MACROMEDIA ColdFusion MX, Develop Manual, Page: 309 / 860

Handling runtime exceptions with ColdFusion tags 309
For example, on a form action page or custom tag used to set a password, the application can
determine whether the password entered is a minimum length, or contains both letters and
number, and throw an error with a message that indicates the password rule that was broken. The
cfcatch
block handles the error and tells the user how to correct the problem.
Using the cfrethrow tag
The
cfrethrow
tag lets you create a hierarchy of error handlers. It tells ColdFusion to exit the
current
cfcatch
block and "rethrow" the exception to the next level of error handler. Thus, if an
error handler designed for a specific type of error cannot handle the error, it can rethrow the error
to a more general-purpose error handler. The
cfrethrow
tag can only be used in a
cfcatch
tag
body.
The cfrethrow tag syntax
The following pseudo-code shows how you can use the
cfrethrow
tag to create an error-handling
hierarchy:
<cftry>
<cftry>
Code that might throw a database error
<cfcatch Type="Database">
<cfif Error is of type I can Handle >
Handle it
<cfelse>
<cfrethrow>
</cfif
</cfcatch>
</cftry>
<cfcatch Type="Any">
General Error Handling code
</cfcatch>
</cftry>
Although this example uses a Database error as an example, you can use any
cfcatch
type
attribute in the innermost error type.
Follow these rules when you use the
cfrethrow
tag:
•
Nest
cftry
tags, with one tag for each level of error handling hierarchy. Each level contains the
cfcatch
tags for that level of error granularity.
•
Place the most general error catching code in the outermost
cftry
block.
•
Place the most specific error catching code in the innermost
cftry
block.
•
Place the code that can cause an exception error at the top of the innermost
cftry
block.
•
End each
cfcatch
block except those in the outermost
cftry
block with a
cfrethrow
tag.
Example: using nested tags, cfthrow, and cfrethrow
The following example shows many of the techniques discussed in this chapter, including nested
cftry
blocks and the
cfthrow
and
cfrethrow
tags. The example includes a simple calling page
and a custom tag page:
•
The calling page does little more than call the custom tag with a single attribute, a name to be
looked up in a database. It does show, however, how a calling page can handle an exception
thrown by the custom tag.