// newio/LockingMappedFiles.java
// (c)2020 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
*/