package io;

import java.nio.*;

import java.nio.channels.*;

import java.io.*;

import java.util.concurrent.*;

/**

* RUN:

* javac io/LockingMappedFiles.java && java io.LockingMappedFiles

*

* OUTPUT:

* Locked: 0 to 50331647

* Locked: 75497471 to 113246206

* Unlocked: 75497471 to 113246206

* Unlocked: 0 to 50331647

*/

public class LockingMappedFiles {

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

static FileChannel fc;

public static void main(String[] args) {

try {

fc = new RandomAccessFile(new File("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);

}

catch (Exception e) {

throw new RuntimeException(e);

}

}

private static class LockAndModify extends Thread {

private ByteBuffer buffer;

private int start, end;

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

this.start = start;

this.end = end;

mbb.limit(end);

mbb.position(start);

buffer = mbb.slice();

start();

}

public void run() {

try {

// exclusive blocking

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

System.out.println("Locked: "+start + " to " + end);

// Change

while (buffer.position() < buffer.limit() - 1) {

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

}

fl.release();

System.out.println("Unlocked: "+start + " to " + end);

}

catch (IOException e) {

throw new RuntimeException(e);

}

}

}

}