Write/Append to File with ByteBuffer

With this is example we are going to demonstrate how to write/append data to a file in Java using a ByteBuffer. Particularly we are going to read data from a source file and append them to the destination file. In short what we do is the following :

• Create a File object to encapsulate an actual file in the file system that we want to read data from
• Allocate a direct (memory-mapped) byte buffer with a byte capacity equal to input file’s length. To do so we use the allocateDirect(int) API method of the ByteBuffer class
• We create an InputStream of the source file so as to read data and put them in the newly created ByteBuffer
• In order to append the data in the ButeBuffer to the destination file all we have to do is create a FileChannel for the destination file and use the write(ByteBuffer) API method of the FileChannel instance providing the ByteBuffer as the input attribute

as described in the code snippet below.

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package com.javacodegeeks.snippets.core;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class WriteAppendToFileWithByteBuffer {

	public static void main(String[] args) {

		try {

			File inFile = new File("in.xml");

			// Allocate a direct (memory-mapped) byte buffer with a byte capacity equal to file's length
			// DO NOT use this approach for copying large files
		    ByteBuffer buf = ByteBuffer.allocateDirect((int)inFile.length());

			InputStream is = new FileInputStream(inFile);

		    int b;

		    while ((b=is.read())!=-1) {
		    	buf.put((byte)b);
		    }

			File file = new File("out.xml");

			// append or overwrite the file
			boolean append = false;

			FileChannel channel = new FileOutputStream(file, append).getChannel();

			// Flips this buffer.  The limit is set to the current position and then
			// the position is set to zero.  If the mark is defined then it is discarded.
			buf.flip();

			// Writes a sequence of bytes to this channel from the given buffer.
			channel.write(buf);

			// close the channel
			channel.close();

		}
		catch (IOException e) {
			System.out.println("I/O Error: " + e.getMessage());
		}

	}

}