以下关于Atomic和volatile的理解是否正确

Question

public class DoubleCheckVolatile {
    private volatile static DoubleCheckVolatile doubleCheckVolatile;

    public static DoubleCheckVolatile getDoubleCheckVolatile() {
        if (doubleCheckVolatile != null)
            return doubleCheckVolatile;
        synchronized (DoubleCheckVolatile.class) {
            if (doubleCheckVolatile != null)
                return doubleCheckVolatile;
            synchronized (DoubleCheckVolatile.class) {
                doubleCheckVolatile = new DoubleCheckVolatile();
                return doubleCheckVolatile;
            }
        }
    }

    private DoubleCheckVolatile() {
    }
}



public class DoubleCheckAtomic {
    private static AtomicReference<DoubleCheckAtomic> doubleCheckAtomicAtomicRef = new AtomicReference<>();

    public static DoubleCheckAtomic getDoubleCheckAtomic() {
        if (doubleCheckAtomicAtomicRef.get() != null)
            return doubleCheckAtomicAtomicRef.get();
        synchronized (DoubleCheckAtomic.class) {
            if (doubleCheckAtomicAtomicRef.get() != null)
                return doubleCheckAtomicAtomicRef.get();
            synchronized (DoubleCheckAtomic.class) {
                doubleCheckAtomicAtomicRef.set(new DoubleCheckAtomic());
                return doubleCheckAtomicAtomicRef.get();
            }
        }
    }

    private DoubleCheckAtomic() {
    }
}

这两个类的做法是否能都能防止对象初始化步骤被指令重排后导致的并发危险？

Answer 1

并发主要解决三个问题，CPU切换线程执导致的原子性问题，缓存导致的可见性问题，指令优化导致的重排序问题

原子类解决的是原子性问题，volatile解决的是变量可见性、禁止重排序；所以你这可以解决指令重排序，但是写的有些繁琐，一般写法如下：

public class DoubleCheckVolatile {
    private volatile static DoubleCheckVolatile doubleCheckVolatile;
 
    public static DoubleCheckVolatile getDoubleCheckVolatile() {
        if (doubleCheckVolatile == null) {
            synchronized (DoubleCheckVolatile.class) {
                if (doubleCheckVolatile == null) {
                    doubleCheckVolatile = new DoubleCheckVolatile();
                }
            }
        }
    }
 
    private DoubleCheckVolatile() {
    }
}