In the Java API, acquiring a lock requires first obtaining the FileChannel for the file to be locked. The lock() method of the FileChannel is used to acquire the lock. The API of the lock() method is
FileLock lock(long begin, long end, boolean shared) where begin and end are the beginning and ending positions of the region being locked. Setting shared to true is for shared locks; setting shared to false acquires the lock exclusively. The lock is released by invoking the release() of the FileLock returned by the lock() operation
import java.io.*;
import java.nio.channels.*;
public class LockingExample {
public static final boolean EXCLUSIVE = false;
public static final boolean SHARED = true;
public static void main(String args[]) throws IOException {
FileLock sharedLock = null;
FileLock exclusiveLock = null;
try {
RandomAccessFile raf = new RandomAccessFile("file.txt","rw");
// get the channel for the file
FileChannel ch = raf.getChannel();
// this locks the first half of the file - exclusive
exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);
/** Now modify the data... */
// release the lock
exclusiveLock.release();
// this locks the second half of the file - shared
sharedLock = ch.lock(raf.length()/2+1,raf.length(),SHARED);
/** Now read the data . . . */
// release the lock
sharedLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}
finally {
if (exclusiveLock != null)
exclusiveLock.release();
if (sharedLock != null)
sharedLock.release();
}
}
}