Java中对位置的内存访问无效

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

public class MemoryMappedFileExample {
    public static void main(String[] args) {
        try {
            File file = new File("example.txt");
            RandomAccessFile raf = new RandomAccessFile(file, "rw");
            FileChannel fc = raf.getChannel();
            MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size());
            mbb.put(0, (byte) 'H');
            mbb.put(1, (byte) 'e');
            mbb.put(2, (byte) 'l');
            mbb.put(3, (byte) 'l');
            mbb.put(4, (byte) 'o');
            fc.close();
            raf.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}