Demonstrate class variable sharing across threads

threaded-class-variable-usage-test-script.txt

$ ruby threaded-class-variable-usage-test.rb
#<Thread:0x007f8a329d6ea8>: New state = #<Thread:0x007f8a329d6ea8>
#<Thread:0x007f8a329d6fe8>: New state = #<Thread:0x007f8a329d6fe8>
#<Thread:0x007f8a329d7240>: New state = #<Thread:0x007f8a329d7240>
#<Thread:0x007f8a329d7128>: New state = #<Thread:0x007f8a329d7128>
#<Thread:0x007f8a329d5eb8>: New state = #<Thread:0x007f8a329d5eb8>
#<Thread:0x007f8a329d6fe8>: State changed! my_name = #<Thread:0x007f8a329d6fe8>; Foo.state = #<Thread:0x007f8a329d5eb8> ; checks = 1
#<Thread:0x007f8a329d7240>: State changed! my_name = #<Thread:0x007f8a329d7240>; Foo.state = #<Thread:0x007f8a329d5eb8> ; checks = 1
#<Thread:0x007f8a329d7128>: State changed! my_name = #<Thread:0x007f8a329d7128>; Foo.state = #<Thread:0x007f8a329d5eb8> ; checks = 1
#<Thread:0x007f8a329d6ea8>: State changed! my_name = #<Thread:0x007f8a329d6ea8>; Foo.state = #<Thread:0x007f8a329d5eb8> ; checks = 2
#<Thread:0x007f8a329d5eb8>: All done (5 checks)

threaded-class-variable-usage-test.rb

#!/usr/bin/env ruby
# This shows how a class variable is shared across threads
# Setting a class variable in thread A will cause it to be
# changed in thread B.

THREAD_COUNT = 5
CHECK_TIMES = 5

class Foo
  @@state = nil

  def self.set_state(val)
    @@state = val
  end

  def self.state
    @@state
  end

end

def my_name
  (Thread.current.name || Thread.current.to_s).sub(/@.*$/, '')
end

def do_stuff
  Foo.set_state(my_name)
  show "New state = #{Foo.state}"
  error = false
  checks = 0
  (1..CHECK_TIMES).each do |num|
    checks += 1
    if Foo.state != my_name
      show "State changed! my_name = #{my_name}; Foo.state = #{Foo.state} ; checks = #{checks}"
      error = true
      break
    end
    sleep 0.5
  end
  show "All done (#{checks} checks)" unless error
end

def show(msg)
  printf "%s: %s\n", my_name, msg
end

threads = []

THREAD_COUNT.times do
  threads << Thread.new { do_stuff }
end

threads.each { |thr| thr.join }

exit

The last thread to set the shared state class variable with it's name "wins".