Transcript Tips and Techniques

Tips and Techniques
Tips re: errors
Obviously, the best approach to reducing bugs is to
code to a design and perform testing at incremental
stages.
However when you don’t know what the problem is
then…
Whenever you come across an error in your JSP
application, you need a mental checklist to help you
track down the problem.
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Tips and Techniques
Files and Folders - locations
• Your .htm, .jsp and database files should be placed in the
following folder:
• At University:
\\fcet11.staffs.ac.uk\studentID\
• At Home:
/tomcat/webapps/ROOT/myJSPfolder/
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Tips and Techniques
HTML Checklist
1. At home ensure the server is installed correctly and has been started.
1. At Uni the server should be running automatically but you always test by
running http://fcet11:8080/index.jsp
2. Are you using the correct URL? (should start with http://fcet11:8080 at
uni or http://localhost:8080 at home)
3. Are your .htm or .html files appearing ok?
4. Do any of your HTML forms contain reserved Java keywords as names
(e.g. not String, int etc.)
5. Do any of your HTML forms contain reserved SQL keywords (e.g.
name, description, password etc.)
http://www.xlinesoft.com/asprunnerpro/articles/sql_access_odbc_reserved_keywords.htm
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Tips and Techniques
JSP Checklist
• Remember that using GET you are restricted to (usually less
than) 1024 chars.
• You should use POST – but GET is good for debugging as
you can see it in the url string in the browser – that is the
recommendation for your early use of JSP
• Use the error page – see later in module
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Database Problems
• Remember we are using DSN-less connections – you can use a
DSN at home if you wish to – available through the ODBC utility
in control panel
• Your connection object must have the following structure:
• At University:
Connection conn =
DriverManager.getConnection("jdbc:odbc:Driver={Microsoft Access
Driver (*.mdb,*.accdb)};DBQ=C:/Program Files/Apache Software
Foundation/Tomcat 7.0/webapps/studentid/databasefilename");
However it is best to use the provided helper DBConnection.JSP which has
the line:
String sMyDatabase = getServletContext().getRealPath(sDatapath);
to work out the path dynamically.
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Database Problems
• At Home:
Connection conn =
DriverManager.getConnection("jdbc:odbc:Driver={Microsoft Access
Driver
(*.mdb,*.accdb)};DBQ=C:/tomcat/webapps/ROOT/studentid/databasefil
ename;");
OR, you can use the IP address of your machine (127.0.1.2 in this
example)
Connection conn =
DriverManager.getConnection("jdbc:odbc:Driver={Microsoft Access
Driver (*.mdb)};DBQ=//127.0.1.2/databasefilename;");
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Database Problems
•
•
Be very careful with your SQL statements!
Remember that you cannot put a record into a database that has the
same primary key value as another record – you will get a nasty
message
– We can gracefully handle this eventually
•
You only put apostrophes around values that are string-based and
your database field is expecting Text values. E.g.
INSERT INTO MyTable (4157, ‘Consummated Ferrets’, ‘B157’);
Note the first field in the table is numeric
INSERT INTO MyTable (‘4157’, ‘Consummated Ferrets’, ‘B157’); // error!
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Database Problems
•
•
•
Be very careful with your SQL statements!
Date values have a pair of # around them, but avoid using DATE types.
If you have a problem with currency fields, use number, set it’s field
size to Decimal and set the format to Currency, then set the number of
decimal places and scale to 2 – see next slide
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Tips and Techniques
Autosequence datatype
• The autosequence column name does not need to be
named in your insert sql statement when writing to a
database.
• We recommend you be careful if you use
autosequence fields – they can be more trouble than
they are worth
– Can be nasty when get out of sequence
– Your mileage may vary – some people do not experience
many problems with autonumbering keyfields
– For example, Nic Shulver always uses them, without issues
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Tips and Techniques
Try…Catch
• In Java (and JSP) we can use a try…catch block
around any piece of code that may cause an
exception.
<%
try
{
// Code which can throw can exception
}
catch(Exception e) // only catch it if the code fails above
{
// Exception handler code here
}
%>
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Exceptions
• For very practical reasons, Java enforces the use of
try…catch blocks around any piece of code that can
cause an exception to be thrown.
• By ‘thrown’, it is meant that the exception has
occurred.
• When an exception is thrown, one of several things
can happen depending on what you want your web
application to do at that point.
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Exception Handling
1) Do nothing… let your program fall over and read the
error message that Java produces on the server.
2) You could handle the exception locally (i.e. in your
code at the point where the exception occurred)
within your catch block.
3) Or, you could redirect the user to an error page and
do something there.
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Tips and Techniques
Form.htm
<html>
<head></head>
<body>
<form action="FormHandler.jsp" method="post">
Enter your age ( in years ) : <input type="text" name="age" />
<input type="submit" value="Submit" />
</form>
</body>
</html>
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FormHandler.jsp
<html>
<head></head>
<body>
<%
int age;
age = Integer.parseInt(request.getParameter("age"));
%>
<p>Your age is : <%= age %> years.</p>
<p><a href="Form.htm">Back</a>.</p>
</body>
</html>
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Tips and Techniques
But……..
• This code works fine until a user enters something
other than an integer via the form.
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Simple Fix - Local Try…Catch
<%
int age;
try
{age = Integer.parseInt(request.getParameter("age"));
out.print("Your age is : " + age + "years. <br>");
}
catch(NumberFormatException e)
{out.print("You must enter a number! <br>");
}
%>
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Tips and Techniques
User-Defined Error Page
<%@ page errorPage="ExceptionHandler.jsp" %>
<html>
<head></head>
<body>
<%
int age;
age = Integer.parseInt(request.getParameter("age"));
%>
<p>Your age is : <%=age %> years.</p>
<p><a href="Form.html">Back</a>.</p>
</body>
</html>
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User-Defined Error Page
<%@ page isErrorPage="true" import="java.io.*" %>
<html><head></head>
<body>
<font color="red"><%= exception.toString() %><br></font>
<%
out.println("<!--");
StringWriter sw = new StringWriter();
PrintWriter pw = new PrintWriter(sw);
exception.printStackTrace(pw);
out.print(sw);
sw.close();
pw.close();
out.println("-->");
%>
</body>
</html>
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Ok, Good, Better!
• This works well but we can do better!
• Currently, the error message that is displayed is a
standard Java message.
• These can be difficult to understand so instead we’ll
pass our own message to our error page for it to
display…
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Tips and Techniques
Combined Version
<%
int age;
try
{
age = Integer.parseInt(request.getParameter("age"));
}
catch (NumberFormatException e)
{
throw new JspException("Please enter a valid integer value!");
}
%>
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Combined Version
• This time we catch the NumberFormatException
locally and throw a new JspException with our own
exception message.
• JspException is a JSP special exception class which
extends java.lang.Exception.
• We need to change the error page code to this:
<font color="red">
<%= exception.getMessage() %>
<br>
</font>
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Recap
• Error investigation does improve with experience
• JSP errors at run time and can be a combination of
<% or } problems
• Validation to catch errors from say user input can be
improved by the use of exception JSPs
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