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 aFile 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 theallocateDirect(int) API method of theByteBuffer class
• We create anInputStream 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 aFileChannel for the destination file and use thewrite(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 fileboolean 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 channelchannel.close();}catch (IOException e) {System.out.println("I/O Error: " + e.getMessage());}}}

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