深拷贝、浅拷贝与Cloneable接口

深拷贝与浅拷贝

浅拷贝

public class Student implements Cloneable{

    Integer a;

    Integer b;

    @Override

    protected Object clone() throws CloneNotSupportedException {

        return super.clone();

    }

    String c;

    Student child;

}

public class Main {

    public static void main(String[] args) throws CloneNotSupportedException {

        Student s = new Student();

        Student s2 = (Student) s.clone();

        System.out.println(s == s2);

        System.out.println(s.child == s2.child);

    }

}

/**********************************************************************/

false

true

由上述代码及运行结果我们可以看出，调用clone方法之后，确实s2是一个新的对象，内存地址已经发生了改变，但s和s2的child属性仍然指向相同的地址，这便是浅拷贝，当然8种基本数据类型是深拷贝，String则是例外。

深拷贝

public class Teacher implements Cloneable{

    @Override

    protected Object clone() throws CloneNotSupportedException {

        return super.clone();

    }

}

public class Student implements Cloneable{

    Integer a;

    Integer b;

    public Student(Teacher t) {

        this.t = t;

    }

    @Override

    protected Object clone() throws CloneNotSupportedException {

        Student s = (Student) super.clone();

        s.t = (Teacher)s.t.clone();//彻底深拷贝关键

        return s;

    }

    String c;

    Teacher t;

}

public class IntersectionofTwoArraysII {

    public static void main(String[] args) throws CloneNotSupportedException {

        Teacher t = new Teacher();

        Student s = new Student(t);

        Student s2 = (Student) s.clone();

        System.out.println(s == s2);

        System.out.println(s.t == s2.t);

    }

}

/**********************************************************************/

false

false

如上代码运行结果所示为深拷贝，想要实现深拷贝，就需要在重写Cloneable接口的clone()方法，并调用属性的clone()方法，因此关联类也要实现Cloneable接口，从而形成拷贝链，实现彻底深拷贝。

实现彻底的深拷贝并不容易，因为它要求涉及的所有类都实现了cloneable接口，但是如StringBuffer由于其没有实现Cloneable接口，想要实现深拷贝效果，则需要做特殊处理从而实现深拷贝的效果。可以在clone方法中实现

sb=new StringBuffer(sb.toString());

Cloneable接口

Cloneable其实就是一个标记接口，只有实现这个接口后，然后在类中重写Object中的clone方法，然后通过类调用clone方法才能克隆成功，如果不实现Cloneable其实就是一个标记接口接口，调用clone方法则会抛出CloneNotSupportedException(克隆不被支持)异常。

如何判断类是否实现了cloneable接口呢？

/**

     * Creates and returns a copy of this {@code Object}. The default

     * implementation returns a so-called "shallow" copy: It creates a new

     * instance of the same class and then copies the field values (including

     * object references) from this instance to the new instance. A "deep" copy,

     * in contrast, would also recursively clone nested objects. A subclass that

     * needs to implement this kind of cloning should call {@code super.clone()}

     * to create the new instance and then create deep copies of the nested,

     * mutable objects.

     *

     * @return a copy of this object.

     * @throws CloneNotSupportedException

     *             if this object's class does not implement the {@code

     *             Cloneable} interface.

     */

    protected Object clone() throws CloneNotSupportedException {

        if (!(this instanceof Cloneable)) {

            throw new CloneNotSupportedException("Class doesn't implement Cloneable");

        }

        return internalClone((Cloneable) this);

    }

    /*

     * Native helper method for cloning.

     */

    private native Object internalClone(Cloneable o);

clone方法首先会判对象是否实现了Cloneable接口，若无则抛出CloneNotSupportedException, 最后会调用internalClone. intervalClone是一个native方法，一般来说native方法的执行效率高于非native方法。

参考资料

详解Java中的clone方法 -- 原型模式

巴特西

深拷贝、浅拷贝与Cloneable接口

深拷贝与浅拷贝

Cloneable接口

参考资料

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