the page is compiled.
• Beware of variable name clashes
• Dont use in confusing ways
• (for instance, included file ending an if
  statement)
• We now know two directives
• Include directive lt_at_include file"filename"gt
• Page directive lt_at_page import"javaclass"gt

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Dynamic Include

• Another way to include
• ltjspinclude page"header.jsp" /gt
• . . .
• ltjspinclude page"footer.html" /gt
• Different from include directive!
• Text of file NOT pasted in before compiling
• Included page is called during execution

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JSP Include Tag

• How it looks
• ltjspinclude page"filename" /gt
• or
• ltjspinclude page"filename"gt
• lt/jspincludegt
• How it works
• Passes the request on to the included page
• When done, returns to previous page

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JSP Include Tag 2

• Also send additional parameters to included page
• ltjspinclude page"printname.jsp"gt
• ltjspparam name"firstname" value"Greg" /gt
• lt/jspincludegt
• Can put JSP expressions into JSP tags
• ltjspinclude page"ltpageVargt"gt
• ltjspparam name"firstname"
  value"ltnameVargt" /gt
• lt/jspincludegt

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Static or Dynamic Include?

• Try to use static include (include directive) if
  possible, because it is faster
• Use dynamic inclusion (JSP include tag) if
• Including large file with a lot of variables
• (avoid potential variable name clashes)
• Passing parameters to included page
• The page you want to include is a variable
• (include the result of a JSP expression)

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Forwarding

• JSP Forward Tag
• ltjspforward page"filename" /gt
• How it works
• Passes the request on to the page forwarded to
• When done, does not return to previous page, just
  stops processing the request.

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JSP Forward Tag

• Like jspinclude, you can . . .
• Send additional parameters to forwarded page
• ltjspforward page"printname.jsp"gt
• ltjspparam name"firstname" value"Greg" /gt
• lt/jspforwardgt
• Put JSP expressions into JSP tags
• ltjspforward page"ltpageVargt"gt
• ltjspparam name"firstname"
  value"ltnameVargt" /gt
• lt/jspforwardgt

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Dynamic Include Example

• print-two-names.jsp
• lthtmlgt
• ltbodygt
• ltjspinclude page"printname.jsp"gt
• ltjspparam name"firstname" value"Greg" /gt
• lt/jspincludegtltpgt
• ltjspinclude page"printname.jsp"gt
• ltjspparam name"firstname" value"Eric" /gt
• lt/jspincludegt
• lt/bodygt
• lt/htmlgt

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Print Two Names Snapshot
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Dynamic Forward Example

• print-one-name.jsp
• lthtmlgt
• ltbodygt
• ltjspforward page"printname.jsp"gt
• ltjspparam name"firstname" value"Greg" /gt
• lt/jspforwardgt
• ltjspforward page"printname.jsp"gt
• ltjspparam name"firstname" valueEric" /gt
• lt/jspforwardgt
• lt/bodygt
• lt/htmlgt

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Print One Name Snapshot
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Request Parameters Limiting

• Request parameters are limiting because they must
  be Strings
• Must convert back and forth from Objects to
  String representation of Objects
• String reps must be unique
• Good news!
• We can put Objects in the request too

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Request Attributes

• Objects stored in the request object
• Add attribute with request.setAttribute
• lt
• request.setAttribute("dataString",
  dataObject)
• gt
• Get attribute with request.getAttribute
• lt
• DataType dataObject
• (DataType)request.getAttribute("dataString")
  
• gt
• getAttribute returns Object, so must cast

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Request Attribute Example

• prepare-date.jsp
• lt
• int month Integer.parseInt(request.getParameter
  ("m"))
• int day Integer.parseInt(request.getParameter("
  d"))
• int year Integer.parseInt(request.getParameter(
  "y"))
• Calendar cal Calendar.getInstance()
• cal.clear()
• cal.set(year, month, day)
• request.setAttribute("date", cal.getDate())
• gt
• ltjspforward page"use-date.jsp" /gt
• use-date.jsp
• lt
• Date date (Date)request.getAttribute("date")
• . . .
• gt

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Include Forward Review

• Static include
• Include directive lt_at_include file"filename"gt
• Pastes text into original file
• Dynamic include
• JSP include tag ltjspinclude page"file.jsp" /gt
• Requests included file at runtime and returns
• Dynamic forward
• JSP forward tag ltjspforward page"file.jsp" /gt
• Requests forwarded file at runtime and does not
  return
• Extra functionality when dynamically loading a
  page
• Send string parameters with jspparam tag
• Use JSP expressions within the tags
• Send object attributes to dynamically loaded page
• Add attribute to the request object with
  request.setAttribute
• Get attributes from the request object with
  request.getAttribute