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。
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：
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。
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();
}
}
}
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;
}
}
}
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。
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
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。
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
http://c.biancheng.net/view/1343.html
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