public class SingletonTest {    private SingletonTest() {    }    public static SingletonTest getInstance() {        return Singleton.INSTANCE.getInstance();    }    private enum Singleton {        INSTANCE;        private SingletonTest singleton;        // JVM保证这个方法绝对只调用一次        Singleton() {            singleton = new SingletonTest();        }        public SingletonTest getInstance() {            return singleton;        }    }}