一、定义：
门面模式（Facade Pattern），是指提供一个统一的接口去访问多个子系统的多个不同的接口，它为子系统中的一组接口提供一个统一的高层接口。使得子系统更容易使用。
二、门面模式的作用：
参考文章：
The facade pattern (also spelled façade) is a software-design pattern commonly used with object-oriented programming. Analogous to a facade in architecture, a facade is an object that serves as a front-facing interface masking more complex underlying or structural code. A facade can:
1、improve the readability and usability of a software library by masking interaction with more complex components behind a single (and often simplified) API；
2、provide a context-specific interface to more generic functionality (complete with context-specific input validation)；
3、serve as a launching point for a broader refactor of monolithic or tightly-coupled systems in favor of more loosely-coupled code；
Developers often use the facade design pattern when a system is very complex or difficult to understand because the system has a large number of interdependent classes or because its source code is unavailable. This pattern hides the complexities of the larger system and provides a simpler interface to the client. It typically involves a single wrapper class that contains a set of members required by the client. These members access the system on behalf of the facade client and hide the implementation details.
简而言之就是门面模式主要有三个作用：
提高了可读性和可用性，屏蔽系统内部复杂的逻辑；
提供了一个更通用的接口；
支持更多松散耦合代码；
三、UML图：
客户端的调用变得非常简单明了。
四、设计模式使用实例：
1、设计组成一台电脑的各个组件：
/*
* 电脑CPU
*/
public class CPU {
public void freeze() {
//...
}
public void jump(long position) {
//...
}
public void execute() {
//...}
}
}
/*
* 硬件驱动
*/
class HardDrive {
public byte[] read(long lba, int size) {
//...
}
}
/*
* 内存
*/
class Memory {
public void load(long position, byte[] data) { ... }
}
2、抽取一个门面接口，便于扩展（不仅仅在于电脑的启动，手机和Pad亦然如此）
/*
* 抽取一个门面接口
*/
interface IComputerFacede{
public void start();
}
3、实现真正被客户端调用的门面对象
/*
* 门面对象
*/
class ComputerFacade implements IComputerFacede {
private CPU cpu;
private Memory memory;
private HardDrive hardDrive;
public ComputerFacade() {
this.cpu = new CPU();
this.memory = new Memory();
this.hardDrive = new HardDrive();
}
public void start() {
cpu.freeze();
memory.load(BOOT_ADDRESS, hardDrive.read(BOOT_SECTOR, SECTOR_SIZE));
cpu.jump(BOOT_ADDRESS);
cpu.execute();
}
}
4、客户端调用
/*
* 客户端调用
*/
class Client {
/* 只要简单的一个调用，就可以完成电脑的启动，无需了解内部逻辑 */
public static void main(String[] args) {
ComputerFacade computer = new ComputerFacade();
computer.start();
}
}