23中设计模式之外观模式

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外观模式是为了解决类与类之家的依赖关系的,像spring一样,可以将类和类之间的关系配置到配置文件中,而外观模式就是将他们的关系放在一个Facade类中,降低了类类之间的耦合度,该模式中没有涉及到接口,看下类图:(我们以一个计算机的启动过程为例)
pic

我们先看下实现类:

[java] view plaincopy1.public class CPU {  2.      3.    public void startup(){  4.        System.out.println("cpu startup!");  5.    }  6.      7.    public void shutdown(){  8.        System.out.println("cpu shutdown!");  9.    }  10.}  [java] view plaincopy1.public class Memory {  2.      3.    public void startup(){  4.        System.out.println("memory startup!");  5.    }  6.      7.    public void shutdown(){  8.        System.out.println("memory shutdown!");  9.    }  10.}  [java] view plaincopy1.public class Disk {  2.      3.    public void startup(){  4.        System.out.println("disk startup!");  5.    }  6.      7.    public void shutdown(){  8.        System.out.println("disk shutdown!");  9.    }  10.}  [java] view plaincopy1.public class Computer {  2.    private CPU cpu;  3.    private Memory memory;  4.    private Disk disk;  5.      6.    public Computer(){  7.        cpu = new CPU();  8.        memory = new Memory();  9.        disk = new Disk();  10.    }  11.      12.    public void startup(){  13.        System.out.println("start the computer!");  14.        cpu.startup();  15.        memory.startup();  16.        disk.startup();  17.        System.out.println("start computer finished!");  18.    }  19.      20.    public void shutdown(){  21.        System.out.println("begin to close the computer!");  22.        cpu.shutdown();  23.        memory.shutdown();  24.        disk.shutdown();  25.        System.out.println("computer closed!");  26.    }  27.}  User类如下:[java] view plaincopy1.public class User {  2.  3.    public static void main(String[] args) {  4.        Computer computer = new Computer();  5.        computer.startup();  6.        computer.shutdown();  7.    }  8.}  输出:start the computer!cpu startup!memory startup!disk startup!start computer finished!begin to close the computer!cpu shutdown!memory shutdown!disk shutdown!computer closed!

如果我们没有Computer类,那么,CPU、Memory、Disk他们之间将会相互持有实例,产生关系,这样会造成严重的依赖,修改一个类,可能会带来其他类的修改,这不是我们想要看到的,有了Computer类,他们之间的关系被放在了Computer类里,这样就起到了解耦的作用,这,就是外观模式!

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