Servlets and HTML Form Data

1
Servlets and HTML Form Data
Vijayan Sugumaran Department of DIS Oakland
University Rochester, MI 48309
Parts of this presentation was provided by
www.coreservlets.com
2
Generating HTML

• Set the Content-Type header
• Use response.setContentType
• Output HTML
• Be sure to include the DOCTYPE
• Use an HTML validation service
• http//validator.w3.org/
• http//www.htmlhelp.com/tools/validator/
• If your servlets are behind a firewall, you can
  run them, save the HTML output, and use a file
  upload form to validate.

3
A Servlet That Generates HTML

• public class HelloWWW extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String docType
• "lt!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML
  4.0 "
• "Transitional//EN\"gt\n"
• out.println(docType
• "ltHTMLgt\n"
• "ltHEADgtltTITLEgtHello
  WWWlt/TITLEgtlt/HEADgt\n"
• "ltBODYgt\n"
• "ltH1gtHello WWWlt/H1gt\n"
• "lt/BODYgtlt/HTMLgt")

4
The Servlet Life Cycle

• init
• Executed once when the servlet is first loaded.
  Not called for each request.
• service
• Called in a new thread by server for each
  request. Dispatches to doGet, doPost, etc. Do
  not override this method!
• doGet, doPost
• Handles GET, POST, etc. requests.
• Override these to provide desired behavior.
• destroy
• Called when server deletes servlet instance. Not
  called after each request.

5
Initializing Servlets

• Common in real-life servlets
• E.g., initializing database connection pools.
• Use ServletConfig.getInitParameter to read
  initialization parameters
• Set init parameters in web.xml (ver 2.2/2.3)
• /WEB-INF/web.xml
• Many servers have custom interfaces to create
  web.xml
• It is common to use init even when you dont
  read init parameters
• See modification date example in Core Servlets
  and JavaServer Pages Chapter 2

6
A Servlet That Uses Initialization Parameters

• public class ShowMessage extends HttpServlet
• private String message
• private String defaultMessage "No message."
• private int repeats 1
• public void init() throws ServletException
• ServletConfig config getServletConfig()
• message config.getInitParameter("message")
• if (message null)
• message defaultMessage
• try
• String repeatString
• config.getInitParameter("repeats")
• repeats Integer.parseInt(repeatString)
• catch(NumberFormatException nfe)

7
ShowMessage Servlet (Continued)

• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title "The ShowMessage Servlet"
• out.println(ServletUtilities.headWithTitle(t
  itle)
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
  
• "ltH1 ALIGNCENTERgt" title
  "lt/H1gt")
• for(int i0 iltrepeats i)
• out.println(message "ltBRgt")
• out.println("lt/BODYgtlt/HTMLgt")

8
Setting Init Parameters

• ...\WEB-INF\web.xml
• tomcat_install_dir\webapps\examples\WEB-INF\web.xm
  l
• ltweb-appgt
• ltservletgt
• ltservlet-namegtShowMsglt/servlet-namegt
• ltservlet-classgtShowMessagelt/servlet-classgt
• ltinit-paramgt
• ltparam-namegtmessagelt/param-namegt
• ltparam-valuegtShibbolethlt/param-valuegt
• lt/init-paramgt
• ltinit-paramgt
• ltparam-namegtrepeatslt/param-namegt
• ltparam-valuegt5lt/param-valuegt
• lt/init-paramgt
• lt/servletgt
• lt/web-appgt

9
ShowMessage Result
10
Debugging Servlets

• Use print statements run server on desktop
• Integrated debugger in IDE
• Look at the HTML source
• Return error pages to the client
• Plan ahead for missing or malformed data
• Use the log file
• log("message") or log("message", Throwable)
• Look at the request data separately.
• See EchoServer at www.coreservlets.com
• Look at the response data separately
• See WebClient at www.coreservlets.com
• Stop and restart the server

11
Handling the Client Request Form Data
12
The Role of Form Data

• Example URL at online travel agent
• http//host/path?userMartyHalloriginbwidestl
  ax
• Names come from HTML author values usually come
  from end user
• Parsing form (query) data in traditional CGI
• Read the data one way (QUERY_STRING) for GET
  requests, another way (standard input) for POST
  requests
• Chop pairs at ampersands, then separate parameter
  names (left of the equal signs) from parameter
  values (right of the equal signs)
• URL decode values (e.g., "7E" becomes "")
• Need special cases for omitted values(param1val1
  param2param3val3) and repeated parameters
  (param1val1param2val2param1val3)

13
Creating Form Data HTML Forms

• lt!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
  Transitional//EN"gt
• ltHTMLgt
• ltHEADgtltTITLEgtA Sample Form Using
  GETlt/TITLEgtlt/HEADgt
• ltBODY BGCOLOR"FDF5E6"gt
• ltH2 ALIGN"CENTER"gtA Sample Form Using GETlt/H2gt
• ltFORM ACTION"http//localhost8088/SomeProgram"gt
• ltCENTERgt
• First name
• ltINPUT TYPE"TEXT" NAME"firstName"
  VALUE"Joe"gtltBRgt
• Last name
• ltINPUT TYPE"TEXT" NAME"lastName"
  VALUE"Hacker"gtltPgt
• ltINPUT TYPE"SUBMIT"gt lt!-- Press this to submit
  form --gt
• lt/CENTERgt
• lt/FORMgt
• lt/BODYgtlt/HTMLgt

14
Aside Installing HTML Files

• Tomcat
• install_dir\webapps\examples\Form.html or
• install_dir\webapps\examples\SomeDir\Form.html
• JRun
• install_dir\servers\default\default-app\Form.html
  or
• install_dir\servers\default\default-app\SomeDir\Fo
  rm.html
• URL
• http//localhost/Form.html or
• http//localhost/SomeDir/Form.html

15
HTML Form Initial Result
16
HTML Form Submission Result(Data Sent to
EchoServer)
17
Sending POST Data

• lt!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
  Transitional//EN"gt
• ltHTMLgt
• ltHEADgtltTITLEgtA Sample Form Using
  POSTlt/TITLEgtlt/HEADgt
• ltBODY BGCOLOR"FDF5E6"gt
• ltH2 ALIGN"CENTER"gtA Sample Form Using POSTlt/H2gt
• ltFORM ACTION"http//localhost8088/SomeProgram"
• METHOD"POST"gt
• ltCENTERgt
• First name
• ltINPUT TYPE"TEXT" NAME"firstName"
  VALUE"Joe"gtltBRgt
• Last name
• ltINPUT TYPE"TEXT" NAME"lastName"
  VALUE"Hacker"gtltPgt
• ltINPUT TYPE"SUBMIT"gt
• lt/CENTERgt
• lt/FORMgt
• lt/BODYgtlt/HTMLgt

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Sending POST Data
19
Reading Form Data In Servlets

• request.getParameter("name")
• Returns URL-decoded value of first occurrence of
  name in query string
• Works identically for GET and POST requests
• Returns null if no such parameter is in query
• request.getParameterValues("name")
• Returns an array of the URL-decoded values of all
  occurrences of name in query string
• Returns a one-element array if param not repeated
• Returns null if no such parameter is in query
• request.getParameterNames()
• Returns Enumeration of request params

20
An HTML Form With Three Parameters

• ltFORM ACTION"/servlet/coreservlets.ThreeParams"gt
• First Parameter ltINPUT TYPE"TEXT"
  NAME"param1"gtltBRgt
• Second Parameter ltINPUT TYPE"TEXT"
  NAME"param2"gtltBRgt
• Third Parameter ltINPUT TYPE"TEXT"
  NAME"param3"gtltBRgt
• ltCENTERgtltINPUT TYPE"SUBMIT"gtlt/CENTERgt
• lt/FORMgt

21
Reading the Three Parameters

• public class ThreeParams extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title "Reading Three Request
  Parameters"
• out.println(ServletUtilities.headWithTitle(tit
  le)
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
• "ltH1 ALIGNCENTERgt" title
  "lt/H1gt\n"
• "ltULgt\n"
• " ltLIgtltBgtparam1lt/Bgt "
• request.getParameter("param1")
  "\n"
• " ltLIgtltBgtparam2lt/Bgt "
• request.getParameter("param2")
  "\n"
• " ltLIgtltBgtparam3lt/Bgt "
• request.getParameter("param3")
  "\n"
• "lt/ULgt\n"
• "lt/BODYgtlt/HTMLgt")

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Reading Three ParametersResult
23
Reading All Parameters

• public class ShowParameters extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String title "Reading All Request
  Parameters"
• out.println(ServletUtilities.headWithTitle(tit
  le)
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
• "ltH1 ALIGNCENTERgt" title
  "lt/H1gt\n"
• "ltTABLE BORDER1 ALIGNCENTERgt\n"
  
• "ltTR BGCOLOR\"FFAD00\"gt\n"
• "ltTHgtParameter NameltTHgtParameter
  Value(s)")

24
Reading All Parameters (cont)

• Enumeration paramNames request.getParameterN
  ames()
• while(paramNames.hasMoreElements())
• String paramName (String)paramNames.nextEl
  ement()
• out.print("ltTRgtltTDgt" paramName
  "\nltTDgt")
• String paramValues
• request.getParameterValues(paramName)
• if (paramValues.length 1)
• String paramValue paramValues0
• if (paramValue.length() 0)
• out.println("ltIgtNo Valuelt/Igt")
• else
• out.println(paramValue)

25
Reading All Parameters (cont)

• else
• out.println("ltULgt")
• for(int i0 iltparamValues.length i)
• out.println("ltLIgt" paramValuesi)
• out.println("lt/ULgt")
• out.println("lt/TABLEgt\nlt/BODYgtlt/HTMLgt")
• public void doPost(HttpServletRequest request,
• HttpServletResponse
  response)
• throws ServletException, IOException
• doGet(request, response)

26
Result of ShowParameters Servlet

• Note that order of parameters in Enumeration does
  not match order they appeared in Web page

27
Summary

• Query data comes from HTML forms as URL-encoded
  name/value pairs
• Servlets read data by calling request.getParameter
  ("name")
• Results in value as entered into form, not as
  sent over network. I.e. not URL-encoded.
• Always check for missing or malformed data
• Special case query data that contains special
  HTML characters
• Need to be filtered if query data will be placed
  into resultant HTML page