java - 还有一个关于 "Double-Checked-Locking"的问题

Question

我根据“effect java”#83修改了“正常”DCL单例情况，如下。

import java.util.Date;

public class MySystem {
    private Date date = new Date();

    private MySystem() {};

    private static volatile MySystem instance;

    public Date getDate() {
        return date;
    }

    public static MySystem getInstance() {
        MySystem my = instance;
        if (my == null) {
            synchronized (MySystem.class) {
                if (instance == null) {
                    instance = my = new MySystem();
                }
            }
        }
        return my;
    }
}

但是当我运行它时，抛出NullpointerException的比例非常高。当我按如下方式修改它时，一切正常。为什么？

import java.util.Date;

public class MySystem {
    private Date date = new Date();

    private MySystem() {};

    private static volatile MySystem instance;

    public Date getDate() {
        return date;
    }

    public static MySystem getInstance() {
        MySystem my = instance;
        if (my == null) {
            synchronized (MySystem.class) {
                my = instance;
                if (my == null) {
                    instance = my = new MySystem();
                }
            }
        }
        return my;
    }
}

主要内容如下。很难弄清楚其中的区别。

public class Main {
    public static void main(String[] args) {
        new Thread() {
            public void run() {
                System.out.println(MySystem.getInstance().getDate());
            }
        }.start();

        new Thread() {
            public void run() {
                System.out.println(MySystem.getInstance().getDate());
            }
        }.start();
    }
}

Answer 1

当发生以下情况时，您会得到 NPE:

public static MySystem getInstance() {
    MySystem my = instance;
    if (my == null) {                            // (1) instance was null => my is null and synchronized block is entered.
        synchronized (MySystem.class) {
            if (instance == null) {              // (2) instance was updated from another thread and is not null anymore.
                instance = my = new MySystem();
            }
        }
    }
    return my;
}

您会注意到，在这种情况下，引用实例不会复制到my中，它仍为null。您可以尝试以下方法来验证:

public static MySystem getInstance() {
    MySystem my = instance;
    if (my == null) {
        synchronized (MySystem.class) {
            if (instance == null) {
                instance = new MySystem();
            }
        }
        my = instance;
    }
    return my;
}