Java: Concurrent reads on an InputStream

Question

Been looking around for a little while now and I\'m a bit confused on this issue. I want to be able to take an input stream and read it concurrently in segments. The segme

Answer 1

First of all, to read the file concurrently starting from different offsets you need random access to the file, this means reading a file from any position. Java allows this with RandomAccessFile in java.in or with SeekableByteChannel in java.nio:

Best Way to Write Bytes in the Middle of a File in Java

http://docs.oracle.com/javase/tutorial/essential/io/rafs.html

I think for the speed reasons you will prefer java.nio. Java NIO FileChannel versus FileOutputstream performance / usefulness

Now you know how to read from any position but you need to do this concurrently. It's not possible with the same file access object because they hold the position in the file. Thus you need as many file access objects as threads. Since you are reading not writing that should be Ok.

Now you know how to read the same file concurrently from many different offsets.

But think about the performance. Despite the number of threads you have only ONE disk drive and random reads (many threads access the same file) performance is much-much slower then sequential reads (one thread reads one file). Even if it's raid 0 or 1 - does not matter. Sequential reading is always much faster. So in you case I would advise you to read the file in one thread and supply other threads with the data from that reading thread.