suppressedException semantics

[Zhong Yu]

I guess that majority of programmers only care about how the stack trace is
printed, and most of us would love to see the "primary exception" printed
first. After all, it is the first exception that occurred, and should be the
first one to be investigated.

Therefore as long as the "primary exception" is printed first, we are
satisfied.

Programmatically: let say e_a of type EA is an exception thrown in the try
block, and e_b of type EB is an exception thrown by close().

If the exception catcher wants to handle different cases differently, e_a
and e_b must be distinguishable. Either they are of different types, or they
have distinguishable field values ("message" doesn't count!). Seriously,
which JDK class does that? File? Socket? SQL? Usually, we are forced to
catch one exception of one type, not knowing what exactly it is. There is
nothing we can do with it but print its stack trace.

Let's say e_a and e_b are distinguishable. The meticulous programmer tries
to handle 3 possible cases differently. He's aware that both e_a and e_b
could have occurred in serial, he needs a way to pattern match this case,
and extract both exceptions. Since there is no such thing as catch(EA e_a,
EB e_b), our hero has to check the spec, and grudgingly does what's
necessary. Which exception suppressed which, it doesn't matter, both choices
produce ugly code.

Facing all kinds of possible combinations of e_a_1, e_a_2... e_a_m, e_b_1
... e_b_n, instead of catching each individually and further checking the
suppressed list of each, the following generic catch code makes things
easier

catch(Ex e_x) // Ex is a common super type of EA_1 ... EA_m, EB_1 ... EB_n
{
    Ex[] exceptions = flat(e_x);  // e_x and suppressed(if any), ordered
*chronically*
    for(Ex e : exceptions)
        if(e instanceof EA_1)
        ...
}

Here, the code doesn't visually depend on whether e_a or e_b was thrown.
Either way works.

If nobody cares programatically, the argument for spec consistency wins. I'm
in the camp of David.

(or more sinisterly - we could change the semantics of try-finally and
switch which exception is suppressed, to line up with Joshua's ARM; no real
world program will be broken...)


Zhong Yu

On Thu, Aug 26, 2010 at 7:10 PM, Neal Gafter <neal at gafter.com> wrote:

> Brian Goetz wrote this on lambda-dev:
>