漫画：什么是 “原型模式” ？

本是古典何须时尚 2023-01-23 04:55 33阅读 0赞

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**—————  第二天  —————**

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————————————

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假如有一天，小灰被外星人抓走了，外星人要拿小灰做实验，想了解小灰在吃得好、睡得好、玩得开心的场景下，与现实中小灰的生存状态有什么区别。

于是，外星人克隆了几个一模一样的小灰：

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就这样，小灰的原型被留在现实中，而三个复制体分别提供了吃得好、睡得好、玩得开心三种不同环境，小灰的原型则不受三个复制体的影响。

过了一段时间，我们来观察一下本体与分身的生存状态：

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在Java语言中，Object类实现了**Cloneable**接口，一个对象可以通过调用**Clone()**方法生成对象，这就是原型模式的典型应用。

但需要注意的是，clone()方法并不是Cloneable接口里的，而是Object类里的，Cloneable是一个标识接口，标识这个类的对象是可被拷贝的，如果没有实现Cloneable接口，却调用了clone()方法，就会报错。

<table> 
 <tbody> 
  <tr> 
   <td><pre><code>// protected native Object clone() throws</code></pre><pre><code> CloneNotSupportedException;protected Object clone() throws</code></pre><pre><code>CloneNotSupportedException {
&nbsp;&nbsp;&nbsp; if (!(this instanceof Cloneable)) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; throw new CloneNotSupportedException(</code></pre><pre><code>"Class " + getClass().getName() +</code></pre><pre><code>" doesn't implement Cloneable");
&nbsp;&nbsp;&nbsp; }
&nbsp;&nbsp;&nbsp; return internalClone();
}
// Native helper method for cloning.

private native Object internalClone();</code></pre></td> 
  </tr> 
 </tbody> 
</table>

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Java中的数据类型，分为基本类型和引用类型。在一个方法里的变量如果是基本类型的话，变量就直接存储在这个方法的栈帧里，例如int、long等；而引用类型则在栈帧里存储这个变量的指针，指向堆中该实体的地址，例如String、Array等。深拷贝和浅拷贝是只针对引用数据类型的。

比如一个方法有一个基本类型参数和一个引用类型参数，在方法体里对参数重新赋值，会影响传入的引用类型参数，而不会影响基本类型参数，因为基本类型参数是值传递，而引用类型参数是引用传递。

先定义一个用户类：

<table> 
 <tbody> 
  <tr> 
   <td><pre><code>// 这是一个非常简单的用户类

public class User {
&nbsp;&nbsp;&nbsp; private String name;
&nbsp;&nbsp;&nbsp; private int age;

&nbsp;&nbsp;&nbsp; public User(String name, int age) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this.name=name;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this.age=age;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public String getName() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return name;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public void setName(String name) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this.name = name;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public int getAge() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return age;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public void setAge(int age) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this.age = age;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; @Override

&nbsp;&nbsp;&nbsp; public String toString() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return "User{name='" + name + ", age=" + age +'}';
&nbsp;&nbsp;&nbsp; }

}</code></pre></td> 
  </tr> 
 </tbody> 
</table>

测试：

<table> 
 <tbody> 
  <tr> 
   <td><pre><code>private int x=10;

public void updateValue(int value){
&nbsp;&nbsp;&nbsp; value = 3 * value;
}

private User user= new User("大黄",20);

public void updateUser(User student){
&nbsp;&nbsp;&nbsp; student.setName("小灰");
&nbsp;&nbsp;&nbsp; student.setAge(18);
}
  public void test(){
&nbsp;&nbsp;&nbsp; System.out.println("调用前x的值："+x);
&nbsp;&nbsp;&nbsp; updateValue(x);
&nbsp;&nbsp;&nbsp; System.out.println("调用后x的值："+x);
&nbsp;&nbsp;&nbsp; System.out.println("调用前user的值："+user.toString());
&nbsp;&nbsp;&nbsp; updateUser(user);
&nbsp;&nbsp;&nbsp; System.out.println("调用后user的值："+user.toString());

}</code></pre></td> 
  </tr> 
 </tbody> 
</table>

Log打印结果如下：

<table> 
 <tbody> 
  <tr> 
   <td><p>调用前x的值：10</p><p>调用后x的值：10</p><p>调用前user的值：User{name='大黄, age=20}</p><p>调用后user的值：User{name='小灰, age=18}</p></td> 
  </tr> 
 </tbody> 
</table>

传递基本类型的方法（updateValue()）流程图：

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传递引用类型的方法（updateUser()）流程图：

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这其中也包含着例外，比如String类型和大小不超过127的Long类型，虽然也是引用类型，却像基本类型一样不受影响。这是因为它们会先比较常量池维护的值，这涉及VM的内容，今天不做过多讨论。

浅拷贝是在按位（bit）拷贝对象，这个对象有着原始对象属性值的一份精确拷贝。我们结合应用场景分析一下，还是刚才的User类，我们增加一个存放地址的内部类Address，我们需要用户信息可以被其他module查询，但是不允许它们被其他module修改，新增代码如下：

<table> 
 <tbody> 
  <tr> 
   <td><pre><code>// 这是一个稍微复杂的、支持拷贝的用户类

public class User implements Cloneable {
        </code></pre><pre><code>// ……省略上文代码……
  &nbsp;&nbsp;&nbsp; private Address address;

&nbsp;&nbsp;&nbsp; @NonNull
&nbsp;&nbsp;&nbsp; @NotNull
&nbsp;&nbsp;&nbsp; @Override

&nbsp;&nbsp;&nbsp; public User clone() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; try{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return (User)super.clone();

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }catch (CloneNotSupportedException e) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e.printStackTrace();
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return null;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public class Address{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // 地市
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; public String city;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // 区县
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; public String county;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // 乡镇街道
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; public String street;
&nbsp;&nbsp;&nbsp; } 
}</code></pre>&nbsp;<p>&nbsp;</p></td> 
  </tr> 
 </tbody> 
</table>

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<table> 
 <tbody> 
  <tr> 
   <td><pre><code>// 这是一个更复杂的、支持深拷贝的用户类

public class User implements Cloneable {

&nbsp;&nbsp;&nbsp; // ……省略上文代码……
&nbsp;&nbsp;&nbsp; @NonNull
&nbsp;&nbsp;&nbsp; @NotNull
&nbsp;&nbsp;&nbsp; @Override
&nbsp;&nbsp;&nbsp; public User clone() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; try{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; User newUser = (User)super.clone();
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newUser.setName(this.name);
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newUser.setAddress(this.address.clone());
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return newUser;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }catch (CloneNotSupportedException e) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e.printStackTrace();
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return null;
&nbsp;&nbsp;&nbsp; }

&nbsp;&nbsp;&nbsp; public class Address implements Cloneable{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // ……省略上文代码……
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; @NonNull
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; @NotNull
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; @Override
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; public Address clone() {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; try{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Address newAddress = (Address)super.clone();
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newAddress.city = this.city;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newAddress.county = this.county;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; newAddress.street = this.street;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return newAddress;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }catch (CloneNotSupportedException e) {
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; e.printStackTrace();
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; return null;
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }
&nbsp;&nbsp;&nbsp; }
}</code></pre></td> 
  </tr> 
 </tbody> 
</table>

需要注意的是，上面代码的深拷贝其实并不彻底，因为彻底的深拷贝几乎是不可能实现的，那样不但可能存在引用关系非常复杂的情况，也可能存在引用链的某一级上引用了一个没有实现Cloneable接口的第三方对象的情况。

绝大多数设计模式都是牺牲性能提升开发效率的，原型模式则是为数不多的**牺牲开发效率提升性能**的设计模式。

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<table> 
 <tbody> 
  <tr> 
   <td><pre><code>private User user= new User("大黄",20);

public void testNew(){

&nbsp;&nbsp;&nbsp; User user1 = new User("小灰",18);

}

public void testClone(){

&nbsp;&nbsp;&nbsp; User user2 = user.clone();

}</code></pre></td> 
  </tr> 
 </tbody> 
</table>

通过ASM工具查看bytecode，可以看出二者对栈资源的消耗：

<table> 
 <tbody> 
  <tr> 
   <td><p>// access flags 0x1</p><p>&nbsp; public &nbsp;testNew()V</p><p>&nbsp;&nbsp;&nbsp;……省略……</p><p>&nbsp;&nbsp;&nbsp; MAXSTACK &nbsp;= 4</p><p>&nbsp;&nbsp;&nbsp; &nbsp;MAXLOCALS = 2</p><p>&nbsp;</p><p>&nbsp; // access &nbsp;flags 0x1</p><p>&nbsp; public &nbsp;testClone()V</p><p>&nbsp;&nbsp;&nbsp;……省略……</p><p>&nbsp;&nbsp;&nbsp; MAXSTACK &nbsp;= 1</p><p>&nbsp;&nbsp;&nbsp; &nbsp;MAXLOCALS = 2</p></td> 
  </tr> 
 </tbody> 
</table>

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<table> 
 <tbody> 
  <tr> 
   <td><pre><code>@Override

public Object clone() {

&nbsp;&nbsp;&nbsp; return new Intent(this);

}</code></pre></td> 
  </tr> 
 </tbody> 
</table>

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最后我们来总结一下原型模式的核心用途：

**1.解决构建复杂对象的资源消耗问题，提升创建对象的效率。**

**2.保护性拷贝，防止外部对只读对象进行需修改。**

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