On Fri, 26 Feb 2021 00:52:51 GMT, Alexander Zvegintsev <azveg...@openjdk.org>
wrote:
>> This bug was reported as a leak of the ImageObserver when the user draws a
>> multiresolution image.
>>
>> The multiresolution image is an image that contains a few images inside,
>> when the app uses the observer to get notifications about image loading we
>> create a special internal observer and use it to track loading
>> resolution-variants. This internal observer forwards notification from the
>> resolution variant to the application observer.
>>
>> The mapping from the application observer to the internal observer is done
>> via "SoftCache" which is a kind of HashMap that uses soft references to the
>> key and value.
>>
>> Here the bug comes, the soft references are cleared only under memory
>> pressure, and unused objects may sit hours in memory before being cleaned.
>> Moreover, the internal observer was implemented using a strong reference to
>> the application observer(it is not obvious since the lambda is used). So the
>> key object refers to the application's observer cannot be clear fast. This
>> causes an even longer delay of the memory cleanup, which was considered by
>> the use as a "leak".
>>
>> The fix changes the usage of SoftCache to the WeakHashMap, so the key(the
>> application observer) will be cleared when the application lost the
>> reference to it.
>
> src/java.desktop/share/classes/sun/awt/image/MultiResolutionToolkitImage.java
> line 130:
>
>> 128:
>> 129: if (observer == null || image == null) {
>> 130: return false;
>
> How about a case when `ObserverCache`'s class instance is not collected by
> GC, but its `observerRef` or `imageRef` is collected. Is it possible?
> If so this class instance will permanently return false on `imageUpdate()`
> call.
observerRef points to the same object as a key in this WeakHashMap, so if
observerRef will point to null means that the key is already collected->at some
point(when any of the methods of this collection will be called) the value of
this map(the ObserverCache object) will be collected as well.
-------------
PR: https://git.openjdk.java.net/jdk/pull/2711
