Custom Exceptions: Differentiate via many subclasses or single class backed with enum?

Answer 1

The chief disadvantage to Option 2 (a common exception + enum) is that you lose some of the utility of checked exceptions. A method pretty much has to say simply "something framework-related might go wrong":

public void foo()
throws MyFrameworkException

...rather than "x or y might go wrong":

public void foo()
throws SomethingWentWrongException, SomethingElseWentWrongException

It means a function that may need to handle one framework exception has to be prepared to handle any of them, whereas if you're specific, a function need only be prepared to handle the exceptions that are thrown by the framework methods it calls.

So for me, a hierarchy such as Option 1 (it needn't be quite so flat, if a structure suggests itself) is the way to go. That said, there are people who don't like checked exceptions at all, and for them I suspect the above is not a compelling argument. :-)

Edit And picking up duffymo's point: I've assumed you were talking about exceptions you really did have to create. Absolutely throw standard exceptions wherever it makes sense (which is nearly everywhere). Don't create your own MyFrameworkIllegalArgumentException, for instance, just use IllegalArgumentException (or its various subclasses).

Answer 2

I would go with exceptions extending java.lang.RuntimeException with descriptive class names.

If you have so many business-specific exceptions that this becomes oppressive, it means you're probably doing it wrong.

See Joshua Bloch's advice about favoring standard exceptions.

Answer 3

I wouldn't inherit from a generic MyFrameworkException, as long as you dont provide any functionality in there common to all projects. Else, always extend Exception.

Normally you should throw meaningful exceptions at domain/layer boundaries. So regarding your question, you should go with option 1, having in mind the point above. Option 2 would increase code complexity, as you will always have to check for the exception type to determine what went wrong. Let the exception class speak for itself.