The simple solution is to use the <CFERROR> tag, which lets you replace the standard error screens with ones of your own. For example:

<!--- Specify error page --->
<CFERROR TYPE="request"
TEMPLATE="/error/standard.cfm">

However, CFML may not be used within <CFERROR> files. After all, if an error condition has occurred, allowing the execution of CFML could in turn generate an additional error, causing the code to execute again, generating yet another error causing...you get the idea. So no CFML tags or functions. Instead, a special set of variables is made available to you within error pages, and you can place them in your code to refer to things like:

• Date and time
• Browser
• Client IP address
• Template name
• Referring page

<CFERROR> can also be used to define other error screens, for example, the screen displayed when form validation fails.

Note: <CFERROR> needs to be executed on each request, so the ideal location for this tag is the Application.cfm page.

Understanding Exception Handling
<CFERROR> and error handling are very useful in presenting a better and more professional end-user experience, but <CFERROR> is not enough. After all, displaying a prettier error screen is wonderful but:

• When an error occurs you still won't know about it.
• If an error occurs, it will likely keep recurring; with <CFERROR> you can't fix or handle the situation.
• When an error occurs, processing stops; you can't gracefully handle the error and keep going.

That's where exception handling comes into play, with an emphasis on handling. Exception handling is a mechanism by which errors (or other conditions) may be trapped and then responded to. Using exception-handling error conditions doesn't halt processing, instead processing is diverted to special blocks of code that you can write - code that can do just about anything you need.

Try and Catch
Exception handling in ColdFusion is implemented using a try/catch interface, implemented (as you'd expect) using tags named <CFTRY> and <CFCATCH>. The basic flow is like this:

start code block
CFML code goes here
error handling block
error handling code goes here
end error handling block
end code block

The CFML code looks like this:

<CFTRY>
... CFML code ...
<CFCATCH>
... CFML code ...
</CFCATCH>
</CFTRY>

For example, if you want to send an e-mail to an administrator when an exception is thrown, you could do something like this:

<CFTRY>
... CFML code ...
<CFCATCH>
<CFMAIL TO="admin@domain.com"
FROM=" admin@domain.com"
SUBJECT="Error on #GetCurrentTemplatePath()#">
TYPE:
#CFCATCH.type#

MESSAGE:
#CFCATCH.message#
</CFMAIL>
</CFCATCH>
</CFTRY>

Handling Specific Exceptions
As you can see, trapping and handling exceptions is not complicated at all. But what if you want to handle different types of exceptions in different ways? That's doable too - all you need is multiple <CFCATCH> blocks, one for each type of exception.

Look at the following code snippet:

<CFTRY>
...
<CFCATCH TYPE="missinginclude">
...
</CFCATCH>
<CFCATCH TYPE="database">
...
</CFCATCH>
<CFCATCH TYPE="lock">
...
</CFCATCH>
<CFCATCH TYPE="any">
...
</CFCATCH>
</CFTRY>

Other exception types are supported, including over 50 advanced types (used to catch very specific exceptions, like <CFPOP> authentication failures) and custom types used to define exception types of your own.

Using Exception Handling
We've only just scratched the surface here; there's lots more to exception handling and what you can do using this functionality. Here are some things to consider:

And that's just the start of it.

Summary
At a minimum, every developer should be using <CFERROR> to override the default error screens with ones that better match the look and feel of the application. However, <CFERROR> is just the beginning. Exception handling is not difficult to implement and should be used to create better applications - better for you and better for your users.

Neo, which I'll be covering extensively starting next month, extends ColdFusion's try/catch handling even further. If you have yet to play with these tags, there couldn't be a better time to start.