原型模式也是建立物件的一種方式，它一般用在這樣的場景：系統中存在大量相同或相似物件的建立問題，如果用傳統的建構函式來建立物件，會比較複雜而且耗費資源。這個時候使用原型模式的克隆方式，能夠節省不少時間。比如Java 類中提供的Object clone()就是原型模式的應用。
一、原型模式介紹
原型設計模式（Prototype Design Pattern）指用一個已經建立的例項作為原型，通過複製該原型物件來建立一個和原型相同或相似的新物件。在Java語言中就存在克隆的方式，比如淺拷貝和深拷貝。
對於一般的物件建立，本身不會花費太多的資源，但是對於負責的物件，比如物件的資料需要經過複雜的計算才能得到（比如排序、計算雜湊值），抑或是需要從 RPC、網路、資料庫、檔案系統等非常慢速的IO中讀取，這個時候就可以利用原型模式從其他物件直接拷貝，從而減少資源的消耗。
二、原型模式的實現
在Java中原型模式的實現方式就是深拷貝和淺拷貝，下面來談談深拷貝和淺拷貝的區別
2.1 深拷貝和淺拷貝
2.1.1 淺拷貝
淺拷貝（Shadow Clone）是把原型物件中的成員變數為值型別的屬性都複製給克隆物件，將為引用類的引用地址複製給克隆物件：
實現程式碼如下：
//實現Cloneable介面
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();
}
}
//呼叫測試
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);
}
}
從最後的測試結果copy == shadowCopy顯示為false，說明為淺拷貝。我們再看看深拷貝：
2.1.2 深拷貝
深拷貝（Deep Clone）是將原型物件中的所有物件，無論值型別還是引用型別，都複製一份給拷貝物件：
那麼深拷貝該如何實現？而且前面我們發現，在拷貝時為何需要重寫 Object的 clone方法？先來看看其原始碼，發現clone方法是一個本地方法：
/**
* 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;
從註釋可以知道，對於所有物件來說：
x.clone()!=x應當返回true，因為克隆物件不能和原物件是同一個物件x.clone().getClass()==x.getClass()應當返回true，因為克隆物件和原物件的型別是相同的x.clone().equals(x)應當返回true，因為使用equals方法比較時，其值都是相同的
Java 實現拷貝主要有兩個步驟：一是實現 Cloneable空介面，二是重寫Object的Clone方法後再呼叫父類的克隆方法super.clone()，那為何這樣做？
拷貝功能不是一個常用的功能，因此在物件需要時實現即可，這樣比較合理，而且在Java語言中一個類也可以實現多個介面。對於呼叫clone方法，因為該方法語義的特殊性，所以要有JVM的直接支援，而clone方法就是這個呼叫介面，一旦有類呼叫這個方法，就可以實現拷貝功能了。
2.1.3 深拷貝的實現方式
深拷貝的實現方式有很多種，大體上有這樣幾種：
1.所有物件都實現深拷貝
這種方式需要讓類中所有引用物件都實現拷貝，從而實現類的深拷貝，程式碼如下：
public class CloneExample {
public static void main(String[] args) throws CloneNotSupportedException {
// 建立被賦值物件
Address address = new Address(110, "北京");
People p1 = new People(1, "Java", address);
// 克隆 p1 物件
People p2 = p1.clone();
// 修改原型物件
p1.getAddress().setCity("西安");
// 輸出 p1 和 p2 地址資訊
System.out.println("p1:" + p1.getAddress().getCity() +
" p2:" + p2.getAddress().getCity());
}
/**
* 使用者類
*/
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;
}
/**
* 重寫 clone 方法
* @throws CloneNotSupportedException
*/
@Override
protected People clone() throws CloneNotSupportedException {
People people = (People) super.clone();
people.setAddress(this.address.clone()); // 引用型別克隆賦值
return people;
}
}
/**
* 地址類
*/
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;
}
/**
* 重寫 clone 方法
* @throws CloneNotSupportedException
*/
@Override
protected Address clone() throws CloneNotSupportedException {
return (Address) super.clone();
}
}
}
2.通過構造方法實現深拷貝
如果構造方法的引數為基本資料型別或者字串型別，直接進行賦值即可，如果是物件型別，則需要重新 new 一個物件，實現程式碼如下：
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.通過位元組流實現深拷貝
可以通過 JDK 自帶的位元組流實現深拷貝的方式，是先將要原型物件寫入到記憶體中的位元組流，然後再從這個位元組流中讀出剛剛儲存的資訊，來作為一個新的物件返回，那麼這個克隆物件和原型物件就不存在任何地址上的共享，實現程式碼如下：
public class CloneExample3 {
public static void main(String[] args) {
Address address = new Address(100, "北京");
People people1 = new People(1, "ethan", address);
//位元組流拷貝物件
People people2 = StreamClone.clone(people1);
}
static class StreamClone {
public static <T extends Serializable> T clone(People obj) {
T cloneObj = null;
try {
//寫入位元組流
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteArrayOutputStream);
objectOutputStream.writeObject(obj);
objectOutputStream.close();
//分配記憶體，寫入原始物件並生成新物件
ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(byteArrayOutputStream.toByteArray());
ObjectInputStream objectInputStream = new ObjectInputStream(byteArrayInputStream);
//返回生成的新物件
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;
}
}
}
在利用位元組流進行拷貝時，要注意每個物件必須實現 Serizlizable介面，標識自己可以被序列化，否則就會丟擲（java.io.NotSerizlizableException）異常。
4.通過Apache Commons Lang實現深拷貝
相對於方法3，這個方法可以直接呼叫，實現程式碼如下：
People people2 = (People)SerizlizationUtils.clone(people1);
//其他部分和方法3相同，省略
5.通過JSON工具類實現深拷貝
Gson gson = new Gson();
People people2 = gson.fromJson(gson.toJson(people1), People.class);
在該方法中，不需要對People和Address類進行標識序列化。使用JSON 工具類會先把物件轉化成字串，然後再從字串轉化成新的物件，因此不會和原型物件有關聯。從而實現了深拷貝，其他類似的 JSON 工具類的實現方式也是如此。
三、總結
原型模式在 Java 中主要有兩種實現方式：深拷貝和淺拷貝，兩者區別是深拷貝會複製引用物件，淺拷貝只會複製引用物件的地址。深拷貝相對於淺拷貝更加耗時和資源。
為何有深拷貝的存在呢？因為對於可變物件來說，淺拷貝對於引用物件的地址拷貝會帶來修改風險。所以在可變物件的場景下，儘量還是選擇深拷貝的方式進行復制。
參考資料
https://time.geekbang.org/column/article/200786
《Java 重學設計模式》
https://kaiwu.lagou.com/course/courseInfo.htm?courseId=59#/detail/pc?id=1767