Three types of memory-mapped buffers

Introduction

FileChannel.MapMode.READ_ONLY
FileChannel.MapMode.READ_WRITE
FileChannel.MapMode.PRIVATE
Demo Code

  
import java.io.File;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class Main {
  public static void main(String[] argv) throws Exception {
    File tempFile = File.createTempFile("test", null);
    RandomAccessFile file = new RandomAccessFile(tempFile, "rw");
    FileChannel channel = file.getChannel();
    ByteBuffer temp = ByteBuffer.allocate(100);

    temp.put("This is the file content".getBytes());
    temp.flip();/*from  w  w  w .j av  a  2  s  . c o m*/
    channel.write(temp, 0);

    temp.clear();
    temp.put("This is more file content".getBytes());
    temp.flip();
    channel.write(temp, 8192);

    MappedByteBuffer ro = channel.map(FileChannel.MapMode.READ_ONLY, 0,
        channel.size());
    MappedByteBuffer rw = channel.map(FileChannel.MapMode.READ_WRITE, 0,
        channel.size());
    MappedByteBuffer cow = channel.map(FileChannel.MapMode.PRIVATE, 0,
        channel.size());

    System.out.println("Begin");
    showBuffers(ro, rw, cow);
    cow.position(8);
    cow.put("test".getBytes());
    showBuffers(ro, rw, cow);
    rw.position(9);
    rw.put(" 11 ".getBytes());
    rw.position(8194);
    rw.put(" 22 ".getBytes());
    rw.force();
    showBuffers(ro, rw, cow);


    temp.clear();
    temp.put("Channel write ".getBytes());
    temp.flip();
    channel.write(temp, 0);
    temp.rewind();
    channel.write(temp, 8202);

    showBuffers(ro, rw, cow);
    cow.position(8207);
    cow.put(" COW2 ".getBytes());

    showBuffers(ro, rw, cow);

    rw.position(0);
    rw.put(" 22 ".getBytes());
    rw.position(8210);
    rw.put(" 22 ".getBytes());
    rw.force();

    showBuffers(ro, rw, cow);

    channel.close();
    file.close();
    tempFile.delete();
  }
  public static void showBuffers(ByteBuffer ro, ByteBuffer rw, ByteBuffer cow)
      throws Exception {
    dumpBuffer("R/O", ro);
    dumpBuffer("R/W", rw);
    dumpBuffer("COW", cow);
    System.out.println("");
  }
  public static void dumpBuffer(String prefix, ByteBuffer buffer)
      throws Exception {
    System.out.print(prefix + ": '");
    int nulls = 0;
    int limit = buffer.limit();
    for (int i = 0; i < limit; i++) {
      char c = (char) buffer.get(i);
      if (c == '\u0000') {
        nulls++;
        continue;
      }
      if (nulls != 0) {
        System.out.print("|[" + nulls + " nulls]|");
        nulls = 0;
      }
      System.out.print(c);
    }
    System.out.println("'");
  }
}
Three types of memory-mapped buffers - Java File Path IO

Introduction

Demo Code

Result

Related Tutorials
