Try/Catch DU Exception

[Tagir Valeev]

Hello!

On Fri, Jul 12, 2024 at 10:44 PM 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`.
>

Thread.stop is not an issue anymore, as it's dysfunctional since Java 18:
https://bugs.openjdk.org/browse/JDK-8277861
It still could be possible to generate an exception in the thread using JVM
TI though. On the other hand, you can update final local variable using JVM
TI as well, so it should not be a problem.


> 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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