单例模式实现

静态内部类(static nested class) 优先考虑

public class LazySingleton{
    private LazySingleton(){}

    private static class Nested{
        private static final LazySingleton single = new LazySingleton();
    }

    public static LazySingleton getInstance(){
        return Nested.single;
    }
}

双重检查锁定(DCL)

public class LazySingleton{
    private LazySingleton(){}

    private static volatile LazySingleton single = null;

    public static LazySingleton getInstance(){
        if(single == null){
            synchronized (LazySingleton.class){
                if(single == null){
                    single = new LazySingleton();   //① 非原子操作
                }
            }
        }
        return single;
    }
}