Is it safe to update jar that are used for dynamic layers?

[Alan Bateman]

On 06/05/2022 12:26, Alex Orlov wrote:
> Hello.
>   
> Please, consider the following situation. We have /home/user/temp/foo.jar that is used in dynamically created jpms
> layer bar.
>   
> Could anyone say if it is safe to do the following: destroy layer bar, update /home/user/temp/foo.jar (modify some code in this jar)and create a new jpms layer (bar_2) that will use /home/user/temp/foo.jar.
>
This isn't really a module system issue, it's more about whether about 
you know if a JAR file is still open (and in use) and whether you can 
replace a JAR file that is open. In general it is not safe/reliable to 
do this. If you search JBS you will find dozens of bug reports on this 
topic, most going back many years. Many of the older bug reports are 
crashes because the central directory of a zip file was historically 
memory mapped and accessing a memory mapped region after the file has 
been replaced will typically SIGBUS. You'll also see bug reports on 
Windows with errors replacing zip or JAR files that are open. For the 
module system then you could use a weak reference and have it be queued 
when the module layer is GC'ed. However, I suspect this may not be 
timely enough for what you are doing and maybe you could look at using a 
unique name/location for each update instead.

-Alan