JavaNIOChannelsBuffer1.java

package org.sydlabz;

import java.io.IOException;
import java.nio.Buffer;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * @author Seyed Sahil
 */
public class Main {
    public static void main(String[] args) throws IOException {
        Path otpFilePath = Paths.get("otp.txt");

        // Allocating a byte buffer of capacity 12, by default capacity and limit will be same.
        ByteBuffer otpBuffer = ByteBuffer.allocate(12);
        printBuffer("After allocation", otpBuffer);

        try (SeekableByteChannel otpFileChannel = Files.newByteChannel(otpFilePath)) {
            System.out.println("Stage 1");

            // Reading data from file to buffer. This will change the position of buffer.
            int numberOfBytes = otpFileChannel.read(otpBuffer);
            System.out.println("Number of bytes read from file is '" + numberOfBytes + "'");
            printBuffer("After loading data", otpBuffer);

            // Rewind is required to read data from buffer. This will reset the position back to zero.
            otpBuffer.rewind();
            printBuffer("After rewind", otpBuffer);

            // Reading 6 digits from the buffer for generating the OTP. This will change the position of buffer.
            byte[] nextOtp = new byte[otpBuffer.capacity() / 2];
            otpBuffer.get(nextOtp);
            System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            printBuffer("After reading OTP", otpBuffer);

            System.out.println("Stage 2");

            // Flip will change the limit of buffer to position of buffer and set position of buffer back to zero.
            otpBuffer.flip();
            printBuffer("After flip", otpBuffer);
            otpBuffer.get(nextOtp);
            System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            printBuffer("After reading OTP", otpBuffer);

            // The following three lines of code wll throw exception, because we are trying to read beyond the limits.
            // otpBuffer.get(nextOtp);
            // System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            // printBuffer("After reading OTP", otpBuffer);

            System.out.println("Stage 3");

            // Setting the limit back to capacity and this let us read the remaining buffer.
            otpBuffer.limit(otpBuffer.capacity());
            printBuffer("After increasing limit", otpBuffer);
            otpBuffer.get(nextOtp);
            System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            printBuffer("After reading OTP", otpBuffer);

            System.out.println("Stage 4");

            // Here the number of bytes read from file will be zero here.
            // This is because the position of buffer is at limit which is 12.
            numberOfBytes = otpFileChannel.read(otpBuffer);
            System.out.println("After loading data to buffer, caret position in file is '" + numberOfBytes + "'");
            printBuffer("After new file read", otpBuffer);

            // Rewind the buffer to set position bvack to zero
            otpBuffer.rewind();
            printBuffer("After rewind", otpBuffer);
            otpBuffer.get(nextOtp);
            System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            printBuffer("After reading OTP", otpBuffer);

            // Flip the limits and position.
            otpBuffer.flip();
            printBuffer("After flip", otpBuffer);

            // Now the limit is half of the capacity of buffer, the number of bytes read should be 6.
            numberOfBytes = otpFileChannel.read(otpBuffer);
            System.out.println("Number of bytes read from file is '" + numberOfBytes + "'");
            printBuffer("After new file read", otpBuffer);

            // We should get a fresh OTP here.
            otpBuffer.rewind();
            otpBuffer.get(nextOtp);
            System.out.println("New OTP '" + new String(nextOtp) + "' created.");
            printBuffer("After reading OTP", otpBuffer);
        }
    }

    private static void printBuffer(String message, Buffer buffer) {
        System.out.println(message + " - Buffer[capacity=" + buffer.capacity() + ", limit=" + buffer.limit() + ", " +
                "position=" + buffer.position() + "]");
    }
}