Structural Patterns Part 7/7: Proxy Pattern

Structural Patterns Part 7/7: Proxy Pattern

目录

Structural Patterns Part 77 Proxy Pattern

目录

Definition

UML Class Diagram

Implementation

Definition

Proxy Pattern提供了一种控制Real Object的创建和访问的方式，它通常代替了Real Object的功能。

一句话：

One class controls the creation of and access to objects in another class.

何时使用？一般当希望隐藏目标对象的创建和访问时(比如缓存)，或者希望在访问目标对象时提供额外的逻辑(动态代理)。

使用频率：

Medium High

UML Class Diagram

Implementation

考虑一个对图片的访问，因为从磁盘读取非常耗时间，所以可以使用代理，把图片缓存起来。

// Image.java
package designpatterns.structuralpatterns.proxy;

public interface Image {
void display();
}

// ImageImpl.java
package designpatterns.structuralpatterns.proxy;

class RealImage implements Image {
private String filename;

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

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

private void loadFromDisk(String filename) {
System.out.println("Load from Disk: " + filename);
}
}

class ProxyImage implements Image {
private RealImage ri;
private String filename;

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

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

ri.display();
}
}

// ProxyDemo.java
package designpatterns.structuralpatterns.proxy;

public class ProxyDemo {
public static void main(String[] args) {
Image image = new ProxyImage("ProxyPattern.jpg");

//image will be loaded from disk
image.display();
System.out.println();

//image will not be loaded from disk
image.display();
}
}

// output, image loaded only once
Load from Disk: ProxyPattern.jpg
Display Image: ProxyPattern.jpg

Display Image: ProxyPattern.jpg