Introduction to JavaServer Pages

1
Introduction to JavaServer Pages

• CMT3332
• Lecture 3

2
What is JavaServer Page (JSP) and Servlet?

• JSPs are web pages with embedded Java code
• The Java code is executed on the server before
  the page is returned to the browser
• Presentation-oriented
• Servlets are pure Java classes
• Servlets allow the programmer to work directly -
  requests are made to a Web Server and responses
  containing the contents are returned to a client
• Designed to perform lower-level functions that
  work with data or implement business models

3
HTML Basic

• The constructing blocks of a web page is HTML
  code
• HTML is HyperText Markup Language
• The format of the web page is determined by HTML
  tags
• An HTML tag is embedding in the angled bracket lt
  gt
• A closing HTML tag will have a stroke following
  the open bracket lt/ gt
• As JSPs are also web pages, we need to know the
  basis of HTML before coding JSPs
• To know more about HTML code, you can visit
• http//www.htmlhelp.com/reference/html40/

4
HTML tags

• ltHTMLgtlt/HTMLgt
• Mark the start and end of an HTML file
• ltHEADgtlt/HEADgt
• The header of an HTML file. The header may
  contain ltTITLEgt, ltMETAgt, ltSTYLEgt, ltBASEgt and
  other details.
• ltBODYgtlt/BODYgt
• The body of an HTML file. We can define the
  following attributes for the body text
• BACKGROUND background pic
• BGCOLOR background color
• TEXT text color
• LINK link color
• VLINK visited link color
• ALINK active link color

5
Example of Basic HTML tags

• ltHTMLgt
• ltHEADgt
• ltTITLEgttest for HTML, HEAD and BODY lt/TITLEgt
• lt/HEADgt
• ltBODY BGCOLOR"YELLOW" TEXT"RED" LINK"BLACK"
  VLINK"BLUE" ALINK"YELLOW"gt
• test for HTML, HEAD and BODY ltBRgt
• ltA HREF"http//www.yahoo.com"gtFrom here to
  Yahoolt/agt
• lt/BODYgt
• lt/HTMLgt

6
Font Tags

• ltFONTgtlt/FONTgt
• Attributes
• SIZE size is based on a seven-point scale. You
  can use absolute value or relative value (e.g. 2
  or 1)
• COLOR you can use named color or hexadecimal
  value starting with (e.g. FFFFFF represents
  white)
• FACE the name of the font (case-sensitive)
• Example
• ltFONT SIZE5 COLOR"BLUE" FACE"Arial"gtArial
  Textlt/FONTgt

7
Styling Tags

• Styling Tags
• ltTTgtlt/TTgt - Monospaced
• ltIgtlt/Igt - Italic
• ltBgtlt/Bgt - Bold
• ltUgtlt/Ugt - Underlined
• ltSgtlt/Sgt - Strikeout
• ltBIGgtlt/BIGgt - Larger font size (size 1)
• ltSMALLgtlt/SMALLgt - Smaller font size (size 1)
• Heading Tags ltH1gtlt/H1gt till ltH6gtlt/H6gt
• Standard headings with automatic line feed
• Attribute ALIGN you can set the alignment to
  LEFT, RIGHT, CENTER and JUSTIFY
• Example ltH1 ALIGN"CENTER"gtWelcomelt/H1gt

8
Paragraph Styling Tags

• ltPgtlt/Pgt used to format a paragraph, you can use
  ALIGN attribute to set the alignment
• ltBRgt jumps to the next line
• ltNOBRgtlt/NOBRgt makes the embedded text keep in
  the same line
• ltHRgtlt/HRgt creates a "horizontal rule"
• You can set ALIGN, SIZE and WIDTH (using absolute
  or percentage value) of the rule
• ltPREgtlt/PREgt keeps the original spacing in the
  text (other format will treat all whitespaces as
  1 single space)
• ltCENTERgtlt/CENTERgt keeps the block of text center
  aligned

9
Example for P, BR, and NOBR

• ltHTMLgt
• ltHEADgt
• ltTITLEgtExample of P/BR/HRlt/TITLEgt
• lt/HEADgt
• ltBODYgt
• Line 1
• ltPgt Line 2 ltBRgt
• Line 3 lt/Pgt
• ltHR ALIGNRIGHT WIDTH50 SIZE10gt
• ltNOBRgt Line 4.
• Line 5. Line 6. Line 7. lt/NOBRgt
• lt/BODYgt
• lt/HTMLgt

10
Image Tag

• ltIMGgt for holding images (GIF, JPEG)
• Attributes SRC (source file), ALT (alternative
  text), WIDTH, HEIGHT, BORDER (if set to 0, no
  border is shown)

11
Table Tags

• ltTABLEgtlt/TABLEgt defines a table
• You can set the attributes BGCOLOR, BORDER,
  ALIGN, WIDTH, CELLSPACING, CELLPADDING
• Cell spacing is the spacing between spaces
• Cell padding is the spacing between the text and
  the border
• ltCAPTIONgtlt/CAPTIONgt forms a caption to describe
  the table
• ltCAPTION ALIGNTOPgt Table 1.1 lt/CAPTIONgt
• Alignment can be TOP, BOTTOM, RIGHT and LEFT
• Within the table, we can define the header, rows
  and cells

12
Header, Rows and Cells

• ltTRgtlt/TRgt defines a row
• You can set the ALIGN, BGCOLOR, VALIGN (vertical
  alignment)
• ltTDgtlt/TDgt defines a cell within a row
• You can set the ALIGN, VALIGN, WIDTH, HEIGHT,
  BGCOLOR, NOWRAP and other attributes in the cell
• ROWSPAN and COLSPAN can set the number of rows
  and columns that the header cell will span
• ltTHgtlt/THgt defines a header cell within a row
• A header cell is similar to an ordinary cell, but
  it serves as the header of a column, with bolded
  fonts

13
Example of Table in HTML

• lthtmlgtltheadgt
• lttitlegtExample of Tablelt/titlegtlt/headgt
• ltbodygt
• lttable Border3 Width50gt
• ltTRgt ltTH ColSpan5gtStudent Listlt/THgtltTH
  RowSpan3 ValignBottom NoWrapTruegtRemarklt/THgt
  lt/TRgt
• ltTRgt ltTH RowSpan2 Width50gt Student Nolt/THgtltTH
  RowSPan2 Width50gtNamelt/THgtltTH
  ColSpan3gtHistorylt/THgtlt/TRgt
• ltTRgtltTHgtPri. Schlt/THgtltTHgtSec.
  Schlt/THgtltTHgtUniversitylt/THgtlt/TRgt
• ltTRgtltTDgt123456lt/TDgtltTDgtJohn Chanlt/TDgtltTD
  NoWrapTruegt1992lt/TDgtltTD NoWrapTruegt1999lt/TDgt
  ltTD NoWrapTruegt2002lt/TDgtltTDgt lt/TDgtlt/TRgt
• lt/bodygtlt/htmlgt

14
The Anchor Tag

• ltAgtlt/Agt Anchor Tag may be the most important tag
  in HTML, represents a hyperlink of the
  destination of a link
• HREF attribute shows the URL of the hyperlink
• TARGET attribute defines in which frame the
  contents of the link should appear value can be
  _blank (new window), _self (current frame),
  _parent and _top
• NAME attributes represents a destination of a
  link, or a bookmark
• You can add XXX to you URL to make the browser
  to go to a specific bookmark in a page (if the
  bookmark exists)

15
JSP behind the Scene

• When the browser asks the Web server for a JSP,
  the Web server passes control to a JSP container
• The container provides the runtime environment
  and other services the JSP needed
• At the first time a JSP is invoked, the JSP
  container will convert it into a servlet
  (HttpJspPage) as an executable unit
• The container will normally check if a servlet
  for that JSP already exists and updated
• The contents of the JSP file will eventually
  convert to codes in a method _jspService (unless
  otherwise specified in the code) in the servlet
• In Tomcat, the servlet files are stored deep in
  the work folder in your Tomcat path.

16
Basic JSP Tags

• lt gt tags (scriptlet tag) in a JSP file
  indicates the presence of Java code within the
  HTML
• Codes inside the scriptlet is sent verbatim
• HTML codes will be encoded to be the printout of
  a Java output statement
• Both scriptlet codes and HTML codes will be
  placed in the _jspService method
• lt gt tag (expression tag) displays a value
  directly to the HTTP client
• It actually calls a out.write command to print
  out the text, i.e., it will be also placed in the
  _jspService method
• Be careful! There is no space between lt and

17
Inserting Comments to a JSP

• lt!-- It is a HTML comment --gt
• The comment will appear in the HTML code, but not
  in the browser
• lt-- It is a JSP comment --gt
• The comment will not appear in the servlet code
  generated.
• It is useful to remove a section of code from a
  JSP
• You can always use Java comments as in an
  ordinary Java code
• Example lt // This is a Java comment gtlt /
  gt This will be commented out. lt / gt

18
Declaring Methods and Variables

• You cannot use lt gt tag to declare a method
• The contents inside lt gt tag will be put in the
  method _jspService
• We can't declare a method within another method
• lt! gt tag (declaration tag) is used to declare a
  method that appears at the top of the resulting
  servlet class
• You may also declare an instance variable that
  may be accessed by all methods using lt! gt tag
• out is an implicit output object (JspWriter)
  defined by JSP specification however, it is
  defined within the _jspService method and cannot
  be used in any method

19
Built-in JSP Objects

• JSP API provides a series of built-in objects
  (implicit objects) to access the information
  provided by the Servlet objects of different
  classes
• They are created at the beginning of a page (in
  the _jspService method) and directly reference
  objects in the underlying servlet
• They are only available to scriptlets and
  expressions
• These JSP objects will be further discussed in
  the coming lecture.

20
Built-in JSP Objects and the Corresponding
Servlet Classes
21
Servlet Basis

• Every servlet must implement the
  javax.servlet.Servlet interface
• Most servlets implement the interface by
  extending javax.servlet.http.HttpServlet
• An HTTP servlet overrides the doPost( ) and/or
  doGet( ) method.
• It is an usual practice that we implements one of
  the above methods and call the method within the
  other one. E.g., doPost will only call doGet
  which have all the implementation

22
A Sample Java Servlet

• import java.io.
• import java.text.
• import java.util.
• import javax.servlet.
• import javax.servlet.http.
• public class MyServlet extends HttpServlet
• public void doGet (HttpServletRequest request,
  HttpServletResponse response) throws
  ServletException, IOException
• PrintWriter out
• String title "MyServlet Output"
• response.setContentType("text/html")
• out.println("ltHTMLgtltHEADgtltTITLEgt")
• out.println(title)
• out.println("lt/TITLEgtlt/HEADgtlt/HTMLgt")
• out.println("ltH1gt" title "lt/H1gt")
• out.println("ltPgtHello World!")
• out.println("lt/BODYgtlt/HTMLgt")
• out.close()

23
Compiling the Servlet

• Make sure the servlet classes are in your
  classpath
• In Tomcat, you use servlet-api.jar in the
  common\lib folder
• Set it into the classpath environment variable or
  use classpath option while compiling
• Compile the servlet using the following command
• java d . XYZServlet.java
• -d option specifies the destination folder

24
Using invoker to Run a Servlet

• By default, Tomcat does not enable the invoker
  servlet
• It is not enable in default because it is
  insecure. A person can force your container to
  load a class that may harm your server
• You need to uncomment some parts in the
  configuration file web.xml in the conf folder in
  order to use invoker
• Uncomment the servlet declaration starts
  withltservletgt ltservlet-namegt invoker
  lt/servlet-namegt
• Uncomment the servlet mapping for the
  invokerltservlet-mappinggtltservlet-namegtinvokerlt/s
  ervlet-namegt
• Place the compiled servlet class file to a path
  defined
• The Servlet can then be invoked through the URL
• http//localhost8080/servlet/XYZServlet

25
doGet and doPost in HttpServlet

• HttpServlet is specially designed with request
  and response object geared towards HTTP
• HttpServlet provides separate methods for
  handling different type of HTTP requests
• doGet(HttpRequest request, HttpResponse response)
  
• doPost(HttpRequest request, HttpResponse
  response)
• Servlet can send back a response of all types
  (even JPG is possible)
• Set the content type before calling getWriter
• Usually, we use PrintWriter for the output of the
  servlet

26
Deploy a Servlet through a Web Application

• One safer way to invoke Servlet is to pack it
  with related classes, JSPs, JARS and other files
  in a single folder and deploy it into the
  container with a deployment descriptor
• The deployment can be done manually or using
  Tomat's administration tool
• The whole process is called a Web Application
  Deployment
• A web application can also be wrapped inside a
  WAR file.

27
JSP and Servlet API Specification

• You can browse the following pages to explore the
  detail API of both JSP and servlet
• Servlet API JavaDoc http//tomcat.apache.org/tomc
  at-5.5-doc/servletapi/index.html
• JSP API JavaDoc http//tomcat.apache.org/tomcat-5
  .5-doc/jspapi/index.html

28
JSP Lifecycle

• Phases of a JSP lifecycle
• Compilation
• Loading
• Initialization
• Execution
• Cleanup

29
JSP Lifecycle - Compilation

• When the browser asks the Web server for a JSP,
  the Web server passes control to a JSP container
  (or a servlet container)
• The container provides the runtime environment
  and other services the JSP needed
• At the first time a JSP is invoked, the JSP
  container will convert it into a servlet as an
  executable unit
• The container will normally check if a servlet
  for that JSP already exists and updated
• The compilation process involves first parsing
  the JSP, turning the JSP into a servlet and then
  compiling the servlet

30
JSP Lifecycle - Loading

• A JSP/servlet can be loaded at three different
  times
• when the server starts up
• when the system admin asks the server to load the
  servlet
• when a browser tries to access the servlet
• You may need to make a change to web.xml in the
  conf folder of Tomcat to make the servlet load at
  start up
• In case of a JSP, when the JSP file is request,
  it will automatically map the resource part of
  the URL to the compiled servlet stored in the
  servlet container

31
JSP Lifecycle - Initialization

• To perform JSP-specific initialization, you need
  to override the jspInit method
• public void jspInit()
• For example, you can initialize database
  connection, open files and etc. in the jspInit
  method
• You may also use the implict config and
  application object to configure your JSP

32
JSP Lifecycle Initialization

• Another method to initialize the JSP file in a
  deployment descriptor
• You can add a "servlet" descriptor for your JSP
  in your deployment descriptor and add your
  init-param elements
• ltservletgt ltservlet-namegt HelloWorldJSP
  lt/servlet-namegt ltjsp-filegt /HellowWorld.jsp
  lt/jsp-filegt ...lt/servletgt

33
JSP Lifecycle Execution Cleanup

• When a browser request a JSP, the JSP invokes the
  jspService method in the servlet representing the
  JSP.
• JSP takes an HttpServletRequest and
  HttpServletResponse as its parameters
• Cleanup is necessary only if the JSP is needed to
  be unloaded from the container
• Override jspDestroy method to perform any
  cleanup, such as releasing database connections
  and close the files public void jspDestroy()

34
The HyperText Transfer Protocol (HTTP)

• HTTP allows the transfering of web contents of
  virtually unlimited types.
• The first version of HTTP, HTTP/0.9, was a simple
  protocol for raw data transfer.
• The most widely used HTTP version is HTTP/1.0
• Since then, an improved protocol, known as
  HTTP/1.1, has been developed and often adopted.
• HTTP/1.1 is a far more extensive protocol than
  HTTP/1.0.
• However, the basics of the protocol is well
  represented in the simpler HTTP/1.0.

35
HTTP Server and Client

• HTTP is a connection-oriented, stateless,
  request-response protocol.
• An HTTP server, or web server, runs on TCP port
  80 by default. (Tomcat uses port 8080 by default)
• HTTP clients (web browsers) are processes which
  implements HTTP to interacts with a web server to
  retrieve documents in HTML.

36
HTTP Request and Response

• HTTP is text-based the request and responses are
  character strings
• Each request and response is composed of these
  parts, in order
• The request/response line
• A header section
• A blank line
• The body

37
The HTTP request

• A client request is sent to the server after the
  client has established a connection to the
  server.
• A request line is of the following form
• ltHTTP methodgtltRequest-URIgtltprotocol
  specificationgt\r\n
• where
• ltHTTP methodgt is the name of a method defined for
  the protocol,
• ltRequest-URIgt is the URI of a web document, or,
  more generally, a web object,
• ltprotocol specificationgt is a specification of
  the protocol observed by the client, and
• An example client request is as follows
• GET /index.html HTTP/1.0

38
HTTP Methods in a client request

• The HTTP method in a client request is a reserved
  word (in uppercase) which specifies an operation
  of the server that the client desires.
• Some of the key client request methods are
  listed below
• GET for retrieving the contents of web object
  referenced by the specified URI
• HEAD for retrieving a header from the server
  only, not the object itself.
• POST used to send data to a process on the
  server host.
• PUT used to request the server to store the
  contents enclosed with the request to the server
  machine in the file location specified by the URI.

39
GET vs. POST

• When you use GET, you are taking advantage of a
  performance hack
• Server needs a little more work to process posted
  data
• However, there is a limit to the length of
  pathname for a GET request (4KB)
• If your form contains sensitive data, use POST
  rather than GET
• A more secure way to send a sensitive data is to
  use HTTPS instead of HTTP

40
Request Header

• The request header fields allow the client to
  pass additional information about the request,
  and about the client itself, to the server.
• These fields act as request modifiers, with
  semantics equivalent to the parameters on a
  programming language method (procedure)
  invocation
• A header is composed of one or more lines, each
  line in the form of
• ltkeywordgt ltvaluegt\r\n

41
Request Header

• Some of the keywords and values that may appear
  in a request header are
• Accept content types acceptable by the client
• User-Agent specifies the type of browser
• Connection Keep-Alive can be specified so that
  the server does not immediately close a
  connection after sending a response.
• Host host name of the server
• An example request header is as follows
• Accept /
• Connection Keep-Alive
• Host www.someU.edu
• User-Agent Generic

42
Accessing Request Header

• Using request.getHeader(HEADER-TYPE) in your
  JSP, you can create a different content according
  to the request header
• Example
• if (request.getHeader(USER-AGENT).indexOf( M
  SIE) gt 0)
• // do IE specific stuff here
• else
• // do Netscape specific stuff here

43
Request Body

• A request optionally ends with a request body,
  which contains data that needs to be transferred
  to the server in association with the request.
• For example, if the POST method is specified in
  the request line, then the body contains data to
  be passed to the target process.

44
Example of HTTP Client Request

• Example1
• GET / HTTP/1.1
• ltblank linegt
• Example2
• HEAD / HTTP/1.1
• Accept /
• Connection Keep-Alive
• Host somehost.com
• User-Agent Generic
• ltblank linegt

• Example3
• POST /servlet/myServer.servlet HTTP/1.0
• Accept /
• Connection Keep-Alive
• Host somehost.com
• User-Agent Generic
• ltblank linegt
• Namedonaldemaildonald_at_someU.edu

45
The HTTP Server Response

• In response to a request received from a client,
  the HTTP server sends it a response.
• Like the request, an HTTP response is composed of
  these parts, in order
• The response or status line
• A header section
• A blank line
• The body

46
The Response Status Line

• The status line is in the form of
• ltprotocolgtltspgtltstatus-codegtltspgtltdescriptiongt\r\n
• The status code designations are as follows
• 100-199 Informational
• 200-299 Client request successful
• 300-399 Client request redirected
• 400-499 Client request incomplete
• 500-599 Server errors
• Example 1 HTTP/1.0 200 OK
• Example 2 HTTP/1.1 404 NOT FOUND
•  

47
HTTP Response Header

• The status line is followed by a response header.
  A response header is composed of one or more
  lines, each line in the form of
• ltkeywordgt ltvaluegt\r\n
• There are two types of response header lines
• Response header lines
• Entity header lines

48
Response and Entity Header Lines

• Response header lines these header lines return
  information about the response, the server, and
  further access to the resource requested
• Age seconds
• Location URI
• Retry-After dateseconds
• Server string
• WWW-Authenticate scheme realm
• Entity header lines these header lines contain
  information about the contents of the object
  requested by the client, as follows
• Content-Encoding
• Content-Length
• Content-Type type/subtype
• Expires date
• Last-Modified date
• Cache-Control maxageseconds

49
HTTP Response Header

• An Example response header is as follows
• Date Mon, 30 Oct 2000 185208 GMT
• Server Apache/1.3.9 (Unix) ApacheJServ/1.0
• Last-modified Mon, 17 June 2001 164513 GMT
• Content-Length 1255
• Connection close
• Content-Type text/html
• The Content-Type specifies the type of the data,
  using the contents type designation of the MIME
  protocol.
• The Content-Encoding specifies the encoding
  scheme (such as uuencode or base64) of the data,
  usually for the purpose of data compression.
• The expiration date gives the date/time
  (specified in a format defined with HTTP)after
  which the web object should be considered stale
• The Last-Modifed date specifies the date that the
  object was last modified.

50
HTTP Response Body

• The body of the response follows the header and a
  blank line, and contains the contents of the web
  object requested.
• HTTP/1.1 200 OK
• Date Sat, 15 Sep 2001 065530 GMT
• Server Apache/1.3.9 (Unix) ApacheJServ/1.0
• Last-Modified Mon, 30 Apr 2001 230236 GMT
• ETag "5b381-ec-3aedef0c"
• Accept-Ranges bytes
• Content-Length 236
• Connection close
• Content-Type text/html
• lthtmlgt
• ltheadgt
• lttitlegtMy web page lt/titlegt
• lt/headgt
• ltbodygt
• Hello world!
• lt/BODYgtlt/HTMLgt

51
HTML Forms

• You create forms to send data from the browser to
  the server
• When the form is used with JSP / servlets, your
  JSP can process the data in the form and return
  the result
• HTML forms are created using ltFORMgtlt/FORMgt tag
• Attributes ACTION, METHOD and TARGET
• ACTION defines the URL of the form handler
  which will be a JSP/Servlet here
• METHOD defines the method that uses to send the
  data to server, GET will append the query string
  to the URL but POST allows non-ASCII character
  and query strings without any length restriction

52
Input for the Form

• Using the ltINPUTgt tag, you can create different
  input components by setting the TYPE attribute to
  different value
• TEXT text box
• PASSWORD text box masked with
• HIDDEN input value which are not shown in
  browser
• CHECKBOX a check box
• RADIO radio buttons, group is determined by the
  NAME attribute
• SUBMIT submit button, submit the form
• RESET cancel button, clear the form
• VALUE attribute can be set to give a default value

53
A Simple HTML Form

• lthtmlgt ltbodygt
• lth1gtPlease tell me about yourselflt/h1gt
• ltform action"SimpleFormHandler.jsp"
  method"get"gt
• Name ltinput type"text" name"firstName"gtltinput
  type"text" name"lastName"gt ltbrgt
• Sex ltinput type"radio" checked name"sex"
  value"male"gtMaleltinput type"radio" name"sex"
  value"female"gtFemale
• ltPgt What Java primitive type best describes your
  personality
• ltselect name"javaType"gt
• ltoption value"boolean"gtbooleanlt/optiongt
• ltoption value"byte"gtbytelt/optiongt
• ltoption value"char" selectedgtcharlt/optiongt
• ltoption value"double"gtdoublelt/optiongt
• ltoption value"float"gtfloatlt/optiongt
• ltoption value"int"gtintlt/optiongt
• ltoption value"long"gtlonglt/optiongt
• lt/selectgtltbrgt
• ltinput type"submit"gt lt/formgt
• lt/bodygt lt/htmlgt

54
Simple Form in Browser
55
JSP Handling the Simple Form

• lthtmlgt ltbodygt
• lt // Grab the variables from the form
• String firstName request.getParameter("firstNam
  e")
• String lastName request.getParameter("lastName"
  )
• String sex request.getParameter("sex")
• String javaType request.getParameter("javaType"
  )
• gt
• lt-- Print out the variables --gt
• lth1gtHello, ltfirstNamegt ltlastNamegt!lt/h1gt
• I see that you are ltsexgt.
• You know, you remind me of a ltjavaTypegt
  variable
• I once knew.
• lt/bodygtlt/htmlgt

56
Build-in Object request in a JSP

• request object is an instance of
  HttpServletRequest
• You can use the getParameter method to retrieve
  the values from the form variables
• Example String firstName request.getParameter(
  firstName)
• The variable names are case-sensitive
• If the form variable in a text box is left empty,
  the value "" (empty string) will be assigned
• If the form variable does not exist, the value
  null will be returned
• In fact, the parameters and their values can be
  appended directly to the URL of the JSP

57
Handling Multiple Form Values

• When we have multiple values for the same form
  variable, you can get all values store in a
  String array by calling getParameterValues method
• getParameterValues return null if the form
  variable does not exist
• An array of size 1 will be returned if only one
  value exists for a parameter
• You can also retrieve the names of all available
  parameters using getParameterNames method, which
  will return you an Enumeration object
• hasMoreElement method in an Enumeration object
  can check if there is any element left in the
  enumeration
• nextElement method is used to retrieve the next
  element in the enumeration

58
Form with Multiple Values

• lthtmlgtltbodygt
• lth1gtPlease enter a list of nameslt/h1gt
• ltform action"MultiFormHandler.jsp" method"get"gt
• ltinput type"text" name"names"gtltbrgt
• ltinput type"text" name"names"gtltbrgt
• ltinput type"text" name"names"gtltbrgt
• ltinput type"text" name"names"gtltbrgt
• ltinput type"text" name"names"gtltbrgt
• ltinput type"submit"gt
• lt/formgt
• lt/bodygt lt/htmlgt

59
JSP Handling the Multiple Values

• lthtmlgt ltbodygt
• The names you entered are
• ltpregt
• lt // Fetch the name values
• String names request.getParameterValues("nam
  es")
• for (int i0 i lt names.length i)
• out.println(namesi)
• gt
• lt/pregt
• lt/bodygt lt/htmlgt

60
Getting Parameter Names and Values

• lthtmlgt ltbodygt You passed me the following
  parameters
• ltpregt
• lt // Find out the names of all the parameters
• java.util.Enumeration params
  request.getParameterNames()
• while (params.hasMoreElements())
• // Get the next parameter name
• String paramName (String) params.nextElement()
  
• // Use getParameterValues in case of multiple
  values
• String paramValues request.getParameterVa
  lues(paramName)
• // If there is only one value, print it out
• if (paramValues.length 1)
• out.println(paramName""paramValues0)
• else
• // For multiple values, loop through them
• out.print(paramName"")
• for (int i0 i lt paramValues.length i)
• // If this isn't the first value, //
  print a comma to separate values
• if (i gt 0) out.print(',')
• out.print(paramValuesi)

61
Different Ways to Call a Form

• Three most common arrangements of forms and JSP
  handler
• Static HTML input form calling a JSP form handler
• A JSP input form calling another JSP form handler
• A single JSP page displaying the form and handles
  the input
• You can also mix servlets with JSP in a web
  application, especially when you are developing a
  multi-tier web application

62
Using the Same JSP for Input Form and Form
Handler

• When the first time the user hits the page, you
  can detect that there is no form data to process
  and show the initial input form
• After that, you need a trick to detect the form
  data
• Look for the presence of form variables
• Pass a special variable indicating that this is a
  form submission
• E.g., ltinput typehidden nameisSubmitted
  valueyesgt
• Submit the form using POST method and examine the
  request method
• To examine the request method, you can use the
  followingif (request.getMethod().equals(POST))
  

63
Multiple Threads in JSP

• When you got instance variables declared in
  declaration tags (lt! gt), you might run into
  threading issues
• Typically, only one instance of the servlet (JSP)
  is loaded at one time
• When a browser requests a JSP, the web server
  spawns another thread to handle the request
• Thus, there may be multiple threads executing the
  same servlet
• When the different threads access and modify the
  same instance variable, your client may display
  some data of another client!! (see Address.jsp
  for example)

64
Solving the Threading Issues

• One solution for the problem is to synchronize
  the variable assignments and method call
• synchronized (this)
• firstName request.getParameter(firstName)
• middleName request.getParameter(middleName)
• formatNameAndAddress(out) // call formatting
  routine
• Solve the solution at the cost of performance
• Only one thread can access the data at one time
• Create a bottleneck for the server
• A better method is to pass the data as method
  parameters rather than using the instance
  variables (see AddressGood.jsp)

65
Advantages and Disadvantages of JSPs

• Advantages
• Looks like HTML
• Easy for web page authors and software developers
  to do their jobs "separately"
• JSP will eventually turn to a servlet and
  automatically load into the servlet container
  and automatically update if the JSP is modified
• Disadvantages
• Due to its capability of mixing Java codes with
  HTML and JavaScript, it is very hard to debug if
  there is any error in the page

66
Advantages and Disadvantages of Servlets

• As Servlets are Java classes, you will not have
  Java, HTML and JavaScript mixed up
• But it ends up with many out.print and
  out.println statements
• When you need to deploy a large web application,
  it will be easier to manage the deployment file
  (web.xml) are oriented to servlet
• Thus, as stated before, servlets are usually used
  to deal with business logic (e.g. database
  fetching, doing computation and etc.)
• JSPs are used to render the output.