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// FileLock.java
import java.io.*;
import java.util.*;
import java.util.concurrent.*;
/**
* "locks" a resource by using the file system as storage
* file has 1 line
* <incarnation> <last ping time in millis>
*/
public class FileLock {
public FileLock( String logicalName )
throws IOException {
_file = new File( "/tmp/java-fileLock-" + logicalName );
_incarnation = "xxx" + Math.random() + "yyy";
if ( ! _file.exists() ) {
FileOutputStream fout = new FileOutputStream( _file );
fout.write( "\n".getBytes() );
fout.close();
}
}
/**
* takes lock
* if someone else has it, blocks until the other one finishes
*/
public void lock()
throws IOException {
if ( _lock != null )
throw new IllegalStateException( "can't lock when you're locked" );
try {
_semaphore.acquire();
}
catch ( InterruptedException ie ) {
throw new RuntimeException( "sad" , ie );
}
_raf = new RandomAccessFile( _file , "rw" );
_lock = _raf.getChannel().lock();
}
public void unlock()
throws IOException {
if ( _lock == null )
throw new IllegalStateException( "can't unlock when you're not locked" );
_lock.release();
_semaphore.release();
_locked = false;
}
final File _file;
final String _incarnation;
private RandomAccessFile _raf;
private java.nio.channels.FileLock _lock;
private boolean _locked;
private static Semaphore _semaphore = new Semaphore(1);
public static void main( final String[] args )
throws Exception {
List<Thread> threads = new ArrayList<Thread>();
for ( int i=0; i<3; i++ ) {
threads.add( new Thread() {
public void run() {
try {
FileLock lock = new FileLock( args[0] );
long start = System.currentTimeMillis();
lock.lock();
System.out.println( "time to lock:\t" + (System.currentTimeMillis()-start) );
Thread.sleep( Integer.parseInt( args[1] ) );
lock.unlock();
System.out.println( "total time:\t" + (System.currentTimeMillis()-start) );
}
catch ( Exception e ) {
e.printStackTrace();
}
}
} );
}
for ( Thread t : threads ) {
t.start();
}
for ( Thread t : threads ) {
t.join();
}
}
}
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