Deadlock between FileHandler and ConsoleHandler when using customized formatter

Peter Levart peter.levart at
Mon Dec 9 14:01:39 UTC 2013

On 12/09/2013 11:12 AM, Daniel Fuchs wrote:
> On 12/9/13 9:58 AM, Peter Levart wrote:
>> On 12/09/2013 09:51 AM, Shi Jun Zhang wrote:
>>> On 12/9/2013 4:28 PM, Peter Levart wrote:
>>>> On 12/09/2013 08:02 AM, Shi Jun Zhang wrote:
>>>>> Peter,
>>>>> I think you are misunderstanding this problem. This deadlock is not
>>>>> related to the formatter synchronization, we can make
>>>>> CustomerFormatter.format not synchronized and not call super.format,
>>>>> the deadlock still happens.
>>>> I'm not saying that your formatters are explicitly synchronized - all
>>>> formatters are currently effectively synchronized by LogHandlers. The
>>>> Formatter is invoked from within LogHandler's publish() method which
>>>> is synchronized (on LogHandler.this). If formatters were invoked out
>>>> of this synchronized section, there would be no danger of deadlocks
>>>> when using Logger.log from within custom formatters. But then other
>>>> issues would arise as a consequence of non-multithreaded formatters
>>>> being invoked concurrently...
>>>> Regards, Peter
>>> Hi Peter,
>>> We have thought about moving formatter out of the synchronized section
>>> of Handler.publish(), it can avoid the deadlock. However, we can
>>> reproduce the similar deadlock by extending the Writer in Handler and
>>> using logger in the customized Writer.
>> That's right. And the remedy for that situation would also be what Jason
>> Mehrens suggested - asynchronous publishing.
> Hi,
> I agree with Peter & Jason - asynchronous publishing seems like a good
> solution. I believe the LogManager will close all handlers on exiting,
> so you might want to make sure that your asynchronous handler flushes
> the queue before quitting - which could still be tricky if flushing
> the queue produces new log messages for the queue - and also because
> you will want Handler.close() to wait until the queue is empty.

You're right, Daniel. There already is a global shut-down hook installed 
in LogManager that close()s all
active handlers when VM is shutting down.

Shi Jun Zhang, here's a quick mock-up of a prototype AsyncHandler that 
might work for you:

Regards, Peter

> Anyway - the best advice still is IMHO - don't call Logger.log while
> publishing a log message. This should save you from a lot of issues,
> like the one you encountered - but also possible stack overflows etc...
> best regards,
> -- daniel
>> Regards, Peter

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