即隐藏系统的复杂性，为子系统的一组接口提供一个一致的界面，并向客户端提供一个可以访问系统的高层接口。
通过外观类为调用者提供统一的调用接口实现，外观类知道如何分配子系统去处理请求，从而将调用者的请求代理给适当的子系统对象。
组建一个家庭影院，有DVD播放器、灯光、爆米花机、投影仪、屏幕和立体声这几种设备。使用家庭影院的过程要求如下所示：
-----------准备工作-----------
DVD player is on
Light is on
Popcorn maker is on
Popcorn maker is running
Projector is on
Screen is going down
Stereo is on
Turn the volume up
-----------正在播放-----------
Light is off
Projector is focusing
DVD player is playing
-----------播放结束-----------
DVD player is off
Light is on
Popcorn maker is off
Projector is off
Screen is rising
Turn the volume down
Stereo is off
问如何编码实现？
public class Theater {
public static void main(String[] args) {
DVDPlayer dvdPlayer = DVDPlayer.getInstance();
Light light = Light.getInstance();
PopcornMaker popcornMaker = PopcornMaker.getInstance();
Projector projector = Projector.getInstance();
Screen screen = Screen.getInstance();
Stereo stereo = Stereo.getInstance();
System.out.println("-----------准备工作-----------");
dvdPlayer.on();
light.on();
popcornMaker.on();
popcornMaker.run();
projector.on();
screen.down();
stereo.on();
stereo.up();
System.out.println("-----------正在播放-----------");
light.off();
projector.focus();
dvdPlayer.play();
System.out.println("-----------播放结束-----------");
dvdPlayer.off();
light.on();
popcornMaker.off();
projector.off();
screen.up();
stereo.down();
stereo.off();
}
}传统方式下是在客户端直接创建各个子系统的对象，并直接去调用子系统的相关方法，对客户端暴露了子系统的细节
public class Facade {
private DVDPlayer dvdPlayer;
private Light light;
private PopcornMaker popcornMaker;
private Projector projector;
private Screen screen;
private Stereo stereo;
public Facade(){
this.dvdPlayer = DVDPlayer.getInstance();
this.light = Light.getInstance();
this.popcornMaker = PopcornMaker.getInstance();
this.projector = Projector.getInstance();
this.screen = Screen.getInstance();
this.stereo = Stereo.getInstance();
}
/**
* 做好播放的准备工作
* */
public void ready(){
System.out.println("-----------准备工作-----------");
this.dvdPlayer.on();
this.light.on();
this.popcornMaker.on();
this.popcornMaker.run();
this.projector.on();
this.screen.down();
this.stereo.on();
this.stereo.up();
}
/**
* 播放
* */
public void play(){
System.out.println("-----------正在播放-----------");
this.light.off();
this.projector.focus();
this.dvdPlayer.play();
}
public void end(){
System.out.println("-----------播放结束-----------");
this.dvdPlayer.off();
this.light.on();
this.popcornMaker.off();
this.projector.off();
this.screen.up();
this.stereo.down();
this.stereo.off();
}
}public class Theater {
public static void main(String[] args) {
Facade theater = new Facade();
theater.ready();
theater.play();
theater.end();
}
}可以看到在使用外观模式设计系统后，客户端只需要使用外观类即可，并不需要知道子系统的内部细节
Configuration类中有一个创建MetaObject对象的方法
查看MetaObject类里的这个方法
发现其调用了一个构造器，查看其代码
这里的Configuration就相当于外观类，而ObjectFactory这些工厂类就相当于子系统类
UML类图为：
更清晰地可以表示为：
