# The Singleton class defines the `instance` method that lets clients access the # unique singleton instance. class Singleton @instance = new private_class_method :new # The static method that controls the access to the singleton instance. # # This implementation let you subclass the Singleton class while keeping just # one instance of each subclass around. def self.instance @instance end # Finally, any singleton should define some business logic, which can be # executed on its instance. def some_business_logic # ... end end # The client code. s1 = Singleton.instance s2 = Singleton.instance if s1.equal?(s2) print 'Singleton works, both variables contain the same instance.' else print 'Singleton failed, variables contain different instances.' end
output.txt: 実行結果
Singleton works, both variables contain the same instance.
スレッド・セーフなシングルトン
問題を可決するには、最初のシングルトン・オブジェクトの生成の間、スレッドを同期する必要があります。
main.rb: 概念的な例
# The Singleton class defines the `instance` method that lets clients access the # unique singleton instance. class Singleton attr_reader :value @instance_mutex = Mutex.new private_class_method :new def initialize(value) @value = value end # The static method that controls the access to the singleton instance. # # This implementation let you subclass the Singleton class while keeping just # one instance of each subclass around. def self.instance(value) return @instance if @instance @instance_mutex.synchronize do @instance ||= new(value) end @instance end # Finally, any singleton should define some business logic, which can be # executed on its instance. def some_business_logic # ... end end # @param [String] value def test_singleton(value) singleton = Singleton.instance(value) puts singleton.value end # The client code. puts "If you see the same value, then singleton was reused (yay!)\n"\ "If you see different values, then 2 singletons were created (booo!!)\n\n"\ "RESULT:\n\n" process1 = Thread.new { test_singleton('FOO') } process2 = Thread.new { test_singleton('BAR') } process1.join process2.join
output.txt: 実行結果
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO FOO
冬のセール! 
