Menu
I have a C++ program that locks files using POSIX advisory locks. That is, it uses the POSIX fcntl system call for lock operations. I want a Java program to interoperate with that C++ program, so I want my Java program to also use POSIX advisory locks. File locking in Java should use the standard FileLock class. But the API documentation is understandably vague on just how locking is implemented:
This file-locking API is intended to map directly to the native locking facility of the underlying operating system. Thus the locks held on a file should be visible to all programs that have access to the file, regardless of the language in which those programs are written.
If I am running a common implementation of Java (Oracles, Open JDK) on a POSIX operating system, or more specifically a GNU/Linux system, is it safe to assume that the Java FileLock class uses POSIX advisory locks?
926
POSIX fcntl locks are locks that are accessed from the fcntl(2) system call with the F_GETLK , F_SETLK and F_SETLKW commands. They provide an advisory locking API that is used by a number of applications, including those running on NFS , since NFS does not support flock(2) .
A token representing a lock on a region of a file. A file-lock object is created each time a lock is acquired on a file via one of the lock or tryLock methods of the FileChannel class, or the lock or tryLock methods of the AsynchronousFileChannel class. A file-lock object is initially valid.
Advisory Locking Advisory locking requires cooperation from the participating processes. Suppose process “A” acquires an WRITE lock, and it started writing into the file, and process “B”, without trying to acquire a lock, it can open the file and write into it. Here process “B” is the non-cooperating process.
Try this:
(1) Write a small java program that locks a file and sleeps (or otherwise stops executing).
(2) cat /proc/locks
(3) You'll see lines like the following:
24: POSIX ADVISORY READ 1784 08:01:27384070 1073742826 1073742335
25: FLOCK ADVISORY WRITE 815 00:0f:9772 0 EOF
Identify your process ID from column 5. If column 2 is FLOCK then flock is being used. If it is POSIX then column 2 will be POSIX, indicating fcntl (or lockf which is build on top of fcntl) is being used.
If java has to choose one or the other then POSIX would be the sensible choice as it supports record locking.
81
Some Unix operating systems, including Linux, provide BSD-style (flock) locks, so it might be thought that Java FileLock could be implemented using BSD-style locks rather than POSIX locks. But that is not possible, because BSD-style locks are whole-file locks rather than record locks, and FileLock is a record lock: each lock is for a range of bytes in a file. Thus there is no real choice on a Unix system, and assuming that the implementation of FileLock uses POSIX fcntl locks is a safe assumption on a Unix operating system.
The resulting FileLock locks might or might not interact with BSD-style locks. BSD-style locks can be implemented using POSIX locks (this was the case for Linux before version 2.0), or the operating system might have the two styles of locking interact (this is the case for FreeBSD). But in general that can not be guaranteed, and BSD-style locks and Java locks might be effectively invisible to each other (this is the case for any version of Linux you are likely to encounter).
37
answered Sep 26 '22 02:09
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
