Before starting deep and shallow clone cloning, we first look at the code below, what is the problem?
class CloneTest {
public static void main(String[] args) throws CloneNotSupportedException {
// 等号赋值（ 基本类型）
int number = 6;
int number2 = number;
// 修改 number2 的值
number2 = 9;
System.out.println("number：" + number);
System.out.println("number2：" + number2);
// 等号赋值（对象）
Dog dog = new Dog();
dog.name = "旺财";
dog.age = 5;
Dog dog2 = dog;
// 修改 dog2 的值
dog2.name = "大黄";
dog2.age = 3;
System.out.println(dog.name + "，" + dog.age + "岁");
System.out.println(dog2.name + "，" + dog2.age + "岁");
}
}Program execution results:
number：6
number2：9
大黄，3岁
大黄，3岁As can be seen, if the equal sign replication, value types, the modifying operation is relatively independent of each other, and for reference types, because the memory address of the copy of the referenced object, wherein a value so modified , another value will change accordingly, as shown in FIG principle:
in order to prevent this problem from occurring, it is necessary to solve the problem using the object type copying cloned references.
One, AsaKatsuTakashi
The default implementation is shallow clone clone (), codes are as follows:
class CloneTest {
public static void main(String[] args) throws CloneNotSupportedException {
Dog dog = new Dog();
dog.name = "旺财";
dog.age = 5;
// 克隆
Dog dog3 = (Dog) dog.clone();
dog3.name = "小白";
dog3.age = 2;
System.out.println(dog.name + "，" + dog.age + "岁");
System.out.println(dog3.name + "，" + dog3.age + "岁");
}
}
class Dog implements Cloneable {
public String name;
public int age;
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}Program execution results:
旺财，5岁
小白，2岁It can be seen on the cloning problem may be solved type copying references, the principle as shown below:
above this replication is called shallow clone.
Realization Condition shallow clones: desired to clone objects must implement Cloneablethe interface and override clone()method, clones this object can be realized.
However, the use of shallow clone also has a problem , refer to the following code.
class CloneTest {
public static void main(String[] args) throws CloneNotSupportedException {
DogChild dogChild = new DogChild();
dogChild.name = "二狗";
Dog dog4 = new Dog();
dog4.name = "大黄";
dog4.dogChild = dogChild;
Dog dog5 = (Dog) dog4.clone();
dog5.name = "旺财";
dog5.dogChild.name = "狗二";
System.out.println("dog name 4："+dog4.name);
System.out.println("dog name 5："+dog5.name);
System.out.println("dog child name 4："+dog4.dogChild.name);
System.out.println("dog child name 5："+dog5.dogChild.name);
}
}
class Dog implements Cloneable {
public String name;
public DogChild dogChild;
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
class DogChild {
public String name;
}Program execution results:
dog name 4：大黄
dog name 5：旺财
dog child name 4：狗二
dog child name 5：狗二That shallow clone, copy only the value of the type of object, but does not copy the object reference type. Reasons shown below:
To deal with the type of reference is not copied, it is necessary to use a deep clone .
Second, deep cloning
It is defined : a deep clone is to copy the entire object information, including values and reference types.
Deep clone implementations typically comprises the following two.
- Realization of deep clone sequence: first the original object is serialized byte stream to the memory, from the byte stream and then deserialize the objects just stored, the original object and the new object does not exist on any of the shared address, this realization of the depth of cloning.
- All references to types implement clone: To copy an object of cloning must achieve all reference types, new objects all objects are replicated in order to achieve a deep clone.
A deep clone implementation: Serialization
实现思路：先将要拷贝对象写入到内存中的字节流中，然后再从这个字节流中读出刚刚存储的信息，作为一个新对象返回，那么这个新对象和原对象就不存在任何地址上的共享，自然实现了深拷贝。请参考以下代码：
class CloneTest {
public static void main(String[] args) throws CloneNotSupportedException {
BirdChild birdChild = new BirdChild();
birdChild.name = "小小鸟";
Bird bird = new Bird();
bird.name = "小鸟";
bird.birdChild = birdChild;
// 使用序列化克隆对象
Bird bird2 = CloneUtils.clone(bird);
bird2.name = "黄雀";
bird2.birdChild.name = "小黄雀";
System.out.println("bird name:" + bird.name);
System.out.println("bird child name:" + bird.birdChild.name);
System.out.println("bird name 2:" + bird2.name);
System.out.println("bird child name 2:" + bird2.birdChild.name);
}
}
class CloneUtils {
public static <T extends Serializable> T clone(T obj) {
T cloneObj = null;
try {
//写入字节流
ByteArrayOutputStream bo = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bo);
oos.writeObject(obj);
oos.close();
//分配内存,写入原始对象,生成新对象
ByteArrayInputStream bi = new ByteArrayInputStream(bo.toByteArray());//获取上面的输出字节流
ObjectInputStream oi = new ObjectInputStream(bi);
//返回生成的新对象
cloneObj = (T) oi.readObject();
oi.close();
} catch (Exception e) {
e.printStackTrace();
}
return cloneObj;
}
}程序执行结果：
bird name:小鸟
bird child name:小小鸟
bird name 2:黄雀
bird child name 2:小黄雀深克隆实现方式二：所有引用类型都实现克隆
class SerializableTest {
public static void main(String[] args) throws IOException, ClassNotFoundException {
ParrotChild parrotChild = new ParrotChild();
parrotChild.name = "小鹦鹉";
Parrot parrot = new Parrot();
parrot.name = "大鹦鹉";
parrot.parrotChild = parrotChild;
// 克隆
Parrot parrot2 = (Parrot) parrot.clone();
parrot2.name = "老鹦鹉";
parrot2.parrotChild.name = "少鹦鹉";
System.out.println("parrot name:" + parrot.name);
System.out.println("parrot child name:" + parrot.parrotChild.name);
System.out.println("parrot name 2:" + parrot2.name);
System.out.println("parrot child name 2:" + parrot2.parrotChild.name);
}
}
class Parrot implements Cloneable {
public String name;
public ParrotChild parrotChild;
@Override
protected Object clone() throws CloneNotSupportedException {
Parrot bird = (Parrot) super.clone();
bird.parrotChild = (ParrotChild) parrotChild.clone();
return bird;
}
}
class ParrotChild implements Cloneable {
public String name;
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}程序执行结果：
parrot name:大鹦鹉
parrot child name:小鹦鹉
parrot name 2:老鹦鹉
parrot child name 2:少鹦鹉三、克隆相关面试题
1.使用克隆有什么好处？
答：好处包含以下几点：
- 使用方便：假如要复制一个对象，但这个对象中的部分属性已经被修改过了，如果不使用克隆的话，需要给属性手动赋值，相比克隆而已麻烦很多；
- 性能高：查看 clone 方法可以知道，它是 native 方法，native 方法是原生函数，使用操作系统底层的语言实现的，因此执行效率更高；
- 隔离性：克隆可以确保对象操作时相互隔离。
clone() 源代码，如下图：
2.浅克隆和深克隆有什么区别？
答：区别主要在对引用类型的复制上，具体信息如下：
- 浅克隆：只会复制对象的值类型，而不会复制对象的引用类型；
深克隆：复制整个对象，包含值类型和引用类型。
3.如何实现浅克隆？
答：克隆的对象实现 Cloneable 接口，并重写 clone() 方法就可以实现浅克隆了。
4.以下代码执行的结果是？
import java.util.Arrays;
class CloneTest {
public static void main(String[] args) throws CloneNotSupportedException {
CloneObj cloneObj = new CloneObj();
cloneObj.name = "老王";
cloneObj.age = 30;
cloneObj.sistersAge = new int[]{18, 19};
CloneObj cloneObj2 = (CloneObj) cloneObj.clone();
cloneObj2.name = "磊哥";
cloneObj2.age = 33;
cloneObj2.sistersAge[0] = 20;
System.out.println(cloneObj.name + "|" + cloneObj2.name);
System.out.println(cloneObj.age + "|" + cloneObj2.age);
System.out.println(Arrays.toString(cloneObj.sistersAge) + "|" + Arrays.toString(cloneObj2.sistersAge));
}
}
class CloneObj implements Cloneable {
public String name;
public int age;
public int[] sistersAge;
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}答：执行结果如下。
老王|磊哥
30|33
[20, 19]|[20, 19]5.深克隆如何实现？有几种实现方式？
答：一般实现方式有以下两种：
- 通过序列化实现深克隆（序列化实现方式：Java 原生序列化、JSON 序列化、Hessian 序列化）；
所有引用类型都实现克隆，从而实现深克隆。
6.为什么不能直接使用 Object 的 Clone 方法，还要重写 clone() 方法之后才能实现克隆？
答：虽然所有类都是 Object 的子类，但因为 Object 中的 clone() 方法被声明为 protected 访问级别，所以非 java.lang 包下的其他类是不能直接使用的。因此要想实现克隆功能，就必须实现 Cloneable，并重写 clone() 方法才行。
7.序列化可不可以实现深克隆？实现的原理是什么？
答：先将原对象序列化到内存的字节流中，再从字节流中反序列化出刚刚存储的对象，这个新对象和原对象就不存在任何地址上的共享，这样就实现了深克隆。
四、总结
调用 Object 类中的 clone() 方法默认是浅克隆，浅克隆只能复制值类型，不能复制引用类型，因此更多的场景下我们需要深克隆，深克隆通常的实现方式有两种：序列化方式或把所有的引用类型都实现克隆。
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