002-支持并发的内部类饿汉单例

public class HungryInnerSingletion {
      // 类级的静态的成员式内部类,该内部类的实例和外部类的实例没有绑定关系,而且只有被调用的时候才会被装载,从而实现了延迟加载
     private static class Inner {
           // 静态初始化器, 由JVM来保证线程安全
            private static HungryInnerSingletion single = new HungryInnerSingletion();
     }

     public static HungryInnerSingletion getInstance(){
           return Inner.single;
     }

     private HungryInnerSingletion(){}

}