Re: Lock a file or somehow make it unwritable

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Thu, 30 Jul 2009 21:05:51 -0400
Lionel van den Berg wrote:

On Jul 31, 8:58 am, Arne Vajh?j <> wrote:

Lionel van den Berg wrote:

I'm building a test harness and would like to lock a particular file
so that when the system under test tries to write to this file it
cannot. I was looking at FileChannel but (un)fortunately that locks it
for the entire VM, so it doesn't affect any later calls.
Any ideas on solutions?

Have you tried opening it for write as a lock?

Yes, here is a sample of the code I have tried:

final File file = new File(filePath);
// Create the file or we won't get a lock.

final FileOutputStream fileOutputStream = new FileOutputStream(file);
final FileLock fileLock = fileOutputStream.getChannel().lock();

if (fileLock == null) {
    writeString("Failed to acquire lock on \"" + filePath + "\".");
} else {

But when you look at the documentation for FileChannel the following
gives away the secret:

"File locks are held on behalf of the entire Java virtual machine.
They are not suitable for controlling access to a file by multiple
threads within the same virtual machine. "

Your original question stated that you wanted to prevent the
test code from writing to the file.

I can not really see how the above correlates to that.

But anyway.

Some experimentation shows that FileOutputStream does
not lock the file exclusively.

But .getChannel().lock() as in your code works (if used
for locking and not for test).

import java.nio.channels.FileLock;

public class Lock {
     public static OutputStream uselessLock(File f) throws IOException {
         return new FileOutputStream(f, true);
     public static FileLock workingLock(File f) throws IOException {
         return new FileOutputStream(f, true).getChannel().lock();
     public static void main(String[] args) throws Exception {
         File f = new File("C:\\z.z");
         OutputStream os = new FileOutputStream(f, true);

did result in:

Exception in thread "main" The process cannot
access the file because another process has locked a portion of the file

which I assume is what you want.


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