Provide API to explicitly unmap MappedByteBuffer?

[Alan Bateman]

On 20/03/2018 10:39, Langer, Christoph wrote:
>
> Hi,
>
> I was just handling a customer issue where Java code tried to delete a 
> file on Windows which was mapped into memory vial FileChannel.map() 
> before. It took me a little while to understand the issue but 
> obviously the problem was that Windows still kept a delete lock on the 
> file as long as the ByteBuffer was still mapped. This behavior is also 
> documented cleanly: 
> https://download.java.net/java/jdk10/docs/api/java/nio/MappedByteBuffer.html.
>
> So far so good, I just started wondering why there isn’t an API to 
> force unmapping (cleanup) on a MappedByteBuffer? The app that opens 
> the mapping simply reads a properties file at this place and I guess 
> they should be able to change this place to use standard io easily and 
> without observable performance problems. However, if they had a way to 
> explicitly unmap the MappedByteBuffer it would also help.
>
> Was this topic already discussed? Is there a hard reason why explicit 
> cleanup should not be offered? I guess people using MappedByteBuffers 
> should know anyway what they are doing…
>
>
This has been discussed many times, it's a really hard problem that many 
smart people have spend time on.

Andrew Haley may be able to say more about this. He has (or had) a 
proposal on this but it involved deep changes in the VM. I don't know he 
has prototyped it yet to see how feasible it is.

-Alan
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