Structural Patterns Part 7/7: Proxy Pattern

Structural Patterns Part 7/7: Proxy Pattern

目录

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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

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UML Class Diagram

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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.javapackage designpatterns.structuralpatterns.proxy;public interface Image {    void display();}// ImageImpl.javapackage 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.javapackage 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 onceLoad from Disk: ProxyPattern.jpgDisplay Image: ProxyPattern.jpgDisplay Image: ProxyPattern.jpg