Java NIO FileChannel transferTo() and transferFrom()

As in normal Java applications where IO happens mostly between an input source and output target, in NIO as well we may need to transfer data from one channel to another channel very frequently.

Bulk transfers of file data from one place to another is so common that a couple of optimization methods have been added to the FileChannel class to make it even more efficient.

1. Data Transfer between Channels

Java NIO provides two methods for transferring the data between two channels:

• FileChannel.transferTo()
• FileChannel.transferFrom()

The transferTo() and transferFrom() methods allow us to cross-connect one channel to another. This eliminates the need to pass the data through an intermediate buffer.

These methods exist only on the FileChannel class, so one of the channels involved in a channel-to-channel transfer must be a FileChannel.

public abstract class FileChannel
        extends AbstractChannel
        implements ByteChannel, GatheringByteChannel, ScatteringByteChannel
{
        // There are more other methods
        public abstract long transferTo (long position, long count, WritableByteChannel target);
        public abstract long transferFrom (ReadableByteChannel src, long position, long count);
}

We can’t do direct transfers between socket channels, but socket channels implement WritableByteChannel and ReadableByteChannel, so the content of a file can be transferred to a socket with transferTo(), or data can be read from a socket directly into a file with transferFrom().

Also, keep in mind that these methods may throw java.io.IOException if any error is encountered during the transfer.

Channel-to-channel transfers can potentially be extremely fast, especially where the underlying operating system provides native support. Some operating systems can perform direct transfers without ever passing the data through user-space. This can be a huge win for high-volume data transfer.

2. FileChannel transferTo() and transferFrom() Example

In this example, we are reading the file content from 3 different files and writing their combined output into a fourth file.

package com.howtodoinjava.nio;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.channels.FileChannel;
import java.nio.channels.WritableByteChannel;

public class ChannelTransferExample 
{
	public static void main(String[] argv) throws Exception 
	{
		//Input files
		String[] inputFiles = new String[]{"inputFile1.txt","inputFile2.txt","inputFile3.txt"};
		
		//Files contents will be written in these files
		String outputFile = "outputFile.txt";
		
		//Get channel for output file
		FileOutputStream fos = new FileOutputStream(new File(outputFile));
		WritableByteChannel targetChannel = fos.getChannel();
		
		for (int i = 0; i < inputFiles.length; i++)
		{
			//Get channel for input files
			FileInputStream fis = new FileInputStream(inputFiles[i]);
			FileChannel inputChannel = fis.getChannel();

			//Transfer data from input channel to output channel
			inputChannel.transferTo(0, inputChannel.size(), targetChannel);

			//close the input channel
			inputChannel.close();
			fis.close();
		}
		
		//finally close the target channel
		targetChannel.close();
		fos.close();
	}
}

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Happy Learning !!