test.rb

t = Thread.new do
  class A
    sleep 10
    def self.a
    end
  end
  sleep 2
end


t2 = Thread.new do
  sleep 1
  p defined?(A)
  p A
  p A.respond_to?(:a)
  sleep 10
  p A.respond_to?(:a)
end

t.join
t2.join