设计模式学习笔记（六）原型模式以及深浅拷贝的区别
原型模式也是创建对象的一种方式，它一般用在这样的场景：系统中存在大量相同或相似对象的创建问题，如果用传统的构造函数来创建对象，会比较复杂而且耗费资源。这个时候使用原型模式的克隆方式，能够节省不少时间。比如Java 类中提供的Object clone()就是原型模式的应用。
一、原型模式介绍
原型设计模式（Prototype Design Pattern）指用一个已经创建的实例作为原型，通过复制该原型对象来创建一个和原型相同或相似的新对象。在Java语言中就存在克隆的方式，比如浅拷贝和深拷贝。
对于一般的对象创建，本身不会花费太多的资源，但是对于负责的对象，比如对象的数据需要经过复杂的计算才能得到（比如排序、计算哈希值），抑或是需要从 RPC、网络、数据库、文件系统等非常慢速的IO中读取，这个时候就可以利用原型模式从其他对象直接拷贝，从而减少资源的消耗。
二、原型模式的实现
在Java中原型模式的实现方式就是深拷贝和浅拷贝，下面来谈谈深拷贝和浅拷贝的区别
2.1 深拷贝和浅拷贝
2.1.1 浅拷贝
浅拷贝（Shadow Clone）是把原型对象中的成员变量为值类型的属性都复制给克隆对象，将为引用类的引用地址复制给克隆对象：
实现代码如下：
//实现Cloneable接口
public class ShadowCopy implements Cloneable{
private String name;
private int id;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public ShadowCopy(String name, int id) {
this.name = name;
this.id = id;
}
@Override
protected Object clone() throws CloneNotSupportedException {
return super.clone();
}
}
//调用测试
public class PrototypeTest {
public static void main(String[] args) throws CloneNotSupportedException {
ShadowCopy shadowCopy = new ShadowCopy("ethan", 01);
ShadowCopy copy = (ShadowCopy) shadowCopy.clone();
System.out.println("name:" + copy.getName() + " " + "id:" + copy.getId());
System.out.println(copy == shadowCopy);
}
}
从最后的测试结果copy == shadowCopy显示为false，说明为浅拷贝。我们再看看深拷贝：
2.1.2 深拷贝
深拷贝（Deep Clone）是将原型对象中的所有对象，无论值类型还是引用类型，都复制一份给拷贝对象：
那么深拷贝该如何实现？而且前面我们发现，在拷贝时为何需要重写 Object的 clone方法？先来看看其源码，发现clone方法是一个本地方法：
/**
* Creates and returns a copy of this object. The precise meaning
* of "copy" may depend on the class of the object. The general
* intent is that, for any object {@code x}, the expression:
* <blockquote>
* <pre>
* x.clone() != x</pre></blockquote>
* will be true, and that the expression:
* <blockquote>
* <pre>
* x.clone().getClass() == x.getClass()</pre></blockquote>
* will be {@code true}, but these are not absolute requirements.
* While it is typically the case that:
* <blockquote>
* <pre>
* x.clone().equals(x)</pre></blockquote>
* will be {@code true}, this is not an absolute requirement.
* <p>
* By convention, the returned object should be obtained by calling
* {@code super.clone}. If a class and all of its superclasses (except
* {@code Object}) obey this convention, it will be the case that
* {@code x.clone().getClass() == x.getClass()}.
* <p>
* By convention, the object returned by this method should be independent
* of this object (which is being cloned). To achieve this independence,
* it may be necessary to modify one or more fields of the object returned
* by {@code super.clone} before returning it. Typically, this means
* copying any mutable objects that comprise the internal "deep structure"
* of the object being cloned and replacing the references to these
* objects with references to the copies. If a class contains only
* primitive fields or references to immutable objects, then it is usually
* the case that no fields in the object returned by {@code super.clone}
* need to be modified.
* <p>
* The method {@code clone} for class {@code Object} performs a
* specific cloning operation. First, if the class of this object does
* not implement the interface {@code Cloneable}, then a
* {@code CloneNotSupportedException} is thrown. Note that all arrays
* are considered to implement the interface {@code Cloneable} and that
* the return type of the {@code clone} method of an array type {@code T[]}
* is {@code T[]} where T is any reference or primitive type.
* Otherwise, this method creates a new instance of the class of this
* object and initializes all its fields with exactly the contents of
* the corresponding fields of this object, as if by assignment; the
* contents of the fields are not themselves cloned. Thus, this method
* performs a "shallow copy" of this object, not a "deep copy" operation.
* <p>
* The class {@code Object} does not itself implement the interface
* {@code Cloneable}, so calling the {@code clone} method on an object
* whose class is {@code Object} will result in throwing an
* exception at run time.
*
* @return a clone of this instance.
* @throws CloneNotSupportedException if the object's class does not
* support the {@code Cloneable} interface. Subclasses
* that override the {@code clone} method can also
* throw this exception to indicate that an instance cannot
* be cloned.
* @see java.lang.Cloneable
*/
protected native Object clone() throws CloneNotSupportedException;
从注释可以知道，对于所有对象来说：
x.clone()!=x应当返回true，因为克隆对象不能和原对象是同一个对象x.clone().getClass()==x.getClass()应当返回true，因为克隆对象和原对象的类型是相同的x.clone().equals(x)应当返回true，因为使用equals方法比较时，其值都是相同的
Java 实现拷贝主要有两个步骤：一是实现 Cloneable空接口，二是重写Object的Clone方法后再调用父类的克隆方法super.clone()，那为何这样做？
拷贝功能不是一个常用的功能，因此在对象需要时实现即可，这样比较合理，而且在Java语言中一个类也可以实现多个接口。对于调用clone方法，因为该方法语义的特殊性，所以要有JVM的直接支持，而clone方法就是这个调用接口，一旦有类调用这个方法，就可以实现拷贝功能了。
2.1.3 深拷贝的实现方式
深拷贝的实现方式有很多种，大体上有这样几种：
1.所有对象都实现深拷贝
这种方式需要让类中所有引用对象都实现拷贝，从而实现类的深拷贝，代码如下：
public class CloneExample {
public static void main(String[] args) throws CloneNotSupportedException {
// 创建被赋值对象
Address address = new Address(110, "北京");
People p1 = new People(1, "Java", address);
// 克隆 p1 对象
People p2 = p1.clone();
// 修改原型对象
p1.getAddress().setCity("西安");
// 输出 p1 和 p2 地址信息
System.out.println("p1:" + p1.getAddress().getCity() +
" p2:" + p2.getAddress().getCity());
}
/**
* 用户类
*/
static class People implements Cloneable {
private Integer id;
private String name;
private Address address;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
public People(Integer id, String name, Address address) {
this.id = id;
this.name = name;
this.address = address;
}
/**
* 重写 clone 方法
* @throws CloneNotSupportedException
*/
@Override
protected People clone() throws CloneNotSupportedException {
People people = (People) super.clone();
people.setAddress(this.address.clone()); // 引用类型克隆赋值
return people;
}
}
/**
* 地址类
*/
static class Address implements Cloneable {
private Integer id;
private String city;
public Address(Integer id, String city) {
this.id = id;
this.city = city;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
/**
* 重写 clone 方法
* @throws CloneNotSupportedException
*/
@Override
protected Address clone() throws CloneNotSupportedException {
return (Address) super.clone();
}
}
}
2.通过构造方法实现深拷贝
如果构造方法的参数为基本数据类型或者字符串类型，直接进行赋值即可，如果是对象类型，则需要重新 new 一个对象，实现代码如下：
public class CloneExample2 {
public static void main(String[] args) {
Address address = new Address(100, "北京");
People people1 = new People(1, "ethan", address);
People people2 = new People(people1.getId(), people1.getName(), new Address(people1.getAddress().getId(), people1.getAddress().getCity()));
}
static class People {
private Integer id;
private String name;
private Address address;
public People(Integer id, String name, Address address) {
this.id = id;
this.name = name;
this.address = address;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
}
static class Address {
private Integer id;
private String city;
public Address(Integer id, String city) {
this.id = id;
this.city = city;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
}
}
3.通过字节流实现深拷贝
可以通过 JDK 自带的字节流实现深拷贝的方式，是先将要原型对象写入到内存中的字节流，然后再从这个字节流中读出刚刚存储的信息，来作为一个新的对象返回，那么这个克隆对象和原型对象就不存在任何地址上的共享，实现代码如下：
public class CloneExample3 {
public static void main(String[] args) {
Address address = new Address(100, "北京");
People people1 = new People(1, "ethan", address);
//字节流拷贝对象
People people2 = StreamClone.clone(people1);
}
static class StreamClone {
public static <T extends Serializable> T clone(People obj) {
T cloneObj = null;
try {
//写入字节流
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteArrayOutputStream);
objectOutputStream.writeObject(obj);
objectOutputStream.close();
//分配内存，写入原始对象并生成新对象
ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(byteArrayOutputStream.toByteArray());
ObjectInputStream objectInputStream = new ObjectInputStream(byteArrayInputStream);
//返回生成的新对象
cloneObj = (T) objectInputStream.readObject();
objectInputStream.close();
} catch (Exception e) {
e.printStackTrace();
}
return cloneObj;
}
}
static class People implements Serializable {
private Integer id;
private String name;
private Address address;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
public People(Integer id, String name, Address address) {
this.id = id;
this.name = name;
this.address = address;
}
}
static class Address implements Serializable {
private Integer id;
private String city;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public Address(Integer id, String city) {
this.id = id;
this.city = city;
}
}
}
在利用字节流进行拷贝时，要注意每个对象必须实现 Serizlizable接口，标识自己可以被序列化，否则就会抛出（java.io.NotSerizlizableException）异常。
4.通过Apache Commons Lang实现深拷贝
相对于方法3，这个方法可以直接调用，实现代码如下：
People people2 = (People)SerizlizationUtils.clone(people1);
//其他部分和方法3相同，省略
5.通过JSON工具类实现深拷贝
Gson gson = new Gson();
People people2 = gson.fromJson(gson.toJson(people1), People.class);
在该方法中，不需要对People和Address类进行标识序列化。使用JSON 工具类会先把对象转化成字符串，然后再从字符串转化成新的对象，因此不会和原型对象有关联。从而实现了深拷贝，其他类似的 JSON 工具类的实现方式也是如此。
三、总结
原型模式在 Java 中主要有两种实现方式：深拷贝和浅拷贝，两者区别是深拷贝会复制引用对象，浅拷贝只会复制引用对象的地址。深拷贝相对于浅拷贝更加耗时和资源。
为何有深拷贝的存在呢？因为对于可变对象来说，浅拷贝对于引用对象的地址拷贝会带来修改风险。所以在可变对象的场景下，尽量还是选择深拷贝的方式进行复制。
参考资料
https://time.geekbang.org/column/article/200786
《Java 重学设计模式》
https://kaiwu.lagou.com/course/courseInfo.htm?courseId=59#/detail/pc?id=1767
