Introduction to J2EE Enterprise Architecture The Web Tier JavaServer Pages

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Introduction to J2EE Enterprise Architecture
The Web TierJavaServer Pages
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The Need for JSP

• With servlets, it is easy to
• Read form data
• Read HTTP request headers
• Set HTTP status codes and response headers
• Use cookies and session tracking
• Share data among servlets
• Remember data between requests
• But tedious to
• Use println() statements to generate HTML
• Maintain that HTML

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JSP Framework

• Idea
• Use regular HTML for most of page
• Mark servlet code with special tags
• Entire JavaServer Page gets translated into a
  servlet (once), and the generated servlet is what
  actually gets invoked (for each request)
• Example
• lt!DOCTYPE gt
• lthtmlgt
• ltheadgt lttitlegtOrder Confirmationlt/titlegt
• ltlink relstylesheet hrefstyles.css
  type"text/css"gt
• lt/headgt
• ltbodygt lth2gtOrder Confirmationlt/h2gt Thanks for
  ordering
• ltigt lt request.getParameter(itemname") gt lt/igt!
  
• lt/bodygt
• lt/htmlgt

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Benefits of JSP

• Although JSP technically can't do anything
  servlets can't do, JSP makes it easier to
• Write HTML
• Read and maintain the HTML
• JSP makes it possible to
• Use standard HTML tools such as Macromedia
  DreamWeaver or Adobe GoLive.
• JSP encourages you to
• Separate the (Java) code that creates the content
  from the (HTML) code that presents it

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Advantages of JSP over competing technologies

• Versus ASP or ColdFusion
• Better language for dynamic part
• Portable to multiple servers and operating
  systems
• Versus PHP
• Better language for dynamic part
• Better tool support
• Versus pure servlets
• More convenient to create HTML
• Can use standard tools (e.g., DreamWeaver)
• Divide and conquer
• JSP programmers still need to know servlet
  programming

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Advantages of JSP over competing technologies

• Versus Velocity or WebMacro
• Standard
• Versus client-side JavaScript (in browser)
• Capabilities mostly do not overlap with JSP, but
• You control server, not client
• Richer language
• Versus server-side JavaScript (e.g., LiveWire,
  BroadVision)
• Richer language
• Versus static HTML
• Dynamic features
• Adding dynamic features no longer all or
  nothing decision

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Example
lt!DOCTYPE HTML PUBLIC "-//W3C//DTD
HTML 4.0 Transitional//EN"gt lthtmlgt ltheadgt
lttitlegtJSP Expressionslt/titlegt ltlink
rel"stylesheet" href"styles.css
type"text/css"gt lt/headgt ltbodygt lth2gtJSP
Expressionslt/h2gt ltulgt ltligtCurrent time
lt new java.util.Date() gt lt/ligt ltligtServer
lt application.getServerInfo() gt lt/ligt
ltligtSession ID lt session.getId() gt lt/ligt
ltligtThe ltcodegttestParamlt/codegt form parameter
lt request.getParameter("testParam") gt
lt/ligt lt/ulgt lt/bodygt lt/htmlgt
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Result
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Common misunderstandings

• Forgetting JSP is server-side
• Very common question
• I cant do such and such with HTML. Will JSP let
  me do it?
• Why doesnt this question make sense?
• JSP runs entirely on server
• It doesnt change content the client (browser)
  can handle
• Similar questions
• How do I put a normal applet in a JSP
  page?Answer send an ltAPPLETgt tag to the client
  
• How do I put an image in a JSP page?Answer send
  an ltIMG gt tag to the client
• How do I use JavaScript/Acrobat/Shockwave/Etc?Ans
  wer send the appropriate HTML tags

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Translation/Request time confusion

• What happens at page translation time?
• JSP constructs get translated into servlet code.
• What happens at request time?
• Servlet code gets executed. No interpretation of
  JSP occurs at request time. The original JSP page
  is totally ignored at request time only the
  servlet that resulted from it is used.
• When does page translation occur?
• Typically, the first time a JSP page is accessed
  after it is modified. This should never happen to
  real users (developers should test all JSP pages
  they install).
• Page translation does not occur for each request.

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JSP/Servlets in the real world

• ofoto.com
• print and manage digital and conventional photos.
  
• delta.com
• Delta Airlines entire Web site, including
  real-time schedule info
• americancentury.com
• American Century Investments more than 70 mutual
  funds, 90 billion under management, two million
  investors
• excite.com
• Excite one of the top five Internet portals one
  of the ten busiest sites on the Web

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Scripting Elements

• Expressions
• Format lt expression gt
• Expression is evaluated and the result converted
  to a String object, then output as part of the
  response to the client
• E.g. lt new java.util.Date() gt
• Scriplets
• Format lt Java code gt
• Any java code can be inserted here which will be
  inserted into the servlets _jspService() method
• E.g.
• lt rb (ia.RequestBean)request.getAttribute("reque
  st")
• if (rb.getType().equalsIgnoreCase("H"))
• gt Hardware
• lt else
• gt Software
• lt gt

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Scripting Elements

• Declarations
• Format lt! codegt
• Inserted verbatim into the body of the servlet
  class, outside of any existing methods
• E,g.
• lt! ia.RequestBean rb
• SimpleDateFormat fm2
• new SimpleDateFormat("dd-MMM-yyyy
  hhmm") gt
• Comments
• JSP supports three types of comments
• XHTML comments
• Format lt!-- and --gt
• Can be placed throughout JSP, but not in
  scriplets
• JSP comments
• Format lt-- and --gt
• Can be placed throughout JSP, but not in
  scriplets
• Java comments
• Standard ones // and / /
• Placed only within scriplets

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Implicit Objects

• Provide programmers with access to many servlet
  capabilities in the context of a Java Server Page
• Four scopes
• Application scope
• Container e.g. Tomcat owns objects with
  application scope
• Any servlet or JSP can manipulate such objects
• Page scope
• Each page has its own instances of the page-scope
  implicit objects
• Request scope
• Exists for the duration of the request
• Go out of scope when request is completed with a
  response to the client
• Session scope
• Exists for the clients entire browsing session

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Implicit Objects
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Example
lt!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN"gt lthtmlgt ltheadgt
lttitlegtJSP Expressions and Declarationslt/titlegt
lt--ltlink rel"stylesheet" href"styles.css"
type"text/css"gt--gt lt/headgt lt! private int
count0 String bgColor "lightgreen" gt
ltbody bgcolor"lt bgColor gt"gt lth2gtJSP
Expressions and Declarationslt/h2gt ltulgt
ltligtCurrent time lt new java.util.Date() gt
lt/ligt ltligtServer lt application.getServerI
nfo() gt lt/ligt ltligtSession ID lt
session.getId() gt lt/ligt ltligtThe
ltCODEgttestParamlt/CODEgt form parameter
lt request.getParameter("testParam") gt lt/ligt
ltligtnumber of times visited ltcountgt
lt/ligt ltligtback ground color is ltbgColorgt
lt/ligt lt/ulgt lt/bodygt lt/htmlgt
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Results
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Standard Actions

• JSPs standard actions provide access to several
  of the most common tasks performed in a JSP, such
  as including content from other resources
  forwarding requests to other resources and
  interacting with JavaBeans.
• Format ltjspactionnamegt and lt/jspactionnamegt
  where actionname is the name of the standard
  action

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Standard Actions
SourceDeitel Deitel
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Further information

• More information about servlets and JavaServer
  Pages can be found on the Internet.
• http//www.fcet.staffs.ac.uk/gdm1/msc/ia/ for the
  Internet Applications module
• http//java.sun.com/products/jsp/ for information
  from Sun
• http//www.google.co.uk/search?hlenqjspmeta
  for a search on Google