| Re: [aspectj-dev] Can aspects throw checked exceptions which are declared on the join point but not referenced on the point cut? |
Probably worth raising an enhancement request to persue this,
otherwise it'll get forgotten on the mailing list...
Andy.
2008/7/3 Ramnivas Laddad <ramnivas@xxxxxxxxxxxxxxx>:
> John,
>
> I definitely agree on the need for something like this. I encountered it
> quite a few times. In fact, AspectJ in Action has a pattern devoted to this
> problem -- Exception Introduction Pattern. I always felt that a proper
> support in AspectJ will be really great.
>
> From the language perspective, some variation of the "rethrow" keyword might
> be appropriate here. Something along the following lines (and here is
> another use case)...
>
> Object around() : transactional() {
> try {
> begin();
> Object ret = proceed();
> commit();
> return ret;
> } catch (Exception ex) {
> rollback();
> rethrow ex; // or rethrow(ex);
> }
> }
>
> In some ways rethrow acts like proceed() -- only in the opposite direction
> of the call stack. Also there is a symmetry between return and rethrow--one
> deals with the return value, other with the thrown exception.
>
> The behavior of rethrow is almost like what you are suggesting, except I
> think rethrow should just throw a runtime exception (say
> IncomaptibleRethrownException) if the exception being rethrown is not
> compatible--since we can assume that correct programming is the common case
> so we can simplify it. The "re" part of rethrow should give sufficient hint
> to the programmer that it is his responsibility to match the expected
> semantics. Of course, the IncomaptibleRethrownException may be caught if the
> return object style is needed.
>
> -Ramnivas
>
> On Mon, Jun 30, 2008 at 5:10 PM, John Watts <john.watts@xxxxxxxxx> wrote:
>>
>> A similar question has been asked many times, but this one is a slightly
>> more limited case which I think could and should be supported.
>>
>> First, what is the use case? Function objects. My function object has a
>> run() method which throws Exception declared on an API I don't control. I
>> want to wrap it in around() advice. By the time any exception thrown from
>> proceed() gets back to my advice, it is typed as a generic Exception. I can
>> determine its type and compare it to the join point but I have no way to let
>> AspectJ know that it is safe to throw.
>>
>> Of course any checked exception thrown from within an aspect must be
>> compatible with the checked exceptions declared for the current join point.
>> However, AspectJ's limitation is currently more restrictive. It requires
>> that any exception thrown from within an aspect must be compatible with the
>> checked exceptions declared for the point cut.
>>
>> If I want to know whether an exception is compatible with the throws
>> clause of the join point, I can use the following code in my advice:
>>
>> boolean isExceptionAllowed(Exception e) {
>> CodeSignature cs = (CodeSignature)
>> thisJoinPointStaticPart.getSignature();
>> Class[] exceptionTypes = cs.getExceptionTypes();
>> for (Class<?> exceptionType : exceptionTypes) {
>> if (exceptionType.isInstance(e)) {
>> return true;
>> }
>> }
>> return false;
>> }
>>
>> After getting true from that method, I know that I should be allowed to
>> throw the exception. However I still can't throw it without casting it to a
>> type which is declared on the point cut. I might not know this type. Or
>> there might be a large number. Or they might be different for different join
>> points (all specified by the same point cut).
>>
>> On the other hand, I could imagine AspectJ providing a function like the
>> following:
>>
>> Exception throwIfAllowed(Exception e) {
>> if (isExceptionAllowed(e)) {
>> throw e;
>> }
>> else {
>> return e;
>> }
>> }
>>
>> Of course this function would have to be declared to throw Exception
>> (defeating the purpose) if I wrote it. But if both functions were declared
>> by AspectJ (in bytecode where necessary) I could safely call them. I could
>> have fallback handling for any exceptions which are not allowed to be
>> thrown.
>>
>> Thanks,
>> John
>> _______________________________________________
>> aspectj-dev mailing list
>> aspectj-dev@xxxxxxxxxxx
>> https://dev.eclipse.org/mailman/listinfo/aspectj-dev
>>
>
>
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