[9] RFR (XS): 8169000: Define reference reachability more precisely in java.lang.ref package

[Zoltán Majó]

Hi Mandy,


On 11/16/2016 07:11 AM, Mandy Chung wrote:
>> On Nov 15, 2016, at 4:41 AM, Zoltán Majó <zoltan.majo at oracle.com> wrote:
>>
>>
>> I think the sentence in webrev.01
>>
>> (1) "If a registered referenceceases to be strongly reachable itself, then it may never be enqueued."
>>
>> is sufficient. I think we want to make a statement only about what happens when a reference ceases to be strongly reachable (i.e., that in that case we can't make a statement about that reference being enqueued or not).
>>
> When a reference becomes unreachable (not by examining the source code but the actual state of the VM at runtime), it will never be enqueued.

I also like that wording, thank you!

>
>> The sentence you suggested
>>
>> (2) "A Java virtual machine may implement optimization that could affects when objects become unreachable."
>>
>> adds too much ambiguity as it leaves room for speculation about what "optimization" might be.
>>
>> For example, statement (2) can be possibly (mis-)read as the JVM making references unreachable *earlier* than what a programmer may think -- based on the source code.(That is clearly not the case: References are available at least until the latest program point where they are used, otherwise the program would encounter an error.)
>>
>> It's indeed a JVM "optimization" keep references alive *longer* than it seems to be from the source code. But (1) already encapsulates that without referring to JVM optimizations explicitly.
>
> That’s fair.
>
> What I’m looking for is something like this [1]:
>
> Reachability is not determined by the statements in your source code but by the actual state of the virtual machine at runtime.

OK, I see now -- thank you for explaining.

Here is a new webrev (webrev.03) that includes what you suggested:

http://cr.openjdk.java.net/~zmajo/8169000/webrev.03/

We could go either with this webrev or with the previous one (webrev.02) 
that changes "will never be enqueued" to "may never be enqueued".

http://cr.openjdk.java.net/~zmajo/8169000/webrev.02/

I'm not sure which webrev is the better option.

Thank you!

Best regards,


Zoltan

> Mandy
> [1] The Java Programming Language, Fourth Edition, section 17.5.4
>