设计模式（十四）外观模式（Facade）-- 结构型

男娘i 2023-03-13 11:57 237阅读 0赞

**项目需求:(影院管理)**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1Mjc1MjMz_size_16_color_FFFFFF_t_70][]

**项目引入：**

传统方式：创建各个子系统的对象，并直接去嗲用子系统的相关方法，会造成调用过程混乱，没有清晰地过程

**解决方案：**

定义一个高层接口，给子系统中的一组接口提供一个一致的界面（比如高层接口提供四个方法ready、play、pause、end），用来访问子系统中的一群接口，用来屏蔽内部子系统的细节，无须关心这个子系统的内部细节 =》外观模式

![20200513152651843.png][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1Mjc1MjMz_size_16_color_FFFFFF_t_70 1][]

**类图说明：**

> 1）外观类（Facade）： 为调用端提供统一的调用接口，外观类知道哪些子系统负责处理请求，从而将调用端的请求代理给适当子系统对象
> 
> 2）调用者（Client）：外观接口的调用者
> 
> 3）子系统的集合：指模块或者子系统，处理Facade 对象指派的任务，是功能的实际提供者

**代码实现：**

//  ================== 子系统集合  ============

//  DVDplayer
    class DVDPlayer {
    
        //   使用单例模式 -- 懒汉式
        private volatile static DVDPlayer lazyInstance;
    
        private DVDPlayer() {
        }
    
        public static DVDPlayer getInstance() {
            if (lazyInstance == null) {
                synchronized (DVDPlayer.class) {
                    if (lazyInstance == null) {
                        lazyInstance = new DVDPlayer();
                    }
                }
            }
            return lazyInstance;
        }
    
        public void on() {
            System.out.println(" dvd on ");
        }
    
        public void off() {
            System.out.println(" dvd off ");
        }
    
        public void play() {
            System.out.println(" dvd is playing ");
        }
    
        public void pause() {
            System.out.println(" dvd pause ..");
        }
    }

popCorn

//  爆米花
    class Popcorn {
    
        private static volatile Popcorn instance;
    
        private Popcorn() {
        }
    
        public static Popcorn getInstance() {
            if (instance == null) {
                synchronized (Popcorn.class) {
                    if (instance == null) {
                        instance = new Popcorn();
                    }
                }
            }
            return instance;
        }
    
        public void on() {
            System.out.println(" popcorn on ");
        }
    
        public void off() {
            System.out.println(" popcorn off ");
        }
    
        public void pop() {
            System.out.println(" popcorn is poping  ");
        }
    }

Projector投影仪

// Projector 投影仪
    class Projector {
    
        private static volatile Projector instance;
    
        private Projector() {
        }
    
        public static Projector getInstance() {
            if (instance == null) {
                synchronized (Projector.class) {
                    if (instance == null) {
                        instance = new Projector();
                    }
                }
            }
            return instance;
        }
    
        public void on() {
            System.out.println(" Projector on ");
        }
    
        public void off() {
            System.out.println(" Projector off ");
        }
    
        public void focus() {
            System.out.println(" Projector is Projector  ");
        }
    }

screen 屏幕

// 屏幕操作
    class Screen {
    
        private static volatile Screen instance;
    
        private Screen() {
        }
    
        public static Screen getInstance() {
            if (instance == null) {
                synchronized (Screen.class) {
                    if (instance == null) {
                        instance = new Screen();
                    }
                }
            }
            return instance;
        }
    
        public void up() {
            System.out.println(" Screen up ");
        }
    
        public void down() {
            System.out.println(" Screen down ");
        }
    }

stereo

//  立体声
    class Stereo {
    
        private static volatile Stereo instance;
    
        private Stereo() {
        }
    
        public static Stereo getInstance() {
            if (instance == null) {
                synchronized (Stereo.class) {
                    if (instance == null) {
                        instance = new Stereo();
                    }
                }
            }
            return instance;
        }
    
        public void on() {
            System.out.println(" Stereo on ");
        }
    
        public void off() {
            System.out.println(" Stereo off ");
        }
    
        public void up() {
            System.out.println(" Stereo up.. ");
        }
    }

TheaterLight灯光

class TheaterLight {
    
        private static volatile TheaterLight instance;
    
        private TheaterLight() {
        }
    
        public static TheaterLight getInstance() {
            if (instance == null) {
                synchronized (TheaterLight.class) {
                    if (instance == null) {
                        instance = new TheaterLight();
                    }
                }
            }
            return instance;
        }
    
        public void on() {
            System.out.println(" TheaterLight on ");
        }
    
        public void off() {
            System.out.println(" TheaterLight off ");
        }
    
        public void dim() {
            System.out.println(" TheaterLight dim.. ");
        }
    
        public void bright() {
            System.out.println(" TheaterLight bright.. ");
        }
    }

// ================= 外观类 =================

//  外观类
    class HomeTheaterFacade {
    
        //定义各个子系统对象
        private TheaterLight theaterLight;
        private Popcorn popcorn;
        private Stereo stereo;
        private Projector projector;
        private Screen screen;
        private DVDPlayer dVDPlayer;
    
        public HomeTheaterFacade () {
            super();
            this.theaterLight = TheaterLight.getInstance();
            this.popcorn = Popcorn.getInstance();
            this.stereo = Stereo.getInstance();
            this.projector = Projector.getInstance();
            this.screen = Screen.getInstance();
            this.dVDPlayer = DVDPlayer.getInstance();
        }
    
        //操作分成 4 步
        public void ready() {
            popcorn.on();
            popcorn.pop();
            screen.down();
            projector.on();
            stereo.on();
            dVDPlayer.on();
            theaterLight.dim();
        }
    
        public void play() {
            dVDPlayer.play();
        }
    
        public void pause() {
            dVDPlayer.pause();
        }
    
        public void end() {
            popcorn.off();
            theaterLight.bright();
            screen.up();
            projector.off();
            stereo.off();
            dVDPlayer.off();
        }
    }

client客户端调用

public static void main(String[] args) {
            HomeTheaterFacade homeTheaterFacade = new HomeTheaterFacade();
            System.out.println("======ready()=====");
            homeTheaterFacade.ready();
            System.out.println("======play()=====");
            homeTheaterFacade.play();
            System.out.println("======pause()=====");
            homeTheaterFacade.pause();
            System.out.println("======end()=======");
            homeTheaterFacade.end();
        }

> ======ready()=====  
>  popcorn on   
>  popcorn is poping    
>  Screen down   
>  Projector on   
>  Stereo on   
>  dvd on   
>  TheaterLight dim..   
> ======play()=====  
>  dvd is playing   
> ======pause()=====  
>  dvd pause ..  
> ======end()=======  
>  popcorn off   
>  TheaterLight bright..   
>  Screen up   
>  Projector off   
>  Stereo off   
>  dvd off

--------------------

外观模式在Mybatis 框架应用的源码分析

Mybatis 中的Configuration 去创建MetaObject 对象使用到外观模式

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1Mjc1MjMz_size_16_color_FFFFFF_t_70 2][]

类图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM1Mjc1MjMz_size_16_color_FFFFFF_t_70 3][]

--------------------

使用中注意事项

1）外观模式屏蔽了子系统的细节，因此降低了客户端堆子系统使用的复杂度

2）外观模式对客户端与子系统的耦合关系 - 解耦，让子系统内部的模块更易维护和扩展

3）更好的划分访问的层次

4）当系统需要进行分层设计时，可以考虑使用Facade 模式

5）不能过多的或者不合理的使用外观模式，使用外观模式好，还是直接调用模块好，要以系统有层次，利于维护为目的

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[20200513152651843.png]: https://img-blog.csdnimg.cn/20200513152651843.png
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