Object.wait returns normally if interrupted

[David Holmes]

On 28/06/2025 4:03 am, Pavel Rappo wrote:
> David,
> 
> Having re-read your replies multiple times, I think I now understand
> it. It finally clicked after I read your "once notified it is no
> longer waiting" for the Nth time. Indeed, even if a thread has not yet
> restored its synchronization claims, it's no longer waiting. I
> suppose, the same applies to interruption: once interrupted, the
> thread is no longer waiting.

Correct. A "wait" places a thread in the monitor's wait-set. A 
notification, or interrupt, removes it from the wait-set, so that it can 
then reclaim the monitor (when it is available) and then return either 
normally or by throwing IE.

> So here's my refined mental model. JLS says [^1] that an
> implementation must not lose notifications or interruptions, even if
> both occur at the same time. Obviously, this is on a best-efforts
> basis because everything is racy.

Not sure what you are getting at here. The implementation synchronizes 
both actions such that it can always respond appropriately - actions 
become serialized internally.

> It's easier not to lose interruptions than notifications. For an
> interruption, either InterruptedException is thrown or the interrupted
> status is set. For a notification (specifically, a notification
> through Object.notify), the implementation must make sure that some
> thread returns normally from Object.wait.

I think you need to look into the implementation, you're overthinking 
this somewhat. There's no inherent reason one is easier than the other.

> If a selected thread cannot return normally because it has been
> interrupted, the implementation selects another thread from the wait
> set and checks if that thread can return normally, and so forth. This
> continues until a suitable thread is found or there are no threads
> left in the wait set. In which case notification will be missed.
> Hence, best-efforts.

If there are no threads to notify then nothing is missed so that really 
isn't "best effort".

Once a thread has been removed from the wait-set due to interrupt, then 
notification never sees it. However, if a thread is removed from the 
wait-set due to notification and then looks at its interrupt state 
(which may have been set after the notification was processed) and 
decides to throw IE, then we would have to pass on the notification. But 
the simplest approach is to not check the interrupt state in that case. 
Hotspot gives preference to notification over interrupt, and implicitly 
to interrupt over timeout.

> So, Object.wait *does not choose* to return normally from an
> interruption to deliver a notification. It just happens so naturally.
> The only thing that accounts for this edge case of simultaneous
> notification-interruption is the iterative search for a thread to
> notify.
> 
> Is my mental model accurate?

If the mental model fits the spec then it is accurate. The Hotspot 
implementation works in a different way.

David
-----

> [^1]: https://docs.oracle.com/javase/specs/jls/se24/html/jls-17.html#jls-17.2.4
> 
> On Fri, Jun 27, 2025 at 1:11 PM Pavel Rappo <pavel.rappo at gmail.com> wrote:
>>
>> So, Object.wait() guarantees to throw InterruptedException if the
>> interrupt status is set upon entrance, yes? Could this be added to
>> javadoc?
>>
>> On Fri, Jun 27, 2025 at 12:59 PM David Holmes <david.holmes at oracle.com> wrote:
>>>
>>> On 27/06/2025 5:58 pm, Pavel Rappo wrote:
>>>> David,
>>>>
>>>> As you correctly identified, the edge case that I mentioned is this:
>>>> concurrent interrupt and notify [^1]. It has nothing to do with
>>>> spurious wake-ups or virtual threads. In fact, I've seen that with
>>>> virtual threads Object.wait behaves exactly the same way as with
>>>> platform threads in that regard. So I cannot report any issue here.
>>>>
>>>> I'm only asking about the accuracy of the javadoc contract. Say, a
>>>> programmer uses this template from Object.wait's javadoc.
>>>>
>>>>       synchronized (obj) {
>>>>           while ( <condition does not hold and timeout not exceeded> ) {
>>>>               long timeoutMillis = ... ; // recompute timeout values
>>>>               int nanos = ... ;
>>>>               obj.wait(timeoutMillis, nanos);
>>>>           }
>>>>           ... // Perform action appropriate to condition or timeout
>>>>       }
>>>>
>>>> Obviously, the programmer handles InterruptedException outside this
>>>> template. Perhaps the programmer relies on that exception being thrown
>>>> on an interrupt. So they think of this code as of interruptible.
>>>>
>>>> Now, if it is generally possible -- and not just in this edge case --
>>>> for Object.wait to return from an interrupt normally, the above code
>>>> is effectively uninterruptible. And the programmer does not know about
>>>> it. To fix that code, the programmer needs to additionally and
>>>> explicitly check the interrupt status.
>>>
>>> The wait() will throw IE on the next loop if the condition is not met.
>>> Otherwise the code proceeds with the knowledge the thread was notified,
>>> irrespective of whether the thread was also interrupted "around the same
>>> time".
>>>
>>> David
>>> -----
>>>
>>>> In contrast, j.u.c.Condition.await, makes a special provision, which
>>>> guarantees that if the method returns normally from an interrupt, it
>>>> will throw an exception on the next iteration of the
>>>> j.u.c.{Lock,Condition} analogue of the above template:
>>>>
>>>>> If the current thread:
>>>>>
>>>>> has its interrupted status set on entry to this method;
>>>>> or ...
>>>>> then InterruptedException is thrown and the current thread's interrupted status is cleared.
>>>>
>>>> [^1]: https://docs.oracle.com/javase/specs/jls/se24/html/jls-17.html#jls-17.2.4
>>>>
>>>> On Fri, Jun 27, 2025 at 3:47 AM David Holmes <david.holmes at oracle.com> wrote:
>>>>>
>>>>> Hi Pavel,
>>>>>
>>>>> On 27/06/2025 8:23 am, Pavel Rappo wrote:
>>>>>> Here's an interesting behaviour. A thread that has been blocked in
>>>>>> Object.wait is interrupted. But instead of throwing an
>>>>>> InterruptedException, Object.wait returns normally with the thread's
>>>>>> interrupt status set.
>>>>>
>>>>> Do you mean it returns normally without being notified? That's allowed
>>>>> by spec as it is just a spurious wakeup.
>>>>>
>>>>> That said, it shouldn't really happen in the current implementation
>>>>> AFAIK, though with recent changes around virtual thread support it is
>>>>> possible some new edge case has crept in. Or a bug.
>>>>>
>>>>> If the interrupt is racing with a notification then what you see is also
>>>>> allowed as you can't tell which occurred first:
>>>>>
>>>>> "If the current thread is interrupted by any thread before or while it
>>>>> is waiting, ...
>>>>>
>>>>> once notified it is no longer waiting.
>>>>>
>>>>>> This behaviour is not mentioned in javadoc, yet I witnessed it.
>>>>>> Granted, I could only trigger it in an edge case described in JLS. I
>>>>>
>>>>> What edge case is described in current JLS?
>>>>>
>>>>>> wonder if the behaviour is reserved for that edge case, or it can be
>>>>>> observed elsewhere and there is a genuine gap in javadoc. FWIW,
>>>>>> javadoc for j.u.c.Condition documents this behaviour.
>>>>>>
>>>>>> Here's a practical implication of this. If some code that uses
>>>>>> Object.wait wants to be interruptible, it must not rely on Object.wait
>>>>>> to throw InterruptedException. It should check interrupt status or
>>>>>> call other methods that are guaranteed to throw InterruptedException
>>>>>> on thread interruption.
>>>>>
>>>>> In typical usage though if you were notified as well then responding to
>>>>> that is generally fine and is what would happen if the notification
>>>>> definitely occurred before the interrupt. If it was a spurious wakeup
>>>>> then you should be re-waiting in a loop and the next wait will throw the
>>>>> IE. So I think no real code would be adversely impacted by these edge cases.
>>>>>
>>>>> Cheers,
>>>>> David
>>>>> -----
>>>>>
>>>>>> -Pavel
>>>>>
>>>