162 lines
4.4 KiB
Markdown
162 lines
4.4 KiB
Markdown
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---
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layout: pattern
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title: Proxy
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folder: proxy
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permalink: /patterns/proxy/
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categories: Structural
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tags:
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- Gang Of Four
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- Decoupling
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---
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## 又被称为
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替代（代孕）模式
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## 目的
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为另一个对象提供代理或占位符以控制对其的访问。
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## 解释
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真实世界例子
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> 想象有一个塔，当地的巫师去那里学习他们的法术。象牙塔只能够通过代理来进入以此来保证只有首先3个巫师才能进入。这里的代理就代表的塔的功能并添加访问控制。
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通俗的说
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> 使用代理模式，一个类代表另一个类的功能。
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维基百科说
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> 在最一般的形式上，代理是一个类，它充当与其他对象的接口。代理是客户端调用的包装器或代理对象，以访问后台的实际服务对象。代理本身可以简单地转发到真实对象，也可以提供其他逻辑。在代理中，可以提供额外的功能，例如在对实对象的操作占用大量资源时进行缓存，或者在对实对象的操作被调用之前检查前提条件。
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**程序示例**
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使用上面的巫师塔为例。首先我们有**巫师塔**接口和**象牙塔**类 。
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```java
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public interface WizardTower {
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void enter(Wizard wizard);
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}
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public class IvoryTower implements WizardTower {
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private static final Logger LOGGER = LoggerFactory.getLogger(IvoryTower.class);
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public void enter(Wizard wizard) {
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LOGGER.info("{} enters the tower.", wizard);
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}
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}
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```
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然后有个简单的巫师类。
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```java
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public class Wizard {
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private final String name;
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public Wizard(String name) {
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this.name = name;
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}
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@Override
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public String toString() {
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return name;
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}
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}
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```
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然后我们有巫师塔代理类为巫师塔添加访问控制。
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```java
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public class WizardTowerProxy implements WizardTower {
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private static final Logger LOGGER = LoggerFactory.getLogger(WizardTowerProxy.class);
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private static final int NUM_WIZARDS_ALLOWED = 3;
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private int numWizards;
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private final WizardTower tower;
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public WizardTowerProxy(WizardTower tower) {
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this.tower = tower;
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}
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@Override
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public void enter(Wizard wizard) {
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if (numWizards < NUM_WIZARDS_ALLOWED) {
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tower.enter(wizard);
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numWizards++;
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} else {
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LOGGER.info("{} is not allowed to enter!", wizard);
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}
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}
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}
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```
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然后这是进入塔的场景。
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```java
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var proxy = new WizardTowerProxy(new IvoryTower());
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proxy.enter(new Wizard("Red wizard"));
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proxy.enter(new Wizard("White wizard"));
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proxy.enter(new Wizard("Black wizard"));
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proxy.enter(new Wizard("Green wizard"));
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proxy.enter(new Wizard("Brown wizard"));
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```
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程序输出：
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```
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Red wizard enters the tower.
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White wizard enters the tower.
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Black wizard enters the tower.
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Green wizard is not allowed to enter!
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Brown wizard is not allowed to enter!
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```
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## 类图
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## 适用性
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代理适用于需要比简单指针更广泛或更复杂的对象引用的情况。这是代理模式适用的几种常见情况。
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* 远程代理为不同地址空间中的对象提供了本地代表。
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* 虚拟代理根据需要创建昂贵的对象。
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* 保护代理控制对原始对象的访问。当对象有不同的接入权限时保护代理很有用。
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## 典型用例
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* 对象的访问控制
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* 懒加载
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* 实现日志记录
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* 简化网络连接
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* 对象的访问计数
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## 教程
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* [Controlling Access With Proxy Pattern](http://java-design-patterns.com/blog/controlling-access-with-proxy-pattern/)
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## 已知使用
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* [java.lang.reflect.Proxy](http://docs.oracle.com/javase/8/docs/api/java/lang/reflect/Proxy.html)
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* [Apache Commons Proxy](https://commons.apache.org/proper/commons-proxy/)
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* Mocking frameworks [Mockito](https://site.mockito.org/),
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[Powermock](https://powermock.github.io/), [EasyMock](https://easymock.org/)
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## 相关设计模式
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* [Ambassador](https://java-design-patterns.com/patterns/ambassador/)
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## 鸣谢
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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