import java.io.*;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.function.Consumer;

public class LockedFile {

    private String filename;

    public LockedFile(String filename){
        this.filename = filename;
    }

    public List<String> read(){
        List<String> lines = new ArrayList<>();

        lockFile(filename, fileChannel -> {
            try(Scanner scanner = new Scanner(fileChannel)){
                while (scanner.hasNextLine()){
                    lines.add(scanner.nextLine());
                }
            }
        });

        return lines;
    }

    public void write(List<String> lines){
        lockFile(filename, fileChannel -> {
            // point to EOF
            try {
                fileChannel.position(fileChannel.size());
            } catch (IOException e) {
                e.printStackTrace();
            }

            for(String line : lines){
                try {
                    fileChannel.write(ByteBuffer.wrap((line + "\r\n").getBytes()));
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
    }

    private synchronized void lockFile(String filename, Consumer<FileChannel> fileChannelConsumer){
        File file = new File(filename);
        try (FileChannel fileChannel = new RandomAccessFile(file, "rw").getChannel()){

            // blocking call - throws when lock can't be acquired
            fileChannel.lock();

            fileChannelConsumer.accept(fileChannel);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}