java.nio.file.Files.isReadable() slow on Windows 7

[Zhong Yu]

However... test it on an unreadable file, checkAccess(path) does not
throw an exception. Please take note of it

The file has Read permission Deny for all users. For this file

Files.isReadable() returns false.

checkAccess(path, AccessMode.READ) throws
    java.nio.file.AccessDeniedException: \tmp\unreadable.txt:
Effective permissions does not allow requested access

AsynchronousFileChannel.open(path, READ) throws
    java.nio.file.AccessDeniedException: C:\tmp\unreadable.txt




On Fri, May 11, 2012 at 9:29 AM, Zhong Yu <zhong.j.yu at gmail.com> wrote:
> On Fri, May 11, 2012 at 2:32 AM, Alan Bateman <Alan.Bateman at oracle.com> wrote:
>> On 11/05/2012 04:09, Zhong Yu wrote:
>>>
>>> java.nio.file.Files.isReadable() seems to be really slow on Windows 7;
>>> on my machine it takes 3ms per call (tested in a tight loop on the
>>> same file). That means only 300 calls per second.
>>>
>>> It's much faster to test readability by opening a file channel for
>>> read then close it.
>>>
>>> Any other workarounds?
>>>
>>> Zhong Yu
>>
>> Oops, I thought we had put in a fast path for the check read case. The
>> background to this is that checking access to a file on Windows is very
>> expensive because it requires reading the DACL and determining the user's
>> effective access.
>>
>> I don't have time to check a Windows machine just at the minute but can you
>> change:
>>
>> Files.isReadable(path)
>>
>> to
>>
>> path.getFileSystem().provider().checkAccess(path)
>
> yes it works, this one takes only 0.02ms!
>
>> in your test, re-run and tell us if this fixes the issue. I suspect it will
>> because checkAccess has a fast path for the check read case, and that fast
>> path doesn't take correctly take access of the isReadable usage.
>>
>> -Alan.