size (i.e. length) of an OS file: gotten from the OS ?

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Sébastien de Mapias - 08 Nov 2007 20:17 GMT

Hi,
I'd like to know how Java knows the size of a file.
When you simply do
File f = new File("/usr/tmp/stuff.txt");
long si = f.length();

it performs a call to its own reference of FileSystem.getLength
on itself, and when I check the getFileSystem() method in the
FileSystem class I see it's native...

So I guess the size (amongst other properties) is a data that
f receives when it's instantiated - or I'm mistaken ?

Thanks.
Seb

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Daniel Pitts - 08 Nov 2007 21:07 GMT

> Hi,
> I'd like to know how Java knows the size of a file.
[quoted text clipped - 11 lines]
> Thanks.
> Seb

It probably isn't when its instantiated, but rather when that method is
invoked... A File object doesn't actually represent a file, but a
file-location. Therefor, the file itself can change over time.

Being that it is a native call, I'm guessing that it is translated to a
call to the OS.

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Roedy Green - 09 Nov 2007 00:13 GMT

On 8 Nov 2007 12:17:59 -0800, Sébastien de Mapias
<sglrigaud@gmail.com> wrote, quoted or indirectly quoted someone who
said :

>So I guess the size (amongst other properties) is a data that
>f receives when it's instantiated - or I'm mistaken ?

nope. The implementation is:
return fs.getLength(this);

where fs is the file system. It does not call getLength until you
call File.length()

the length can change over time.

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The Java Glossary
http://mindprod.com

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size (i.e. length) of an OS file: gotten from the OS ?

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