Exceptions: When Things Go Wrong

Log the Error and Resume

Of course, the advice to stop the program assumes that the program can be terminated reasonably. Some programs—such as programs that monitor the space shuttle or programs that control a nuclear magnetic resonance (NMR) machine—cannot (and should not) be terminated because of such an error.

Such programs are called failsafe because they are designed to run without termination. For these programs, the exception should be reported in whatever manner is most appropriate, but the program should continue running. If the exceptional condition invalidates the program’s computations, then the exception handler should make it clear that the results are tainted.

Other programs—such as programs that analyze a large transaction database—should be designed to continue processing after catching such errors. For example, suppose a large airline runs a program once a day to analyze the ticketing transactions that took place. This kind of program might use exceptions to identify erroneous transactions or transactions that involve invalid data of some sort. Because there are bound to be many errors of this kind in the database, it is not reasonable to stop the program. This kind of program shouldn’t stop until it has finished processing all of the transactions. An appropriate action for this kind of program is to log the exceptions into some kind of file and continue processing the transactions.

Suppose a divide-by-zero error happened in one of these programs. In that case, you would override Java’s default exception handling to ensure that the program is not terminated. More generally, it’s important that these types of programs be designed to catch and report such exceptions. This type of exception handling should be built right into the program’s design.