Singleton pattern in Ruby

The following is the singleton pattern in Ruby:

ruby 代码

class Singleton
    private_class_method :new
    @@instance = nil


    def Singleton.instance
        if(@instance == nil)
            @instance = new
        end
        return @instance
    end
end

and you can define two instances of this class by the instance method like this:

ruby 代码

s = Singleton.instance
s2 = Singleton.instance

then you can use this code to test whether the two instances are the same reference or not:

ruby 代码

puts s.equal?(s2)

If you run these codes,you can get 'true' in your console.

But there is a problem,this class can't work in mutl-thread environment,is it right?