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

Sergey Bylokhov Sergey.Bylokhov at oracle.com
Wed Nov 16 16:43:52 UTC 2016

On 16.11.16 19:21, Semyon Sadetsky wrote:
> I can give you an example:
> CheckboxMenuItem.java
>  243     public synchronized void addItemListener(ItemListener l) {
>  244         if (l == null) {
>  245             return;
>  246         }
>  247         itemListener = AWTEventMulticaster.add(itemListener, l);
>  248         newEventsOnly = true;
>  249     }
> we are enabling event by calling the method above on thread A. The
> method is synchronized.
> at the same time on thread B dispatchEventImpl(AWTEvent e) is called
> which is not synchronized at all.
> If thread A is paused by the scheduler at line 248 then thread B may be
> executed and the event will be skipped in dispatchEventImpl(AWTEvent e)
> because of newEventsOnly=false still.
> By making newEventsOnly and itemListener fields volatile you only
> guarantee their visibility for thread B but this doesn't make
> newEventsOnly change atomic along with adding an item listener.

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.

> Same for enableEvents(long eventsToEnable) method.

>> 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.

>>> Note that Component appContext field can be changed and this
>>> multithreading issue should be resolved. Since the filed is accessed
>>> only using the component accessor it should be enough to synchronize the
>>> corresponding getter and setter.
> 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()?

>>> --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

Best regards, Sergey.

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