public class Singleton
{
	private Singleton(){ }
	
	public static Singleton getInstance()
	{
		return Nested.instance;		
	}
	
	//在第一次被引用時被載入
	static class Nested
	{
		private static Singleton instance = new Singleton();
	}
	
	public static void main(String args[])
	{
		Singleton instance = Singleton.getInstance();
		Singleton instance2 = Singleton.getInstance();
		System.out.println(instance == instance2);
	}
}
 

總結：

      由於內部靜態類只會被載入一次，故該實現方式時執行緒安全的！