User Guide

Handling runtime exceptions with ColdFusion tags 333
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 pseudocode 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.