WeakReference with null referent
peter.levart at gmail.com
Tue Jul 10 06:59:00 UTC 2018
On 07/10/2018 12:18 AM, mandy chung wrote:
> On 7/9/18 12:59 PM, Per Liden wrote:
>> On 2018-07-09 20:49, mandy chung wrote:
>>> On 7/9/18 11:31 AM, Zheka Kozlov wrote:
>>>> It is possible to create a
>>>> with a null value in which case the Reference will never be
>>>> enqueued. This
>>>> is quite obvious (since null cannot be weakly/softly/phantom
>>>> But I think it's worth being mentioned in the JavaDoc. What do you
>>> Alternatively, the constructor should require non-null referent and
>>> throws NPE if null.
>>> I created https://bugs.openjdk.java.net/browse/JDK-8206933 to track
>> It's not completely obvious to me that throwing NPE or otherwise
>> blocking this is the right thing to do. Sure, creating a Reference
>> with a null referent seems pretty useless, but it's also very similar
>> to creating a Reference and immediately calling its clear() method,
>> which is perfectly valid (and equally useless).
>> Are you saying we should block this because we can easily detect this
>> particular case/misuse, as opposed to the immediately-called-clear
>> case? Or is there some other rationale?
> I don't expect any one wants to create a reference with null referent
> Throwing NPE would be one way to catch if a reference object is created
> with null referent. I think the compatibility risk is low.
> I agree that Reference::clear is equally useless. Are you thinking
> something should be done with Reference::clear?
There are situations where being able to create a XxxReference without a
referent is useful. For example, imagine a doubly-linked list of
WeakReference subclasses - a special instance without a referent can be
used as the head of such list that never goes away and simplifies
insertion/deletion logic, etc...
JDK's java.lang.ref.Cleaner API uses such scheme in its implementation
(see jdk.internal.ref.CleanerImpl.[Phantom|Weak|Soft]CleanableRef no-arg
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