Exception Handling Strategies

Adoption of an exception handling strategy is an essential part of the JADE object‑oriented design process. For the best results, your designer or project leader needs to define and communicate a strategy to your development team before implementation begins, not as an add‑on later in the project.

Major aspects of the strategy should address the following objectives.

• The overall strategy should be one of Optimistic Error Handling; that is, correct system behavior is assumed. Errors are dealt with separately, as exceptions.

  This approach enhances system efficiency, because checking for errors (for example, via method return codes) is greatly reduced or eliminated. Error handling code is kept separate from normal application logic, which simplifies maintenance.

• For editing, error detection code should be encapsulated within the class whose data is being edited; that is, classes should be responsible for precondition checks (that is, verifying inputs from clients).

  When errors are detected, the class method should raise an exception. For example, editing code for a persistent or model class should reside in that class, not in the client GUI or other client location. Not only does this ensure the integrity of the data in the persistent class, but the code needs only to exist in one place.

  This provides another benefit, where multiple developers work on the same project. The reduced coupling of application classes arising from the use of exceptions allows fewer dependent, parallel development activities to take place. For example, when the persistent database classes have been designed and their method signatures defined, the implementation of those classes can be worked on reasonably independently of the GUI classes, for example.

• Define application exception classes, if necessary, as subclasses of NormalException.

  If you want to add additional information to the exception objects that you raise, you need your own exception subclasses and attendant properties. It is also useful to define your own exception classes so that your exception arming method (using the on <Exception Class Name> syntax) can specify a specific type of exception that is to be directed to your handler, rather than all exceptions.

• Identify which application exceptions are resumable, continuable, or both, and set these properties appropriately when raising exceptions.

  When you instantiate NormalException or a subclass of it, the properties resumable and continuable are both false, by default. If you want to allow the client's exception handler to return Ex_Resume_Next or Ex_Continue after handling your exception, you must set these properties accordingly.

  If your exception handler attempts to continue a non‑continuable exception or to resume a non‑resumable exception, JADE raises a 1238 exception (Invalid exception handler return code). For more details, see the Ex_Continue return value, earlier in this document.