Locking portions of a mapped file : MappedByteBuffer « File « Java Tutorial





// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
// Locking portions of a mapped file.
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;

public class MainClass {
  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();
    }

    public void run() {
      try {
        FileLock fl = fc.lock(start, end, false);
        System.out.println("Locked: " + start + " to " + end);

        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);
      }
    }
  }
}










11.49.MappedByteBuffer
11.49.1.Create MappedByteBuffer from FileInputStream
11.49.2.Get MappedByteBuffer from FileChannel
11.49.3.What happens when the entire file isn't in your mapping region?
11.49.4.Create Read only buffer
11.49.5.Locking portions of a mapped file
11.49.6.Creating a very large file using mapping
11.49.7.Read file upside/down with RandomAccessFile
11.49.8.Mapping an entire file into memory for reading