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How can I use ReadableByteChannel to get file contents and store it in byteBuffer?
File file = new File(fileName);
try {
    ReadableByteChannel channel = new FileInputStream(fileName).getChannel();
    ByteBuffer buf = ByteBuffer.allocateDirect(file.length());

    // How can use buf.read to get all the contents?

} catch (Exception e){

}

FileChannel channel = new FileInputStream(fileName).getChannel();
ByteBuffer buf = channel.map(MapMode.READ_ONLY,0,channel.size());