Mega Code Archive
Demonstrates file locking and simple file read and write operations using java nio channels FileChannel
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/*
* Copyright (c) 2004 David Flanagan. All rights reserved.
* This code is from the book Java Examples in a Nutshell, 3nd Edition.
* It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
* You may study, use, and modify it for any non-commercial purpose,
* including teaching and use in open-source projects.
* You may distribute it non-commercially as long as you retain this notice.
* For a commercial use license, or to purchase the book,
* please visit http://www.davidflanagan.com/javaexamples3.
*/
//package je3.nio;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
/**
* Lock.java: this class demonstrates file locking and simple file read and
* write operations using java.nio.channels.FileChannel. It uses file locking to
* prevent two instances of the program from running at the same time.
*/
public class Lock {
public static void main(String args[]) throws IOException, InterruptedException {
RandomAccessFile file = null; // The file we'll lock
FileChannel f = null; // The channel to the file
FileLock lock = null; // The lock object we hold
try { // The finally clause closes the channel and releases the lock
// We use a temporary file as the lock file.
String tmpdir = System.getProperty("java.io.tmpdir");
String filename = Lock.class.getName() + ".lock";
File lockfile = new File(tmpdir, filename);
// Create a FileChannel that can read and write that file.
// Note that we rely on the java.io package to open the file,
// in read/write mode, and then just get a channel from it.
// This will create the file if it doesn't exit. We'll arrange
// for it to be deleted below, if we succeed in locking it.
file = new RandomAccessFile(lockfile, "rw");
f = file.getChannel();
// Try to get an exclusive lock on the file.
// This method will return a lock or null, but will not block.
// See also FileChannel.lock() for a blocking variant.
lock = f.tryLock();
if (lock != null) {
// We obtained the lock, so arrange to delete the file when
// we're done, and then write the approximate time at which
// we'll relinquish the lock into the file.
lockfile.deleteOnExit(); // Just a temporary file
// First, we need a buffer to hold the timestamp
ByteBuffer bytes = ByteBuffer.allocate(8); // a long is 8 bytes
// Put the time in the buffer and flip to prepare for writing
// Note that many Buffer methods can be "chained" like this.
bytes.putLong(System.currentTimeMillis() + 10000).flip();
f.write(bytes); // Write the buffer contents to the channel
f.force(false); // Force them out to the disk
} else {
// We didn't get the lock, which means another instance is
// running. First, let the user know this.
System.out.println("Another instance is already running");
// Next, we attempt to read the file to figure out how much
// longer the other instance will be running. Since we don't
// have a lock, the read may fail or return inconsistent data.
try {
ByteBuffer bytes = ByteBuffer.allocate(8);
f.read(bytes); // Read 8 bytes from the file
bytes.flip(); // Flip buffer before extracting bytes
long exittime = bytes.getLong(); // Read bytes as a long
// Figure out how long that time is from now and round
// it to the nearest second.
long secs = (exittime - System.currentTimeMillis() + 500) / 1000;
// And tell the user about it.
System.out.println("Try again in about " + secs + " seconds");
} catch (IOException e) {
// This probably means that locking is enforced by the OS
// and we were prevented from reading the file.
}
// This is an abnormal exit, so set an exit code.
System.exit(1);
}
// Simulate a real application by sleeping for 10 seconds.
System.out.println("Starting...");
Thread.sleep(10000);
System.out.println("Exiting.");
} finally {
// Always release the lock and close the file
// Closing the RandomAccessFile also closes its FileChannel.
if (lock != null && lock.isValid())
lock.release();
if (file != null)
file.close();
}
}
}
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