Memory-Mapped Files in Java

try (FileChannel ch = FileChannel.open(path, READ, WRITE)) {
    MappedByteBuffer map = ch.map(FileChannel.MapMode.READ_WRITE, 0, size);
    map.put(index, value);       // writes go straight to the mapped file
}

import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
 
public class Mapped {
    public static void main(String[] args) throws IOException {
        Path file = Path.of("counters.dat");
        int counters = 4;
 
        try (FileChannel ch = FileChannel.open(file,
                StandardOpenOption.CREATE, StandardOpenOption.READ,
                StandardOpenOption.WRITE)) {
            MappedByteBuffer map = ch.map(
                FileChannel.MapMode.READ_WRITE, 0, counters * 4L);
 
            for (int round = 0; round < 1000; round++) {
                int slot = round % counters;
                map.putInt(slot * 4, map.getInt(slot * 4) + 1);
            }
            map.force();
            System.out.println("File size: " + ch.size() + " bytes");
        }
 
        try (FileChannel ch = FileChannel.open(file)) {
            MappedByteBuffer map = ch.map(FileChannel.MapMode.READ_ONLY,
                                          0, ch.size());
            StringBuilder values = new StringBuilder();
            for (int i = 0; i < counters; i++) {
                values.append(map.getInt(i * 4)).append(' ');
            }
            System.out.println("Counters: " + values.toString().strip());
        }
    }
}