<AWT Dev> [9] Review Request: 8165769 Hang in the help menu item

Alexey Ivanov alexey.ivanov at oracle.com
Mon Nov 28 08:12:46 UTC 2016

Hi Semyon, Sergey,

On 17.11.2016 11:27, Semyon Sadetsky wrote:
> On 16.11.2016 20:54, Sergey Bylokhov wrote:
>> On 16.11.16 20:25, Semyon Sadetsky wrote:
>>>> The example above produce the same result as if the thread B will call
>>>> dispatchEventImpl() early than addItemListener() was called by thread
>>>> A. And this is correct behavior(the new events will be proceeded only
>>>> when we set newEventsOnly to true).
>>>> addItemListener is synchronized, because we need to synchronize access
>>>> to the list of listeners "itemListener" when we add/remove listener.
>>> Then explain to me why to make all those fields volatile? Because the
>>> cache for the changed fields is guaranteed to be flushed upon exit from
>>> the synchronized block, so the changes will be visible to other threads
>>> when the method returns.
>> The statement above is incorrect, there is no "cache". I do not know 
>> where you get "changed fields is guaranteed to be flushed upon exit 
>> from the synchronized block". Also there is no guarantee that the 
>> reader will see the latest version of the field if the reader will 
>> use another mutex or will not use synchronization at all. In the fix 
>> "volatile" will guarantee that the readers will see the latest 
>> version which was set.
> Flushing the cache is reality. By adding volatile to all Menu* fields 
> you didn't make the methods effects predictable in case of they are 
> called concurrently because they are not synchronized between each 
> other. After this change it still may not be recommended to use menus 
> from different threads arbitrarily.

Semyon is right that the updated values will be written to memory upon 
exit from synchronized block.

And Sergey is right that the above statement does not guarantee the 
updated value will be read from memory. That is getter called on another 
thread could still see the previous value. For getter to read the 
updated value, it also has to be synchronized (on the same object monitor).

To avoid synchronized in getters, volatile could be used as it 
guarantees the reader will always see the latest written value.

> Or if I'm wrong and this change is a real improvement why you don't 
> add volatile to all AWT classes' fields? The rest AWT classes are not 
> better synchronized than the menu onces.

Unfortunately, you're right other AWT classes are not well-suited for 
multi-threaded environment…


>>>>> Same for enableEvents(long eventsToEnable) method.
>>> Also, the state field setter is synchronized by this object monitor
>>> while the peer object which it should notify is protected by another
>>> monitor and may be reset concurrently.
>>>>>> But in some corner cases we change this value, so it cannot be 
>>>>>> final.
>>>>> What is that corner case? The comment clearly states that it is never
>>>>> changed.
>>>> We have a setter and we call it in applets, trayicon and in X11.
>>> But TryIcon is not a MenuComponent. It seems the comment is correct.
>> But we still have a setter which is called by other code. Also it 
>> cannot be made final because it is updated during de-serialization.
> And you still did not answer where it is really called. Probably the 
> setter may be removed? At least please update the comment statement 
> since in the change you assumes that the opposite is true.
> Why you left without synchronization the analogous field in the 
> Component class? This field is really modified I can give you examples.
>>>>> Also it seems Menu#isHelpMenu field is never used except for 
>>>>> toString()
>>>>> and may be removed.
>>>> Why it can be removed since it is used in the toString()?
>>> Because in this case it looks like cache anti-pattern and it should be
>>> replaced with the real value. For which purpose the toString() may be
>>> used? for debugging? But it seems one cannot guarantee that the 
>>> cache is
>>> updated in each moment of time in case of multi-threading.
>> It is used to provide an information in the "string" that this menu 
>> is a helpmenu. It is used in some tests as well. We also should take 
>> care since this object is Serializable.
>>>>>>> --Semyon
>>>>>>>>  - When the submenu is removed from Menu/MenuBar we do not 
>>>>>>>> reset its
>>>>>>>> parent field if the Menu/MenuBar have peer==null. So if later we
>>>>>>>> tried
>>>>>>>> to call MenuBar.setHelpMenu(submenu) we skip this submenu 
>>>>>>>> because we
>>>>>>>> think it was added already.
>>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8165769
>>>>>>>> Webrev can be found at:
>>>>>>>> http://cr.openjdk.java.net/~serb/8165769/webrev.00

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