Advanced Java Server Pages

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Advanced Java Server Pages

• An more detailed look at JSPs

2
Custom methods in JSPs

• Just as Servlets, JSPs have methods for
  initialization end destruction
• JspInit() corresponds to init() in Servlets
• JspDestroy corresponds to destroy() in Servlets
• Both JspInit() and JspDestroy() can implemented
  in your JSPs, i.e you can override the base class
• Think before you use them
• If you need the init and destroy, is it really a
  JSP you should be writing? Probably not!

3
Servlets and JSPs

• Servlets should be used as front ends for JSPs
• The Servlet should handle the control logic
• The Servlet can initialize Beans that the JSP can
  use
• The Beans can talk to databases, EJBs and so on
• This is basic MVC
• But how is this done?

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Before the explanation

• Assume there is a Bean, DataBean that fetches
  records from a database
• The records are fetched to the bean with the
  fetch() method
• The records are returned from the bean with the
  getRecords() method

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The Servlet part

• Create the Bean instance
• Call your methods on the Bean
• Add the bean to the request or the session
• Get a RequestDispatcher
• Use the forward() method in the RequestDispatcher

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The Servlet part, using the request

• public void doGet(HttpServletRequest request,
  HttpServletResponse response) throws
  ServletException, IOException
• DataBean db new DataBean()
• db.fetch()
• request.setAttribute("dbBean", db)
• RequestDispatcher rd request.getRequestDispa
  tcher("jsppage.jsp")
• rd.forward(request, response)

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The Servlet part, using the session

• public void doGet(HttpServletRequest request,
  HttpServletResponse response) throws
  ServletException, IOException
• HttpSession ses request.getSession()
• DataBean db new DataBean()
• db.fetch()
• ses.setAttribute("dbBean", db)
• RequestDispatcher rd request.getRequestDispa
  tcher("jsppage.jsp")
• rd.forward(request, response)

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The JSP part

• The JSP uses the ltjspuseBean ..gt to define the
  bean, but it should not result in a new instance.
• The instance that was added in the Servlet is
  used
• The same scope in ltjspuseBean gt as in the
  Servlet (request and session in our examples)
• The same id in ltjspuseBean gt as the name in the
  Servlet
• The JSP uses ltjspgetProperty gt to call methods
  in the Bean

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The JSP part, with request

• ltjspuseBean iddbBean" classbeans.DataBean"
  scoperequest"gt
• Error, the bean should have been created in
  the servlet!
• lt/jspuseBeangt
• ltjspgetProperty namedbBean propertyrecords
  /gt
• lt-- propertyrecords will look for
  getRecords() in the bean --gt

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The JSP part, with session

• ltjspuseBean iddbBean" classbeans.DataBean"
  scopesession"gt
• Error, the bean should have been created in
  the servlet!
• lt/jspuseBeangt
• ltjspgetProperty namedbBean propertyrecords
  /gt
• lt-- propertyrecords will look for
  getRecords() in the bean --gt

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Custom Tags

• We dont want Java code in our JSPs
• The built in tags ltjspxxx gt is quite limited in
  functionality
• What we need is a way to extend the built in tags
  with our own
• Custom Tags

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Custom Tags

• Custom Tags separates the logic from the
  presentation even more that ltjspuseBean gt
• In our BookShop for example, the JSP with a book
  list and the shopping cart can consist of two
  lines of code
• ltbookshopbookList /gt
• ltbookshopshoppingCart /gt

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Two types of Custom Tags

• Simple Tag (Tag)
• A tag with or without arguments
• Doesnt use the Tag body
• Implements javax.servlet.jsp.tagext.Tag
• Body Tag
• With or without arguments
• Evaluate the body of the tag
• ltbookshopaTaggtThis is the bodylt/bookshopaTaggt
• Implements javax.servlet.jsp.tagext.BodyTag

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Simple Tag, an example

• import javax.servlet.jsp.
• import javax.servlet.jsp.tagext.
• public class HelloWorldTag implements Tag
• private PageContext pageContext
• private Tag parent
• /
• Constructor
• /
• public HelloWorldTag()
• super()
• /
• Method called at start of Tag
• _at_return either a EVAL_BODY_INCLUDE or a
  SKIP_BODY

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Simple Tag, an example

• /
• Method Called at end of Tag
• _at_return either EVAL_PAGE or SKIP_PAGE
• /
• public int doEndTag() throws javax.servlet.jsp.J
  spTagException
• try
• pageContext.getOut().write("Hello World!")
• catch(java.io.IOException e)
• throw new JspTagException("IO Error "
  e.getMessage())
• return EVAL_PAGE
• /
• Method called to releases all resources

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Simple Tag, an example

• / Method used by the JSP container to set the
  parent of the Tag
• _at_param parent, the parent Tag
• /
• public void setParent(final javax.servlet.jsp.ta
  gext.Tag parent)
• this.parentparent
• / Method for retrieving the parent
• _at_return the parent
• /
• public javax.servlet.jsp.tagext.Tag getParent()
  
• return parent

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Simple Tag

• All methods in the interface must be implemented!
• All work is done in the doStartTag() and
  doEndTag()
• The doStartTag() is called at the start of the
  tag
• The doEndTag() is called at the start of the tag
• Instead of implementing Tag, extend TagSupport
• A helper class included in the package that has
  implementation for all required methods. Just
  implement the one that you will use!

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Simple Tag, with TagSupport (complete)

• import javax.servlet.jsp.
• import javax.servlet.jsp.tagext.
• public class HelloWorldTag extends TagSupport
• private PageContext pageContext
• private Tag parent
• /
• Method Called at end of Tag
• _at_return either EVAL_PAGE or SKIP_PAGE
• /
• public int doEndTag() throws javax.servlet.jsp.J
  spTagException
• try
• pageContext.getOut().write("Hello World!")
• catch(java.io.IOException e)

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Simple Tags

• In other words, use TagSupport!
• The HelloTag implements our simple tag
• Custom Tags are defined in a Tag Library
  Descriptor (tld)

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Tag Library Descriptor

• lttaglibgt
• lttlibversiongt1.0lt/tlibversiongt
• ltjspversiongt1.1lt/jspversiongt
• ltshortnamegtmtlt/shortnamegt
• lturigt/mytaglt/urigt
• ltinfogtMy first Tag librarylt/infogt
• lttaggt
• ltnamegthelloWorldlt/namegt
• lttagclassgttags.HelloWorldTaglt/tagclassgt
• ltbodycontentgtemptylt/bodycontentgt
• ltinfogtA Hello world Taglt/infogt
• lt/taggt
• lt/taglibgt

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Web.xml

• The tld is referenced in web.xml
• Web.xml binds a tag library to a web application
• lttaglibgt
• lttaglib-urigtmytagslt/taglib-urigt
• lttaglib-locationgt/WEB-INF/taglib.tldlt/taglib-loca
  tiongt
• lt/taglibgt

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Using your tag

• The uri specified in web.xml is used in the JSP
• lt_at_ taglib uri"mytags" prefix"mt" gt
• At the start of the page
• ltmthelloTag /gt
• Prints Hello World

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Parameterized tags

• Both Tag and BodyTag can take parameters
• ltmthelloTag nameFredrik /gt
• Just as in JavaBeans, use a set-method and a
  member variable
• private String name
• public void setName(_name) name_name

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Parameterized tags

• import javax.servlet.jsp.
• import javax.servlet.jsp.tagext.
• public class HelloNameTag extends TagSupport
• private String name
• public void setName(String _name)name_name
• public int doEndTag() throws javax.servlet.jsp.Jsp
  TagException
• try
• pageContext.getOut().write("Hello
  name)
• catch(java.io.IOException e)
• throw new JspTagException("IO Error "
  e.getMessage())
• return EVAL_PAGE

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TLD for HelloName

• lttaggtltnamegthellolt/namegtlttagclassgtHelloNameTaglt/t
  agclassgtltbodycontentgtemptylt/bodycontentgtltinfogtA
  Hello Taglt/infogtltattributegtltnamegtnamelt/namegtltre
  quiredgtfalselt/requiredgtltrtexprvaluegtfalselt/rtexpr
  valuegtlt/attributegt
• lt/taggt

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Using the parameterized tag

• ltmthello nameFredrik/gt

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BodyTag

• Just as with Tag, there are a lot of methods to
  write if implementing BodyTag
• There is a helper class call BodyTagSupport just
  as with TagSupport
• BodyTag is often used for looping over some result

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Body Tag

• package se.upright.education.uu.pvk.tags
• import javax.servlet.jsp.
• import javax.servlet.jsp.tagext.
• public class LoopTag extends BodyTagSupport
• private int iterations 5
• /
• Method used by the JSP container to set the
  parameter Name.
• /
• public void setIterations(int _iterations)
• this.iterations_iterations
• /

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Body Tag

• public int doAfterBody() throws JspTagException
• if(iterationsgt1)
• //decrease the number of iterations left to
  do
• iterations--
• return EVAL_BODY_TAG
• else
• return SKIP_BODY

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Body Tag

• /
• Method Called at end of tag
• _at_return either EVAL_PAGE or SKIP_PAGE
• /
• public int doEndTag() throws JspTagException
• try
• if(bodyContent ! null) // Check if we even
  entered the body
• bodyContent.writeOut(bodyContent.getEnclos
  ingWriter())
• catch(java.io.IOException e)
• throw new JspTagException("IO Error "
  e.getMessage())
• return EVAL_PAGE

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Body Tag tld

• lttaggt
• ltnamegtlooplt/namegt
• lttag-classgtse.upright.education.uu.pvk.tag
  s.LoopTaglt/tag-classgt
• ltbody-contentgtJSPlt/body-contentgt
• ltattributegt
• ltnamegtiterationslt/namegt
• ltrequiredgttruelt/requiredgt
• ltrtexprvaluegttruelt/rtexprvaluegt
• lt/attributegt
• lt/taggt

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Using Body Tag

• ltuuloop iterations"10"gt
• Looping ltbr /gt
• lt/uuloopgt

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Next Time

• Java Standard Tag Library
• A Tag Library with common functionallity
• XML
• XSLT