Performance of JrtFileSystemProvider.exists

[Alan Bateman]

On 16/02/2017 08:15, Jason Zaugg wrote:

> Recently I modified the Scala compiler to read class files
> jrt:/ filesystem, rather than relying on rt.jar.
>
> I noticed a slowdown when running on JDK 9 ea and exercising this code.
> Profiles suggest that the bottleneck is in calls to Files.exists.
> JrtFileSystemProvider inherits an implementation which needs to construct,
> throw and catch NoSuchFileException to answer in the negative.
>
> It is possible that I'm calling this more often than needed, and could
> implement a caching layer. But it seems worthwhile for jrtfs to implement a
> fast path for exists, as is done in UnixFileSystemProvider.
>
> Possibly related bug "Startup regression with Jython":
> https://bugs.openjdk.java.net/browse/JDK-8166236
>
As it happens, we a similar issue with the zip provider. It's something 
that I was looking at with Sherman recently and we have a proposal to 
push the exists down to the provider interface so that it can be overridden.

So a different issues to JDK-8166236 - that one is that Jython seems to 
create a cache on first usage of all classes that it finds in rt.jar. I 
assume that if/when Jython adds support for JDK 9 and jrtfs that they 
will be able to integrate it with their caching.

-Alan