设计模式之单例模式

public class  {
	private volatile static Singleton singleton;
	private () {
	}
	public static Singleton getSingleton() {
		if (singleton == null) {
			synchronized (Singleton.class) {
				if (singleton == null) {
					singleton = new Singleton();
				}
			}
		}
		return singleton;
	}
}

public class  {
	private static Singleton instance = new Singleton();
	private () {
	}
	public static Singleton getInstance() {
		return instance;
	}
}

public class  {
	private static class SingletonHolder{
		private static Singleton instance = new Singleton();
	}
	private Singleton() {
	}
	public static Singleton getInstance() {
		return SingletonHolder.instance;
	}
}