using java FileChannel FileLock to prevent file writes but allow reads

Question

I think I'm misunderstanding how the FileChannel's locking features work.

I want to have an exclusive write lock on a file, but allow reads from any process.

On a Windows 7 machine running Java 7, I can get FileChannel's lock to work, but it prevents both reads and writes from other processes.

How can I achieve a file lock that disallows writes but allows reads by other processes?

apangin · Answer 1 · 2014-03-25T23:40:42.780

5

FileChannel.lock() deals with file regions, not with the file itself.
The lock can be either shared (many readers, no writers) or exclusive (one writer and no readers).

I guess you are looking for a bit different feature - to open a file for writing while allowing other processes to open it for reading but not for writing.

This can be achieved by Java 7 FileChannel.open API with non-standard open option:

import static java.nio.file.StandardOpenOption.*;
import static com.sun.nio.file.ExtendedOpenOption.*;
...
Path path = FileSystems.getDefault().getPath("noshared.tmp");
FileChannel fc = FileChannel.open(path, CREATE, WRITE, NOSHARE_WRITE);

Note ExtendedOpenOption.NOSHARE_WRITE which is a non-standard option existing in Oracle JDK.

edited Mar 25 '14 at 23:40

answered Mar 25 '14 at 23:31

apangin

92,924
10
193
247

1

bah :-( why do they make it exist in `com.sun.*` but not available to the user? – Jason S Mar 26 '14 at 15:57
@JasonS It IS available to user (if you are running Oracle JDK or OpenJDK). – apangin Mar 26 '14 at 16:05
1

You'll need `NOSHARE_DELETE`, too - so the File can't be deleted by other processes (`FileChannel.lock()` prevents deletions) – hinneLinks Jul 26 '17 at 09:38

using java FileChannel FileLock to prevent file writes but allow reads

1 Answers1

Linked