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Example of inter-process and inter-thread file locking in Java
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileLock;
// The class demonstrates how to get an exclusive file lock that prevents other threads and processes / JVMs from
// obtaining a lock on the same file. To do this, you need to synchronize a block on a file and acquire FileLock. See
// comments below for more details. Run this class in multiple JVMs and see each thread of each JVM acquires a lock in
// an orderly fasion.
public class FileLocking extends Thread
{
private static final File file = new File("lock.test");
public static void main(String[] args) throws Exception
{
for (int i = 0; i < 5; i++) {
new FileLocking().start();
}
}
@Override
public void run()
{
// synchronized on a static variable "file" so that threads don't try to acquire a lock on it at the same
// time. Javadoc of FileLock states "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." What this
// actually means is that if two threads tries to get a lock on the same file, one fails with
// OverlappingFileLockException instead of waiting for a release of the lock.
synchronized(file) {
FileOutputStream fos = null;
try {
fos = new FileOutputStream(file);
// Synchronization doesn't have any effect on threads running on a different JVM, of
// course. FileChannel.lock() acquires a lock on a file that prevents another process from getting a
// lock on it. The method waits until a lock is released if the lock is held by a different process, as
// opposed to throwing exception when the lock is held by a thread in the same JVM.
FileLock lock = fos.getChannel().lock();
System.out.println("Got a lock in " + getName());
// do some file write operation
sleep(1000);
fos.close();
} catch (Exception e) {
e.printStackTrace();
} finally {
if (fos != null) {
try {
fos.close();
} catch (IOException ex) {
}
}
}
}
}
}
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