懒汉式，线程不安全

//懒汉式，线程不安全
public class Singleton2 {
        private static  Singleton2 singleton;
        private Singleton2(){

        }

        public static Singleton2 getInstance(){
            if(singleton == null){
                singleton = new Singleton2();
            }
            return singleton;
        }

    public static void main(String[] args) {
        Singleton2 instance1 = Singleton2.getInstance();
        Singleton2 instance2 = Singleton2.getInstance();
        System.out.println(instance1 == instance2);
        System.out.println("****");
        System.out.println(instance1.hashCode());
        System.out.println(instance2.hashCode());
    }
}