Try/Catch DU Exception

[Olexandr Rotan]

I do not agree with Attila on this one. I think there is nothing "very
smart" about compiler knowing can some variable be possibly/for sure
uninitialized, especially in distant advents of non-null types from
valhalla. I would love to see this feature in language. Moreover, this new
rule may be event more loose if talking about checked exceptions: variable
could possibly be initialized if and only if it's initialization at least
in one execution branch happens before last call to method that throws
checked exception in catch block, i.e.

// assuming "created" is a final variable
try {
created = iThrowIOException();
// blah blah blah that does not throw IOException
} catch (IOEception _) {
created = false;
}

In this example, the last possible operation to throw IOException happens
before variable initialization, so variable is definitely unassigned.

Also note that this does not only apply to final local variables, but also
to final fields assignment in constructors or static blocks.

I see this smartening of the compiler as a good step ahead in direction of
making language more friendly and simultaneously flexible, without
introducing any complex rules.

PS: I am currently unable to test provided code myself, but if  ode would
compile fine without assignment in catch block, this is a serious
vulnerability because compiler let's through possibly uninitialized final
variable.

On Fri, Jul 12, 2024, 23:44 Attila Kelemen <attila.kelemen85 at gmail.com>
wrote:

> I personally wouldn't like too much if the compiler tried to be very smart
> with this. The current behavior is very easy to reason about, and this
> proposal looks too special to me. Especially because then it raises more
> questions about `Thread.stop`.
>
> Also, the way this issue is presented is not much of a concern (at least
> for the example code). Not making the local variable final is kinda
> whatever. Where this is annoying is when you want to capture the variable
> (mostly in a lambda). However, if that is the issue that you are trying to
> address, then I think this is the wrong solution to that issue. I think the
> real solution to the issue would be relaxing the constraint to be able to
> capture a local variable: We should be able to capture it, iff the variable
> is definitely assigned before capturing, and there is definitely no
> assignment after it.
>
> Archie Cobbs <archie.cobbs at gmail.com> ezt írta (időpont: 2024. júl. 12.,
> P, 22:24):
>
>> I'd like to propose a very minor language improvement and would
>> appreciate any feedback.
>>
>> This is a true corner case, but I bet most developers have tripped over
>> it a few times. It's easy to work around... but still...
>>
>> Here's a simple example:
>>
>>     void describeMyThread(Thread thread) {
>>         final String description;
>>         try {
>>             description = '"' + thread.getName() + "'";
>>         } catch (NullPointerException e) {
>>             description = "null";
>>         }
>>         System.out.println("the thread is " + description);
>>     }
>>
>> This doesn't compile:
>>
>>     DUTest.java:8: error: variable description might already have been
>> assigned
>>                 description = "(null)";
>>                 ^
>>
>> The error is a false positive: there is no way an exception can be thrown
>> in the try block *after* description is assigned, because description
>> being assigned is literally the last thing that occurs in the try block.
>>
>> Developers intuitively know that description will be DU at the start of
>> the catch block, so the error feels surprising and makes the compiler seem
>> less smart than it should be.
>>
>> My proposal is to fix this by adding a "try/catch DU exception" to
>> §16.2.15:
>>
>> V is definitely unassigned before a catch block iff all of the following
>> are true:
>>     V is definitely unassigned after the try block *or the try block
>> ends with an assignment expression statement V=b and V is definitely
>> unassigned after b*
>>     V is definitely unassigned before every return statement ...
>>
>> A prototype compiler implementation is here
>> <https://github.com/openjdk/jdk/compare/master...archiecobbs:jdk:TryCatchDUException>
>> .
>>
>> Thoughts?
>>
>> -Archie
>>
>> --
>> Archie L. Cobbs
>>
>
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