Java的“影子克隆”和“深度克隆”

今天来学习学习java对象的克隆，在写代码的时候，有时候我们会这样写：对象1=对象2，也就是把对象2赋值给对象1了，但是这样做有个问题，就是如果我们修改了对象2的属性值，对象1的相同属性值也被修改了，反过来亦如此，让我来证明一下：

public class A {
    private String msg;

    public A() {
        msg = "Hello";
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

public class Main {

    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new A();
        System.out.println("a1.msg: " + a1.getMsg());
        a1 = a2;
        a2.setMsg("World");
        System.out.println("a1.msg: " + a1.getMsg());
    }

}

运行结果：

aaarticlea/png;base64,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" alt="" />

从Main类可以看到，一开始我们new了两个对象，也就是在JVM堆内存开辟了两块内存空间，接着把a2赋值给a1,这就意味着a2和a1对象指向了栈内存中同一个引用，这个时候我们修改a2对象的属性，a1对象的属性也同时会改变，反过来亦如此，例如我现在修改a1的属性来试试看a2的属性是否被改变了：

public class Main {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new A();
        System.out.println("a1.msg: " + a1.getMsg());
        a1 = a2;
        a1.setMsg("Java");
        System.out.println("a2.msg: " + a2.getMsg());
    }
}

aaarticlea/png;base64,iVBORw0KGgoAAAANSUhEUgAAAKkAAAAyCAIAAACoFmz4AAADHklEQVR4nO2a2YGFIAxFqYuCqIcSpgqaoZg3Hy5AAsgmisn9mxERciDm4RW/mP5YBCSi7FkUxOzpitnTFbOnK2ZPVyPYWy2FEMoM6GptGSV2SW0rrz6gPvb7dKTW6hPsTzxS22NNi+p1bbXM0M1fnalBOd98g/0PzaRlYl9h79Z+NlnVhGjvUup9lynj8ofF7fY28AlhBj26tKhBbZwv2LvHCiGkTk2wib3f94ydVLfvtUyMqnZ7uBJhm7Ay2/9cNI3yHuUaubuPv6yWQkoczxvYWy39izYRjhb2oC+jJlQFlTnfahld7C3s9xvcrUHnRiUnjx62rZoxkQo2drgFY5M0Cj+4gX2km/tfo9fsg6SfSnQN7I9+EuzB68YLTWz9jXuHpve9hZFIped69tHw3Q7/gj3MagP3/SV70P7Efyt7kNYB+9TwUB/rs8ezfYp9ELI7c36GffEKa8r5KH5P5/zgpXvW5/GGqYGmavgse4gAjySsDIbVeln2uARD4zzn0VDrgec+Xuv5pY/UB//cKxAOGTHwbnKFvtTW7xx3DCIc/gCM7cgG9uDkDQw0OmVvJ+Aq0b8zfxW3mPAj7yPn+a85L1lJn2BvVOqYhZXRsuyDDPmR8+TJWpY9q1vMnq6YPV0xe7pi9nTF7OlqAHt30MU/spdSJ/vggHv7o39MzytyAPvBI4Rur2YYk9injZVV/N12RQ3y6x0q+fD4br8eGmvSThJ0Db12fsuth8pITtAgv96m8nP19/r10DjTM4LRCIa8N0g9/iKSUzTIr/dLfMrO9PNSvx4aZ2Y1o6tBCPIBecHbZJBfrzb6b/XrwWUVo5uJhp+6UBYoi+REjfDrNRiK3+rXy7O/joYbJ0Rf6nycqF6/XsqjfqG3+vUiFeU5pDL34v6uAqVPufNxorr8emF1tt8Q8RxGa/h3+vXQgUXo1itwLxolhJJwMRY7Hyeqy68X9aBdsn+vXw91nfAexqMRNkqNJ3/vVH3kPJ/9eg36BHv26zVpWfbs1+vWsuxZ3WL2dMXs6YrZ0xWzpytmT1fMnq6YPV0xe7pi9nT1DywNBMGxkHl4AAAAAElFTkSuQmCC" alt="" />

毫无悬念地改变了，也就是说使用“=”操作符会让两个对象指向同一个引用，两个对象的属性值是互通的，一个对象的改变必然会同步到另一个对象上面，但是有时候我们并不想一个对象的改变引起另一个对象的改变，我们希望a2对象被创建后含有a1对象初始的值，但是a2对象属性的改变不会影响a1对象，这样我们就应该使用对象的克隆，对象的克隆又分浅克隆和深克隆，我们先来看看浅克隆的实现方式：

public class A implements Cloneable {
    private String msg;

    public A() {
        msg = "Hello";
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

public class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        A a1 = new A();
        System.out.println("a1.msg: " + a1.getMsg());
        A a2 = (A) a1.clone();
        System.out.println("a2.msg: " + a2.getMsg());
        a2.setMsg("Java");
        System.out.println("a1.msg: " + a1.getMsg());
        System.out.println("a2.msg: " + a2.getMsg());
    }
}

aaarticlea/png;base64,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" alt="" />

可以看到实现对象的克隆比较简单，首先该类应该实现Cloneable接口并覆盖Object根类的clone方法，注意Cloneable只是一个标记接口，里面并没有定义任何方法，clone方法里面的实现也很简单，调用父类Object的clone方法就好了。现在看看Main类就知道，a2对象的msg初始化是"Hello",把a2的msg属性的值改为"Java"后，并没有影响a1对象的msg值，其值仍然是"Hello". 我们刚才看到的是浅克隆，没有集合属性，如果有集合属性的话，浅克隆就应付不了这种情况了，我们来看看代码：

public class A implements Cloneable {
    private List<String> list;

    public A() {
        list = new ArrayList<String>();
        list.add("Hello");
        list.add("World");
    }

    public List<String> getList() {
        return list;
    }

    public void setList(List<String> list) {
        this.list = list;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

public class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        A a1 = new A();
        System.out.println("a1.list: " + a1.getList());
        A a2 = (A) a1.clone();
        System.out.println("a2.list: " + a2.getList());
        a2.getList().add("Java");
        System.out.println("a1.list: " + a1.getList());
        System.out.println("a2.list: " + a2.getList());
    }
}

aaarticlea/png;base64,iVBORw0KGgoAAAANSUhEUgAAAU0AAABkCAIAAACNRvXZAAAHFUlEQVR4nO2d26GjIBRFrYuCrMcSpgqbsZjMR3INb/BAIpys9TMzUXmerYiwZ3nE+AcAiliiOgcATaBzAP2gcwD9oHMA/Yyu880sZjvuLgXA3PTQ+bGZZVnW/eJl+7q8yCh5TJ0/a7xIap0i3xpVbTUVF2r0bGz7tPNiLa3xcdp0/mpvs22rOOKPzfTvrWMzZuucpp+8V2Y39qR3gnxrfKStEpwVOKsQ/tJOZY2ip32zNWan07h9R+ePoBUkjTKOzlPl71sEdP4dSjo/rJt4ZpjUX+elcd3hPF2s/N8XWgSFO0ciojiR6dwpWeI2JNW5nXavh+27/Pv6LrBbhHS+f0fW/d1ZF7RqJWy2A503cu15vplEFH3seZ65C5wZPkPCyb/4PP+6zo/N2AePRFNKWsNL6ynKHvF/5mZNwFiVqsh3X5d1e8+xhMVPCdhOeDOLMei8iYvj9pR+btB5totvG7enxhGxBtrXq7EbPRpJpqE7IskcmzGb/ff6fPe1cDNNzHT4pY9Ox6Lzeso6dwbuqQHnt3XuvVAEp4z2fn74rZgaYl9vjWjT9xH6M5XXSOT8x6thq/ItliNSo1jfRe/r6Lyegs790eUoz/PgJC8MbtG5NzT3dF5XnqF0/szuVavnn847OzqfhqzOwxYfU+fhaaPpvHqaWjRuD9o++On4+wZdLoGfznqmtRmzru8q1uQr0Tnj9g+Qf547t9FcrGT6szTjJYhsX1Dh7d59wu9raja46zxcVufhNFVQi3TKl/P1z9qMqLLuDOexGWc+rCJfkc5fvWcXnnm4Rkrv5/bUktn+tJ577Yx0debnxPtq/miYbSSY8l+auuvc+w5oFdGdc3daqktrhGf41RUuWHxd6N3p058KnUPRyKisr9dUz+9qYcXQeT2jr28fk8kirN+XtqGYrBduBZ1L+MD69g/iLDZQAevbr4LOAfSDzgH0g84B9IPOAfSDzgH0g84B9DO6zsf0jQKYiw46f69curZ8Gn84G/zhkuAP106jzp2VVrbrSD1afKPwh5OAb9R3aPaBdHs8tjWjgBadP/CHE4DOv0Mnf7g/BKGNP1xFysWj+MNBjk7+cE/2tfu4HX+48lH84aBEJ3+4R2I7dW2S+MPlU84dxR8OinTyhwudm6rBH64i5cxR/OGgTA9/uLaBIv5w5ZRzR/GHgzKt/nCp98x68Icrp5w9ij8cFGnyh/N8vB6pvs/OeAkiO5gKwB8OfzjI0eQPV/UJC384/OEu1tdrKvzh2hl9ffuYTBZh+MP9POhcAv5w98L69qugcwD9oHMA/aBzAP2gcwD9oHMA/aBzAP2MrvMxfaMA5qKHzoXLrfCHs8EfLknoDzcj937zb/aNepZ8k++P0uIbhT+chBbfKDmRZbdfWkh01xq+TuP2hn2QWnT+wB9OwD06d9Ptn2whwwF17m69SBaxv87xh7t09Bf94Trg6Twd7X493X02+WuDDAfUuUtys/nHnueZuwD+cPG0fsYfrgPZOPG7KNgnkJk8SillDp0n2+UGneMPl07mN/zhOpCPk+Coc6+O+geUUh5X51X+cN/WuTdMwh8uXxIhg/vDCfDLE1NyJtptA4xwF2CNUgbVeZU/3OMOnXsn4Q9X/vEyx9j+cALyOi9H+zvSfJlXKmVInUfrOaDOw9NG0zn+cAnE43ZZjSL3oTOFumh/3XF2978qqFbKkDov+cM5J6b6E3+4d474w0XKLvOHa6jReZX3TK6L9n1dltX4951apYyp84I/XPS1M9LVmZ8T76v4w9W3RniGfn84cY3cpHNBGUa7c1J6nVTu2mF1DjHu6i0h+vzhpq0ROp+J9yNhBt81nf5ws9Vo5vXtADAD6BxAP+gcQD/oHEA/6BxAP+gcQD+j63xM3yiAueigc3t50ZXr8IezwR8uCf5w7TTqPFgufH33yEdWCN2wjwV/OAnCfUqN4A93jWCbQnbrfRwtOn/gDyfgHp276fZPtpDhgDp3t14UiygI7aI3S9lnAn84/OFkeDpPRzv+cBbeptw6RJGNP5z7K/5wMrJxgj9c4pDoSSLVeba5bhu3p8YR+MPFEftM9CEfJ8HRn/eHO+QdIRypOsOk4JTR3s9jG/WjQ+zrrYE/3AXwh4tT5XrVNlDsMPMU3mVu0XnGT6a6PEPp/MAfDn+484fGiZ8uM8z+aaPpHH+4BOJxu6xG+MNFKbhe2cOY84JI32dnvASRHbwZ4Q+HP1wd+MPFybpeVX3Cwh8Of7iL9fWayuAP18zo69vH5K7eEjKtm1qSaWuEzmfi/UiYwaVsRje1PDPWaOr17QAwAegcQD/oHEA/6BxAP+gcQD/oHEA/6BxAP+gcQD/oHEA/6BxAP+gcQD/oHEA/6BxAP+gcQD/oHEA/6BxAP+gcQD/oHEA/6BxAP+gcQD/oHEA//wE7ph1C/MwiTgAAAABJRU5ErkJggg==" alt="" />

大家看看，神奇的魔咒又出现了，即使我们实现了对象的克隆，但是因为对象的属性是集合类，这样我们修改a2对象的集合类中的值，也一定会影响a1对象集合类的值, 这是因为a1和a2对象的list属性是指向同一个内存地址的，面对这种情况，我们需要使用到对象的深克隆，具体怎么实现呢，来看代码：

public class A implements Cloneable {
    private List<String> list;

    public A() {
        list = new ArrayList<String>();
        list.add("Hello");
        list.add("World");
    }

    public List<String> getList() {
        return list;
    }

    public void setList(List<String> list) {
        this.list = list;
    }

    @Override
    protected A clone() throws CloneNotSupportedException {
        A other = (A) super.clone();
        List<String> otherList = new ArrayList<String>();
        for (String s : list) {
            otherList.add(s);
        }
        other.setList(otherList);
        return other;
    }
}

public class Main {
    public static void main(String[] args) throws CloneNotSupportedException {
        A a1 = new A();
        System.out.println("a1.list: " + a1.getList());
        A a2 = (A) a1.clone();
        System.out.println("a2.list: " + a2.getList());
        a2.getList().add("Java");
        System.out.println("a1.list: " + a1.getList());
        System.out.println("a2.list: " + a2.getList());
    }
}

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大家可以看到，深克隆的实现也就是要在clone方法里面new一个list，并把原来对象list中的值赋值到新list中并把新list设置到新对象，说起来好拗口啊，这样a2.list值得改变不会影响a1.list了，想想如果原始类的属性是一个对象，而对象的属性又有对象和集合类，那么clone方法里面的代码就会很大，因为深克隆必须保证全面的克隆。