While the best efforts are made to stop errors from happening, they do happen. Sometimes the errors might be due to some bad code; other times it may be due to external resources that are out of one's control. During the development process, these errors hold valuable information that allows us to improve and fix our code, but in production environments, these errors hold information that can make our servers vulnerable to attack. It is important that as a developer you anticipate errors and gracefully handle them. Thankfully, ColdFusion offers a number of ways to trap those errors and even allow developers to react to those errors and call alternative functionality.
learncfinaweek
learncfinaweek
/ gist:4121405
Created
November 20, 2012 21:46
Error Handling and Debugging - Error Handling
By now you've learned the basics of ColdFusion, script vs. tag syntax, scopes, how to deal with data, and even some code-reuse techniques. You're now able to write something useful, so it's time we introduce you to the Request Lifecycle.
You see, when someone requests a ColdFusion page, CF doesn't just start executing your code. There are several events that first take place, which you can be waiting for, and to which you can react. This is not strictly necessary, but you'll find that any complex application will eventually want to make use of some or all of these features, so it's best that you know about them. In order to react to these events, you need to have an Application.cfc file. ColdFusion has designated Application.cfc as a special component that it will automatically look for, and in which we can put our event listeners for request lifecycle events.
A Note on Terminolog
Programming is all about doing stuff to things. Generally, the stuff is the execution of a process or algorithm and the things are information or data.
An algorithm is just a fancy name for a series of steps, like tying your shoelaces. Information or data is just values. Your name is a piece of information and so is your birth date. Since it's shorter, in this chapter we'll use the word data to describe a piece of information.
In ColdFusion, data is held in variables. Think of a variable like a mailbox. You can stuff things like letters and packages into a mailbox and get them out later to do stuff. ColdFusion makes storing information very easy because it's a loosely typed Language. You can stick any kind of data into a ColdFusion variable without having to tell ColdFusion what kind of data it is.
As you write more and larger ColdFusion applications, you will start looking for ways to improve the performance of your applications. There are many ways to do this, but perhaps the easiest is to use ColdFusion's caching mechanisms to reduce the amount of work your application has to do over and over. Caching simply refers to the idea that you create a piece of content or data once and hold it in application memory for some period of time. During that time frame, any part of your application that needs that content or data uses the copy that was previously generated rather than regenerating it.
ColdFusion has several different caching mechanisms built in, but they generally fall into two main categories--programmatic caching and application server caching.
The ARTISTS table in the cfartgallery datasource used for examples is an excellent example of how NOT to store passwords. First, they are stored in clear text and the column is limited to only 8 characters.
| ARTISTID |
|---|
Dealing with XML in ColdFusion is fairly straightforward. ColdFusion provides many built-in functions for doing all sorts of XML based operations. Some of those functions provide streamlined access to create, read, and search an xml document. You can also validate XML data against a DTD or schema and transform XML using XSLT.
Looping is employed by programming languages to repeat a behavior defined by a section of code either while a condition is maintained true (conditional looping), or a certain number of times (iterative looping). As with other programming languages, ColdFusion allows you to perform both iterative and conditional looping.
ColdFusion uses the cfloop tag to enable looping in tag-based syntax. cfloop permits you to loop over a variable such as a structure or array, loop while a condition remains true, or loop for a given iteration. ColdFusion permits looping in cfscript using for loops and do while loops.
cfdocument will take your combination of CFML and HTML and convert it to a PDF. At its simplest, you can stick some text between the opening and closing tags of cfdocument (there is currently no built-in cfdocument script equivalent) and it will render a PDF to the screen. Here is some sample code:
<cfdocument format="PDF">
ColdFusion groups variables together in scopes, or a range in which a variable can be accessed. Think of a scope as a bucket of memory that can store variables. Each scope has its own purpose, and each scope has its own lifecycle.
The following table shows major scopes available in a running ColdFusion application:
- Variables: Default scope available in ColdFusion templates. Variables are available only during the execution of the template.