Java - File Input Output SeekableByteChannel

Introduction

SeekableByteChannel can random access a file using the channel API.

You can create a SeekableByteChannel object for a Path using the newByteChannel() method of the Files class.

Path src = Paths.get("data.txt");
SeekableByteChannel seekableChannel = Files.newByteChannel(src, READ, WRITE, CREATE, TRUNCATE_EXISTING);

The following code uses a SeekableByteChannel to Read Data from and Write Data to a File

Demo

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class Main {
  public static void main(String[] args) throws Exception {
    Path src = Paths.get("data.txt");
    // Get the file encoding for the system
    String encoding = System.getProperty("file.encoding");
    Charset cs = Charset.forName(encoding);

    try (SeekableByteChannel seekableChannel = Files.newByteChannel(src, StandardOpenOption.READ,
        StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING)) {

      // Print the details
      printDetails(seekableChannel, "Before writing data");

      // First, write some data to the file
      writeData(seekableChannel, cs);//from w  w  w .j a v  a  2  s  .  c o m

      // Print the details
      printDetails(seekableChannel, "After writing data");

      // Reset the position of the seekable channel to 0,
      // so we can read the data from the beginning
      seekableChannel.position(0);

      // Print the details
      printDetails(seekableChannel, "After resetting position to 0");

      // Read the data from the file
      readData(seekableChannel, cs);

      // Print the details
      printDetails(seekableChannel, "After reading data");
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  public static void writeData(SeekableByteChannel seekableChannel, Charset cs) throws IOException {
    // Get the platform-dependent line separator
    String separator = System.getProperty("line.separator");

    // Prepare the text to write to the file
    StringBuilder sb = new StringBuilder();
    sb.append("Data");
    sb.append(separator);
    sb.append("data");
    sb.append(separator);
    sb.append("data,");
    sb.append(separator);
    sb.append("data");
    sb.append(separator);
    // Wrap the text into a char buffer
    CharBuffer charBuffer = CharBuffer.wrap(sb);

    // Encode the char buffer data into a byte buffer
    ByteBuffer byteBuffer = cs.encode(charBuffer);

    // Write the data to the file
    seekableChannel.write(byteBuffer);
  }

  public static void readData(SeekableByteChannel seekableChannel, Charset cs) throws IOException {
    ByteBuffer byteBuffer = ByteBuffer.allocate(128);

    while (seekableChannel.read(byteBuffer) > 0) {
      byteBuffer.rewind();
      CharBuffer charBuffer = cs.decode(byteBuffer);
      System.out.print(charBuffer);
      byteBuffer.flip();
    }
  }

  public static void printDetails(SeekableByteChannel seekableChannel, String msg) throws Exception {

    System.out.println(msg + ": Size = " + seekableChannel.size() + ", Position = " + seekableChannel.position());

  }
}

Result