Provide API to explicitly unmap MappedByteBuffer?

[Paul Sandoz]

Also, we may have another swing at this problem in Project Panama, where memory regions are more explicitly managed via scopes. However, there will still be similar safety and performance challenges.

Paul.

> On Mar 20, 2018, at 7:54 AM, Langer, Christoph <christoph.langer at sap.com> wrote:
> 
> Thanks Alan,
>  
> I can see the history in bug https://bugs.openjdk.java.net/browse/JDK-4724038 <https://bugs.openjdk.java.net/browse/JDK-4724038>...
>  
> Best regards
> Christoph
>  
>  
>   <>
> From: Alan Bateman [mailto:Alan.Bateman at oracle.com] 
> Sent: Dienstag, 20. März 2018 14:59
> To: Langer, Christoph <christoph.langer at sap.com>; nio-dev at openjdk.java.net
> Subject: Re: Provide API to explicitly unmap MappedByteBuffer?
>  
> 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 <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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