February 2010
Beginner
255 pages
6h 10m
English
Content preview from 97 Things Every Programmer Should Know
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Chapter 21. Distinguish Business Exceptions from Technical
THERE ARE BASICALLY TWO REASONS that things go wrong at runtime: technical problems that prevent us from using the application and business logic that prevents us from misusing the application. Most modern languages, such as LISP, Java, Smalltalk, and C#, use exceptions to signal both these situations. However, the two situations are so different that they should be carefully held apart. It is a potential source of confusion to represent them both using the same exception hierarchy, not to mention the same exception class.
An unresolvable technical problem can occur when there is a programming error. For example, if you try to access element 83 from an array of size 17, then the program is clearly off track, and some exception should result. The subtler version is calling some library code with inappropriate arguments, causing the same situation on the inside of the library.
It would be a mistake to attempt to resolve these situations you caused yourself. Instead, we let the exception bubble up to the highest architectural level and let some general exception-handling mechanism do what it can to ensure that the system is in a safe state, such as rolling back a transaction, logging and alerting administration, and reporting back (politely) to the user.
A variant of this situation is when you are in the “library ...
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