gistfile1.txt

~/projects/jruby ➔ jruby memory_mapped.rb 
Reading and writing memory-mapped
  2.355000   0.000000   2.355000 (  2.299000)
  1.943000   0.000000   1.943000 (  1.942000)
  2.014000   0.000000   2.014000 (  2.014000)
  1.899000   0.000000   1.899000 (  1.899000)
  1.870000   0.000000   1.870000 (  1.870000)
Reading from disk
  2.982000   0.000000   2.982000 (  2.982000)
  3.031000   0.000000   3.031000 (  3.031000)
  3.027000   0.000000   3.027000 (  3.027000)
  3.404000   0.000000   3.404000 (  3.405000)
  3.775000   0.000000   3.775000 (  3.775000)

memory_mapped.rb

require 'java'
require 'benchmark'
require 'fileutils'

java_import java.nio.channels.FileChannel

# copy jruby.jar to a tmp location
FileUtils.cp 'lib/jruby.jar', '/tmp/jruby.jar'
tmp_file = File.open('/tmp/jruby.jar', 'r+')

# get the channel and memory map
channel = tmp_file.to_channel
map = channel.map FileChannel::MapMode::READ_WRITE, 0, channel.size
map.load # load into memory

# perform many reads and writes
puts "Reading and writing memory-mapped"
5.times {
  puts Benchmark.measure {
    100.times {
      bytes = Java::byte[channel.size].new
      map.position 0
      map.get bytes
      map.position 0
      map.put bytes
    }
  }
}

puts "Reading and writing disk"
5.times {
  puts Benchmark.measure {
    100.times {
      tmp_file.seek 0
      str = tmp_file.read
      tmp_file.seek 0
      tmp_file.write str
    }
  }
}

If I'm trying to randomly access portions of a large file (2-10GB) with JRuby, would there be a big incentive to leverage a method like this over the standard Ruby IO library? Isn't that using NIO under the covers?