java clone 中的浅复制和深复制

什么是浅复制？

首先，浅复制会创建一个新对象，这个新的对象各个字段的值会从原始对象复制过来，如果某个字段是引用其他的对象，那么仅仅复制此对象在内存中的引用地址。

图1

如图1 所示，对象 MainObject1 有一个 int 类型的字段 Field1 和一个字段引用 ContainObject1对象，被克隆的新对象是 MainObject2, MainObject2 有一个 int 类型字段 Field2, Field2 的值是从对象MainObject1的字段 Field1 复制过来，而MainObject2的一个字段和MainObject1 中的某个字段仍然指向同一个对象ContainObject1，也就是说只要ContainObject1 发生任何变化，MainObject1 和MainObject2所引用的ContainObject1 都会用变化。

什么是深复制？

深复制也会创建一个新的对象，除了复制这个新对象里的原始类型字段的值，还要对此对象的引用字段再做克隆，而不是仅仅复制此引用字段再内存中的引用地址。

图2

如图2 所示，MainObject1 是原始对象，MainObject2 是被克隆出的对象，从图2和图1的差别可以看出，克隆对象MainObject2 字段ContainObject2和 MainObject1 里的字段 ContainObject1 指向的是不同的对象，也就是说，MainObject1 里的 ContainObject1 发生任何变化，都不会影响到 MainObject2 里的 ContainObject2。

用 java 实现浅复制示例：

class Subject {
    private String name;
 
    public String getName() {
        return name;
    }
 
    public void setName(String s) {
        name = s;
    }
 
    public Subject(String s) {
        name = s;
    }
}
 
class Student implements Cloneable {
    //Contained object
    private Subject subj;
 
    private String name;
 
    public Subject getSubj() {
        return subj;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String s) {
        name = s;
    }
 
    public Student(String s, String sub) {
        name = s;
        subj = new Subject(sub);
    }
 
    public Object clone() {
        //shallow copy
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            return null;
        }
    }
}
 
public class CopyTest {
    public static void main(String[] args) {
        //Original Object
        Student stud = new Student("John", "Algebra");
        System.out.println("Original Object: " + stud.getName() + " - "
                + stud.getSubj().getName());
        //Clone Object
        Student clonedStud = (Student) stud.clone();
        System.out.println("Cloned Object: " + clonedStud.getName() + " - "
                + clonedStud.getSubj().getName());
        stud.setName("Dan");
        stud.getSubj().setName("Physics");
        System.out.println("Original Object after it is updated: "
                + stud.getName() + " - " + stud.getSubj().getName());
        System.out.println("Cloned Object after updating original object: "
                + clonedStud.getName() + " - " + clonedStud.getSubj().getName());
 
    }
}

输出结果是：

Original Object: John - Algebra
Cloned Object: John - Algebra
Original Object after it is updated: Dan - Physics
Cloned Object after updating original object: John - Physics

用java 实现深复制示例

只需要把上例的Student类的clone 方法修改如下：

class Student implements Cloneable {
  //Contained object
  private Subject subj;
 
  private String name;
 
  public Subject getSubj() {
	return subj;
  }
 
  public String getName() {
	return name;
  }
 
  public void setName(String s) {
	name = s;
  }
 
  public Student(String s, String sub) {
	name = s;
	subj = new Subject(sub);
  }
 
  public Object clone() {
	//Deep copy
	Student s = new Student(name, subj.getName());
	return s;
  }
}

修改后运行结果如下:

Original Object: John - Algebra
Cloned Object: John - Algebra
Original Object after it is updated: Dan - Physics
Cloned Object after updating original object: John - Algebra

使用序列化实现深复制

请看如下示例：

public class ColoredCircle implements Serializable {
    private int x;
    private int y;
 
    public ColoredCircle(int x, int y) {
        this.x = x;
        this.y = y;
    }
 
    public int getX() {
        return x;
    }
 
    public void setX(int x) {
        this.x = x;
    }
 
    public int getY() {
        return y;
    }
 
    public void setY(int y) {
        this.y = y;
    }
 
    @Override
    public String toString() {
        return "x=" + x + ",y=" + y;
    }
}

public class DeepCopy {
 
    static public void main(String[] args) {
        ObjectOutputStream oos = null;
        ObjectInputStream ois = null;
 
        try {
            // create original serializable object
            ColoredCircle c1 = new ColoredCircle(100, 100);
            // print it
            System.out.println("Original = " + c1);
 
            ColoredCircle c2 = null;
 
            // deep copy
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            oos = new ObjectOutputStream(bos);
            // serialize and pass the object
            oos.writeObject(c1);
            oos.flush();
            ByteArrayInputStream bin =
                    new ByteArrayInputStream(bos.toByteArray());
            ois = new ObjectInputStream(bin);
            // return the new object
            c2 = (ColoredCircle) ois.readObject();
 
            // verify it is the same
            System.out.println("Copied   = " + c2);
            // change the original object's contents
            c1.setX(200);
            c1.setY(200);
            // see what is in each one now
            System.out.println("Original = " + c1);
            System.out.println("Copied   = " + c2);
        } catch (Exception e) {
            System.out.println("Exception in main = " + e);
        } finally {
            try {
                oos.close();
                ois.close();
            } catch (IOException e) {
                System.out.println(e);
            }
 
        }
    }
}

程序输出结果：

Original = x=100,y=100
Copied   = x=100,y=100
Original = x=200,y=200
Copied   = x=100,y=100