As we know that Java NIO is a more optimized API for data IO operations as compared to the conventional IO API of Java.One more additional support which Java NIO provides is to read/write data from/to multiple buffers to channel.This multiple read and write support is termed as Scatter and Gather in which data is scattered to multiple buffers from single channel in case of read data while data is gathered from multiple buffers to single channel in case of write data.

In order to achieve this multiple read and write from channel there is ScatteringByteChannel and GatheringByteChannel API which Java NIO provides for read and write the data as illustrate in below example.

GatheringByteChannel

write to multiple channels − In this we made to write data from multiple buffers into a single channel.For this again multiple buffers are allocated and are added to a buffer type array.Then this array is passed as parameter to the GatheringByteChannel write() method which then writes data from the multiple buffers in the sequence the buffers occur in the array.One point to remember here is only the data between the position and the limit of the buffers are written.

The following example shows how data gathering is performed in Java NIO

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.GatheringByteChannel;

public class GatherExample {
   private static String FILENAME = "C:/Test/temp.txt";
   public static void main(String[] args) {
      String stream1 = "Gather data stream first";
      String stream2 = "Gather data stream second";
      ByteBuffer bLen1 = ByteBuffer.allocate(1024);
      ByteBuffer bLen2 = ByteBuffer.allocate(1024);
      // Next two buffer hold the data we want to write
      ByteBuffer bstream1 = ByteBuffer.wrap(stream1.getBytes());
      ByteBuffer bstream2 = ByteBuffer.wrap(stream2.getBytes());
      int len1 = stream1.length();
      int len2 = stream2.length();
      // Writing length(data) to the Buffer
      bLen1.asIntBuffer().put(len1);
      bLen2.asIntBuffer().put(len2);
      System.out.println("Gathering : Len1 = " + len1);
      System.out.println("Gathering : Len2 = " + len2);
      // Write data to the file
      try { 
         FileOutputStream out = new FileOutputStream(FILENAME);
         GatheringByteChannel gather = out.getChannel();						
         gather.write(new ByteBuffer[] {bLen1, bLen2, bstream1, bstream2});
         out.close();
         gather.close();
      }
      catch (FileNotFoundException exObj) {
         exObj.printStackTrace();
      }
      catch(IOException ioObj) {
         ioObj.printStackTrace();
      }
   }
}

Output

Gathering : Len1 = 24
Gathering : Len2 = 25

In last it can be concluded that scatter/gather approach in Java NIO is introduced as an optimized and multitasked when used properly.It allows you to delegate to the operating system the grunt work of separating out the data you read into multiple buckets, or assembling disparate chunks of data into a whole.No doubt this saves time and uses operating system more efficiently by avoiding buffer copies, and reduces the amount of code need to write and debug.