// newio/LockingMappedFiles.java

// (c)2017 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://OnJava8.com for more book information.

// Locking portions of a mapped file

import java.nio.*;

import java.nio.channels.*;

import java.io.*;

public class LockingMappedFiles {

static final int LENGTH = 0x8FFFFFF; // 128 MB

static FileChannel fc;

public static void

main(String[] args) throws Exception {

fc = new RandomAccessFile("test.dat", "rw")

.getChannel();

MappedByteBuffer out = fc.map(

FileChannel.MapMode.READ_WRITE, 0, LENGTH);

for(int i = 0; i < LENGTH; i++)

out.put((byte)'x');

new LockAndModify(out, 0, 0 + LENGTH/3);

new LockAndModify(

out, LENGTH/2, LENGTH/2 + LENGTH/4);

}

private static class LockAndModify extends Thread {

private ByteBuffer buff;

private int start, end;

LockAndModify(ByteBuffer mbb, int start, int end) {

this.start = start;

this.end = end;

mbb.limit(end);

mbb.position(start);

buff = mbb.slice();

start();

}

@Override

public void run() {

try {

// Exclusive lock with no overlap:

FileLock fl = fc.lock(start, end, false);

System.out.println(

"Locked: "+ start +" to "+ end);

// Perform modification:

while(buff.position() < buff.limit() - 1)

buff.put((byte)(buff.get() + 1));

fl.release();

System.out.println(

"Released: " + start + " to " + end);

} catch(IOException e) {

throw new RuntimeException(e);

}

}

}

}

/* Output:

Locked: 75497471 to 113246206

Locked: 0 to 50331647

Released: 75497471 to 113246206

Released: 0 to 50331647

*/