FT228/3 Web Development

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FT228/3 Web Development
Error processing
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Introduction

• READ Chapter 9 of Java Server Pages from Oreilly
  2nd Edition
• Need to be able to
• 1) Diagnose and fix syntax errors
• 2) Deal with runtime errors

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Some Tips on errors and debugging in JSP

• Typical errors are
• - wrong or missing brackets (e.g. for EL)
• - Tags not closed either with a closing tag
  (e.g. ltcifgt.lt/cifgt
• OR if an empty element ltcout . /gt
• 3. taglib directive missing ? tags wont be
  recognised
• incorrectly used when setting values of an
  action

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Some Tips on errors and debugging in JSP

• Compilation errors generated by Apache Tomcat
  vary in helpfulness, depending on the error
• If location of error is given e.g..
  /ch9/error.jsp (0, 33) unterminated tag lt_at_
• The first index (0) refers to the line number in
  the JSP page, 0 is the first line number. The
  second index 33 refers to the position within
  the line. Not all errors are so meaningfulTo
  debug a JSP, can use ltcoutgt actions to output
  values if you need to see the values of
  variables

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Runtime errors

• In java have used try/catch blocks to trap
  errors gracefully
• Two techniques are useful in JSP
• (1) Catch exceptions (similar to java).
• (2) Using error pages - your application will
  default to an error page specified by you, if an
  uncaught exception is encountered

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Runtime errors catching exceptions ltccatchgt

• JSTL provides a ltccatchgt action to catch
  exceptions that occur when a JSP page is run
• Similar to java .. but no try needed ?
  ltccatchgt is placed around the code that is
  likely to cause the error
• The ltccatchgt tag provides an attribute var
  which you must name in order to hold the error
  message.
• Example
• ltccatch var "errormsg"gt
• ltcout value "The result of your calculation
  is param.num1/param.num2"/gt
• lt/ccatchgt

Error msg put in here if error thrown
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Runtime errors catching exceptions ltccatchgt

• To then test if an error has occurred just check
  the contents of your error message variable
• Example
• ltccatch var "error"gt
• ltcout value "The result of your calculation
  is param.num1/param.num2"/gt
• lt/ccatchgt
• ltcif test "error!null"gt
• ltcout value "You have an error
  error"/gt
• lt/cifgt

Checks the error variable to see if an error
occurred
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JSP page using ccatch
Enter two numbers and click divide to get
result of number 1 / number 2
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JSP page using ccatch
lt_at_ page contentType "text/html" gt lt_at_ taglib
prefix "c" uri"http//java.sun.com/jsp/jstl/cor
e" gt lthtmlgt ltbodygt ltform method "post"
action "catch_example.jsp"gt Please enter
numbers ltbrgt ltinput type hidden name
submitted value "true"/gt Number 1 ltInput
type text Name "num1" gtltbrgt Number 2
ltInput type text Name "num2"gtltbrgt
-? continued overleaf
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JSP page using ccatch
ltinput type "submit" value
"divide"gt lt/formgt ltcif test "!empty
param.submitted"gt ltccatch var "error"gt
ltcout value "The result of your
calculation is param.num1/param.num2"/gt
lt/ccatchgt ltcif test
"error!null"gt ltcout value "You have
an error error"/gt lt/cifgt lt/cifgt
lt/bodygt lt/htmlgt
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JSP page using ccatch

• ltccatch gt doesnt allow you to specify a
  specific exception just allows errors to be
  handled gracefully within the page
• Prevents an un-userfriendly exception stack trace
  from being displayed to the user

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Error page

• Can specify a default error page for your
  application
• When an unhandled error occurs (i.e. not handled
  by ccatch, this page will be displayed
• Use page directive attribute errorpage to
  specify a default error page that should be
  display each time an unhandled runtime error
  occurs
• lt_at_ page errorPage errorpage.jsp

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Error page

• Must have error page (e.g errorpage.jsp)
  available.
• This error page must declare itself as an error,
  also using page directive

• lt_at_ page iserrorPage true

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Runtime errors

• Using JSTL Runtime errors are less likely. JSTL
  tries to anticipate or handle the error
  gracefully..e.g. if Divide by zero error is
  encountered JSTL will simply print out a result
  as Divide by zeroTo find the runtime errors
  that may occur, test your code!

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Anticipate errors

• Design your code so that errors are anticipated
  and managed before they occur
• E.g. User registration adding a new user. User
  is identified by username which they enter.
  ---- before adding the user, check whether the
  username has already been used before..how?