Checked exceptions

[Brian Goetz]

There are two questions here:
 - Are checked exceptions, in fact, a failed experiment?
 - If so, is there something that can be done about it?

The first is, as StackOverflow would say, “primarily opinion-based”, and opinions run strong and varied.  (PLEASE DO NOT weigh in with your opinion on this here; that’s not the point, and it wouldn’t be helpful.  All the opinions are on the record thousands of times over.)  There are surely some — even many — who believe strongly that checked exceptions are a failed experiment.  And I don’t want to say these people are wrong — it’s their opinion — but in my discussions with them, they are nearly universally wrong about one thing: *they all assume that everyone agrees with them.*  And that is completely wrong.  

We had a dramatic demonstration of this at an Ask the Architects at Devoxx a few years ago, when the topic came up.  It was clear what the opinion of the asker was (“When are you guys going to get rid of checked exceptions already”), and also clear he thought that this was “settled case law.”  So we did a show of hands, and found (in this particular audience), that about 20% of the audience thought CEs were definitely a failed experiment, and about 40% of the audience was willing to raise their hand to something like “They’re not great, but they’re not the end of the world, and I am pretty much able to get on with my life.”  The moral of this story was: when the only voices you hear vocally are “X sucks”, it’s easy to mistakenly assume the whole world thinks the same, when in fact its just a vocal minority, and there’s a silent majority who doesn’t fully agree but doesn’t want to argue, and would rather just  get on with their work.

We saw something similar, by the way, when we did local variable type inference.  For years, we heard people consistently asking for it, but as soon as we did it, a vocal counter-faction came out of the woodwork to declare that this was the dumbest thing they ever heard.  Again, moral of the story is, don’t assume that your opinions are universally held, *even if you can’t find anyone to take up the opposite position.*  

On the other question, even if we assume that checked exceptions are a failed experiment (and my discussions with developers do not bear this out as being a settled question), and therefore we would do something different if we were starting over, would the cure be worse than the disease to try and change course now.  And, as Andrew states, “it’s hard to see a good way to transition away from them.”  

IMO, I think the problem is not so much with the concept of checked exceptions in general, but with the fact that many libraries — especially those developed early in the game — used them inappropriately, and in ways we wouldn’t were we doing those libraries now.  The good news here is that it’s easier to do new libraries than to change the underpinnings of the programming model, so over time, this problem may resolve itself.  (Though, IOException will likely be a thorn in our side forever.)

The one case where it is indeed painful — and where we would most likely consider doing something — is with the interaction of exceptions and lambdas, which I understand is painful for people.  But, there’s a good reason we haven’t done something about this yet — we don’t know what the right thing is.  We tried exploring a form of exception transparency, which seemed initially promising, but turned out to be flawed.  We then tried another direction, which was promising and looked to be sound, but extremely complex.  We’ve invested a great number of expert-hours in the problem, and we don’t yet have a solution that doesn’t suck, so we haven’t done anything yet.  But there are developments on the horizon that might enable us to take another run at the problem, so I wouldn’t count it out yet.  





> On Oct 15, 2019, at 5:00 AM, Andrew Haley <aph at redhat.com> wrote:
> 
> On 10/14/19 11:45 PM, Ryan Schmitt wrote:
> 
>> I'm curious to know what the current thinking is on checked
>> exceptions. Is there agreement that the current situation is less
>> than ideal? If so, how might they be addressed, and are they even
>> enough of a problem to be worth addressing?
> 
> Opinions are divided, but when you ask experienced OpenJDK developers
> about their least-favourite Java feature, checked exceptions are
> likely to be high on the list. In hindsight, it would have been better
> not to have them, at least in this form.
> 
> However, it's hard to see a good way to transition away from them. I
> suppose we could simply declare exception specifications to be
> optional in some future Java release, but it would be substantial work
> for a lot of people and we'd have to very carefully review the
> compatibility implications.
> 
> -- 
> Andrew Haley  (he/him)
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com>
> https://keybase.io/andrewhaley
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671