Wednesday, December 17, 2003

Exceptions (again)

13 Exceptional Exception Handling Techniques notes "Declare Unchecked Exceptions in the Throws Clause" and "Soften Checked Exceptions" (always use RuntimeExceptions). This lead to JDO and its JDO Transaction class that uses runtime exceptions (although it does document them) instead of JDBC's use of checked exceptions. Similarly, the Spring Framework and in Chapter 4 of Expert One-on-One J2EE Design and Development the author discusses the usual reasons given to avoid checked exceptions:
"Checked exceptions are much superior to error return codes...However, I don't recommend using checked exceptions unless callers are likely to be able to handle them. In particular, checked exceptions shouldn't be used to indicate that something went horribly wrong, which the caller can't be expected to handle...Use an unchecked exception if the exception is fatal."

With both JDO and Spring the contract offered by the framework tells the client what they can and cannot handle. In my experience, this is not an either or situation. For example in JDO they use "CanRetryException" and "FatalException" - an exception that can be retried, could actually be fatal depending on the context and vice-versa. This often occurs when large frameworks are used in conjunction with one another - at the system integration level. Preventing the developer the choice, when integrating into larger frameworks, what exceptions can and cannot be caught often leads to unexpected exceptions tunneling through layers.
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