Class Variables and initialisation time

Question

If I create a class variable like so:

class Song
  @@plays = 0

  class << self

    def plays=( plays )
      @@plays += plays
    end

    def plays
      @@plays
    end

  end

end

An I have multiple threads accessing this class method and setting it in jruby:

t1 = Thread.new {st1 = Song.plays = 1}
t2 = Thread.new {st2 = Song.plays = 5}
t3 = Thread.new {st3 = Song.plays = 3}

Is it possible to have 2 threads initialise @@plays to 0 at the same time? At what stage in execution are class variables created?

Answer 1

@@plays = 0 is set when Ruby evaluates your class definition. This should only happen once and before starting your threads.

The assignment method plays= on the other hand can be executed concurrently. You should therefore wrap it in a synchronize call, e.g.:

require 'thread'
require 'song'   # <- @@plays is set to 0 here

Song.plays #=> 0

semaphore = Mutex.new

t1 = Thread.new { semaphore.synchronize { Song.plays = 1 } }
t2 = Thread.new { semaphore.synchronize { Song.plays = 5 } }
t3 = Thread.new { semaphore.synchronize { Song.plays = 3 } }

[t1, t2, t3].each(&:join)
Song.plays #=> 9

Another option is to make Song#plays= thread-safe by moving the mutex into the Song class:

class Song
  @@plays = 0
  @@semaphore = Mutex.new

  def self.plays=(plays)
    @@semaphore.synchronize { @@plays += plays }
  end

  # ...
end