确保类的单个实例的正确方法是什么?[复制品]

在Java中,我会创建这样的东西:

private static MyClass instance;

public static MyClass getInstance() {
  if(instance != null) {
    return instance;
  }
  instance = new MyClass();
  return instance;
}

在Ruby中获得相同功能的适当方法是什么?

更新:我已经读过“include singleton”,但是当我在Ruby1.9的IRB中尝试这样做时,我得到了:

[vertis@raven:~/workspace/test]$ ruby -v
ruby 1.9.1p378 (2010-01-10 revision 26273) [i386-darwin9.4.0]
[vertis@raven:~/workspace/test]$ irb
irb(main):001:0> class TestTest
irb(main):002:1>   include Singleton
irb(main):003:1> end
NameError: uninitialized constant TestTest::Singleton
    from (irb):2:in `<class:TestTest>'
    from (irb):1
    from /usr/local/bin/irb:12:in `<main>'