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How safe is it to use java.nio.channels.FileLock for locking files among processes? It is stated that other processes can not access the file if we have an exclusive lock. However, the below answer on another SO question states other processes have to check for the filelock too in order for our process to be safe.

(a) Are you aware that locking the file won't keep other processes from touching it unless they also use locks?

So I tested my code and tried to change, a file which I have the lock already, with Windows Text Editor and I was safe from harm but not when I test with Notepad++..

Is there a solution for locking a file appropriately in Java 6?

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Guillotine1789
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3 Answers3

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Java FileLock uses advisory (not mandatory) locks on many platforms. That means it may only provide locking against other applications that also use FileLock (or the equivalent in other languages).

Neither Linux or Windows implement mandatory locking across the board. For instance:

  • For Linux and similar, file locking is advisory only.

  • For Windows, according to Wikipedia:

    "For applications that use the file read/write APIs in Windows, byte-range locks are enforced .... by the file systems that execute within Windows. For applications that use the file mapping APIs in Windows, byte-range locks are not enforced ..."

    In other words, locking on Windows can be either mandatory or advisory, depending on which API an Windows application uses to access files.

How safe is it to use Java FileLock?

If you are actually asking if it is safe to assume that FileLock provides mandatory file locking with respect to all other applications (Java & non-Java) irrespective of how they are written, the answer is No. It is NOT safe to make that assumption.

Is there a solution for locking a file appropriately in Java 6?

Only if all of the applications (Java & other) cooperate; e.g. by using FileLock or the equivalent.

If you can't make that assumption, there is no solution using portable Java. Indeed, on most (if not all) common OS platforms, there is no solution at all, AFAIK ... because the platform itself doesn't support mandatory file locking independent of the application.

Stephen C
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    On Unix systems it is safe to assume they are POSIX advisory locks: http://stackoverflow.com/questions/23562369/is-a-java-filelock-a-posix-advisory-fcntl-lock – Raedwald Jan 14 '16 at 09:39
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From the Javadoc of java.nio.channels.FileLock under Platform Dependencies:

The native file-locking facilities of some systems are merely advisory, meaning that programs must cooperatively observe a known locking protocol in order to guarantee data integrity. On other systems native file locks are mandatory, meaning that if one program locks a region of a file then other programs are actually prevented from accessing that region in a way that would violate the lock. On yet other systems, whether native file locks are advisory or mandatory is configurable on a per-file basis. To ensure consistent and correct behavior across platforms, it is strongly recommended that the locks provided by this API be used as if they were advisory locks.

As you discovered from your testing, other non-Java code running on your version of Windows does not have to honor your exclusive lock.

Your only solution is to read the file into memory as fast as you can, take your time processing the information, then write the file to disk as fast as you can.

Gilbert Le Blanc
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It is stated that other processes can not access the file if we have an exclusive lock.

It is stated where? Not in the Javadoc.

However, the below answer on another [SO question][2] states other processes have to check for the filelock too in order for our process to be safe.

That is correct.

So I tested my code and tried to change, a file which I have the lock already, with Windows Text Editor and I was safe from harm but not when I test with Notepad++.

You're already doing something invalid by testing on the one platform where file locks affect ordinary opens, but the only conclusion to be drawn from this is that it isn't safe. Notepad++ keeps the file open, and so encounters your locks, but Windows Text Editor doesn't, and so doesn't see the locks either, until you try to save.

Is there a solution for locking a file appropriately in Java 6?

Not unless the applications you're locking against also use file locks.

user207421
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  • In java.nio.channels.FileChannel the method tryLock returns :A lock object representing the newly-acquired lock, or null if the lock could not be acquired because another program holds an overlapping lock. but throws an exception when If a lock that overlaps the requested region is already held by this Java virtual machine, or if another thread is already blocked in this method and is attempting to lock an overlapping region – Guillotine1789 Jan 14 '16 at 08:08
  • @Guillotine1789 If that is intended to support your statement beginning 'it is stated ...', it doesn't. If not, the relevance escapes me. – user207421 Jan 14 '16 at 23:04
  • Sorry for late reply how is it not related? Thought of another program as another process? – Guillotine1789 Jan 19 '16 at 20:33
  • You wrote 'It is stated that other processes can not access the file if we have an exclusive lock'. There is nothing in your quotation above that states anything whatsoever about access, or about what other processes can and can't do. – user207421 Feb 01 '16 at 04:32