> For the complete documentation index, see [llms.txt](https://lewiszlw.gitbook.io/notebooks/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lewiszlw.gitbook.io/notebooks/blogs/history/she-ji-mo-shi-zhi-dai-li-mo-shi.md).

# 设计模式之代理模式

在代理模式（Proxy Pattern）中，一个类代表另一个类的功能。这种类型的设计模式属于结构型模式。

## 介绍

在代理模式中，我们创建具有现有对象的对象，以便向外界提供功能接口。

**意图**：为其他对象提供一种代理以控制对这个对象的访问。

**主要解决**：在直接访问对象时带来的问题，比如说：要访问的对象在远程的机器上。在面向对象系统中，有些对象由于某些原因（比如对象创建开销很大，或者某些操作需要安全控制，或者需要进程外的访问），直接访问会给使用者或者系统结构带来很多麻烦，我们可以在访问此对象时加上一个对此对象的访问层。

**何时使用**：想在访问一个类时做一些控制。

**如何解决**：增加中间层。

**关键代码**：实现与被代理类组合。

## Demo

1.Image接口

```java
package com.zlw.demo;

public interface Image {
    void display();
}
```

2.被代理的类RealImage

```java
package com.zlw.demo;

public class RealImage implements Image{

    private String filename;

    public RealImage(String filename) {
        this.filename = filename;
        loadFromDisk(filename);
    }

    @Override
    public void display() {
        System.out.println("Displaying: " + filename);
    }

    private void loadFromDisk(String filename) {
        System.out.println("Loading: " + filename);
    }
}
```

3.代理类ProxyImage

```java
package com.zlw.demo;

public class ProxyImage implements Image{

    private RealImage realImage;
    private String filename;

    public ProxyImage(String filename) {
        this.filename = filename;
    }

    @Override
    public void display() {
        if(realImage == null) {
            realImage = new RealImage(filename);
        }
        realImage.display();
    }
}
```

4.测试用例

```java
package com.zlw.demo;

public class ProxyPatternDemo {
    public static void main(String[] args) {
        Image image = new ProxyImage("image001.jpg");
        image.display();
    }
}
```

**理解**：使用代理类ProxyImage来控制真实类RealImage的行为。当我们想要调用RealImage类的display行为，使用RealImage类的唯一标示（即filename）来创建属于这个RealImage的ProxyImage，然后利用ProxyImage的display行为来间接调用RealImage的display行为。

## 参考资料

<http://www.runoob.com/design-pattern/proxy-pattern.html>


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