Fastest way of reading relatively huge byte-files in Java

Question

what's the probably fastest way of reading relatively huge files with Java's I/O-methods? My current solution uses the BufferedInputStream saving to an byte-array with 1024 bytes allocated to it. Each buffer is than saved in an ArrayList for later use. The whole process is called via a separate thread (callable-interface).

Not very fast though.

    ArrayList<byte[]> outputArr = new ArrayList<byte[]>();      
    try {
        BufferedInputStream reader = new BufferedInputStream(new FileInputStream (dir+filename));

        byte[] buffer = new byte[LIMIT]; // == 1024 
            int i = 0;
            while (reader.available() != 0) {
                reader.read(buffer);
                i++;
                if (i <= LIMIT){
                    outputArr.add(buffer);
                    i = 0;
                    buffer = null;
                    buffer = new byte[LIMIT];
                }
                else continue;              
            }

         System.out.println("FileReader-Elements: "+outputArr.size()+" w. "+buffer.length+" byte each.");   

Answer 1

I would use a memory mapped file which is fast enough to do in the same thread.

final FileChannel channel = new FileInputStream(fileName).getChannel();
MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

// when finished
channel.close();

This assumes the file is smaller than 2 GB and will take 10 milli-seconds or less.

Answer 2

Don't use available() : it's not reliable. And don't ignore the result of the read() method: it tells you how many bytes were actually read. And if you want to read everything in memory, use a ByteArrayOutputStream rather than using a List<byte[]> :

ByteArrayOutputStream baos = new ByteArrayOutputStream();
int read;
while ((read = reader.read(buffer)) >= 0) {
    baos.write(buffer, 0, read);
}
byte[] everything = baos.toByteArray();

I think 1024 is a bit small as a buffer size. I would use a larger buffer (something like 16 KB or 32KB)

Note that Apache commons IO and Guava have utility methods that do this for you, and have been optimized already.

Answer 3

Have a look at Java NIO (Non-Blocking Input/Output) API. Also, this question might prove being useful.

I don't have much experience with IO, but I've heard that NIO is much more efficient way of handling large sets of data.