一. 定义与类型
定义：门面模式，提供一个统一的接口，用来访问子系统中的一群接口，门面模式定义了一个高层接口，让子系统更容易使用
类型：结构性
二. 使用场景
子系统越来越复杂，增加外观模式提供简单调用接口
构建多层系统结构，利用外观对象作为每层的入口，简化层间调用
三. 优缺点
优点：简化了调用过程，无需了解深入子系统，防止带来风险。
减少系统依赖，松散耦合
更好的划分访问层次
符合迪米特法则，即最少知道原则
缺点：增加子系统，扩展子系统行为容易引入风险
不符合开闭原则
四. 相关设计模式
外观模式和中介者模式
外观模式和单例模式
外观模式和抽象工厂模式
五. Coding
假设一个场景，在商城中用积分退换商品时，有几个步骤：
(1) 校验：判断当前积分是否能够兑换商品
(2) 支付：使用积分支付兑换商品
(3)物流：将兑换的商品，送到目的地
在实际的场景中，一般正常的步骤是，用户直接用积分兑换商品，并没有校验，支付，物流等细节。而是提供了一个统一的兑换接口，来访问这些子系统。
/**
* @program: designModel
* @description: 积分礼物
* @author: YuKai Fan
* @create: 2018-12-17 10:11
**/
public class PointsGift {
private String name; public PointsGift(String name) {
this.name = name;
} public String getName() {
return name;
} public void setName(String name) {
this.name = name;
}
}
/**
* @program: designModel
* @description: 校验系统
* @author: YuKai Fan
* @create: 2018-12-17 10:12
**/
public class QualifyService {
public boolean isAvailable(PointsGift pointsGift) {
System.out.println("校验" + pointsGift.getName() + " 积分资格通过，库存通过");
return true;
}
}
/**
* @program: designModel
* @description: 校验系统
* @author: YuKai Fan
* @create: 2018-12-17 10:12
**/
public class QualifyService {
public boolean isAvailable(PointsGift pointsGift) {
System.out.println("校验" + pointsGift.getName() + " 积分资格通过，库存通过");
return true;
}
}
/**
* @program: designModel
* @description: 积分支付系统
* @author: YuKai Fan
* @create: 2018-12-17 10:13
**/
public class PoingtsPaymentService {
public boolean pay(PointsGift pointsGift) {
//扣减积分
System.out.println("支付" + pointsGift.getName() + " 积分成功");
return true;
}
}
/**
* @program: designModel
* @description: 物流系统
* @author: YuKai Fan
* @create: 2018-12-17 10:15
**/
public class ShippingService {
public String shipGift(PointsGift pointsGift) {
//物流系统的对接逻辑
System.out.println(pointsGift.getName() + "进入物流系统");
String shippingOrderNo = "666";
return shippingOrderNo;
}
}
统一对外开放的接口
/**
* @program: designModel
* @description: 礼物兑换
* @author: YuKai Fan
* @create: 2018-12-17 10:17
**/
public class GiftExchangeService {
private QualifyService qualifyService;
private PoingtsPaymentService poingtsPaymentService ;
private ShippingService shippingService; public void giftExchange(PointsGift pointsGift) {
if (qualifyService.isAvailable(pointsGift)) {
//资格校验通过
if (poingtsPaymentService.pay(pointsGift)) {
//如果支付积分成功
String shippingOrderNo = shippingService.shipGift(pointsGift);
System.out.println("物流系统下班成功，订单号：" + shippingOrderNo);
}
}
} public QualifyService getQualifyService() {
return qualifyService;
} public void setQualifyService(QualifyService qualifyService) {
this.qualifyService = qualifyService;
} public PoingtsPaymentService getPoingtsPaymentService() {
return poingtsPaymentService;
} public void setPoingtsPaymentService(PoingtsPaymentService poingtsPaymentService) {
this.poingtsPaymentService = poingtsPaymentService;
} public ShippingService getShippingService() {
return shippingService;
} public void setShippingService(ShippingService shippingService) {
this.shippingService = shippingService;
}
}
客户端：
/**
* @program: designModel
* @description:
* @author: YuKai Fan
* @create: 2018-12-17 10:21
**/
public class Test {
public static void main(String[] args) {
PointsGift pointsGift = new PointsGift("T shirt");
GiftExchangeService giftExchangeService = new GiftExchangeService(); giftExchangeService.setQualifyService(new QualifyService());
giftExchangeService.setPoingtsPaymentService(new PoingtsPaymentService());
giftExchangeService.setShippingService(new ShippingService()); giftExchangeService.giftExchange(pointsGift);
}
}
UML类图：
看上面的UML类图，应用层与子系统依旧存在着联系，这是因为没有集成spring的依赖注入的原因，而是直接在应用层中创建了子系统的对象注入到外观对象中。
进一步完善后的代码：
/**
* @program: designModel
* @description: 礼物兑换
* @author: YuKai Fan
* @create: 2018-12-17 10:17
**/
public class GiftExchangeService {
private QualifyService qualifyService = new QualifyService();
private PoingtsPaymentService poingtsPaymentService = new PoingtsPaymentService();
private ShippingService shippingService = new ShippingService(); public void giftExchange(PointsGift pointsGift) {
if (qualifyService.isAvailable(pointsGift)) {
//资格校验通过
if (poingtsPaymentService.pay(pointsGift)) {
//如果支付积分成功
String shippingOrderNo = shippingService.shipGift(pointsGift);
System.out.println("物流系统下班成功，订单号：" + shippingOrderNo);
}
}
} public QualifyService getQualifyService() {
return qualifyService;
} public void setQualifyService(QualifyService qualifyService) {
this.qualifyService = qualifyService;
} public PoingtsPaymentService getPoingtsPaymentService() {
return poingtsPaymentService;
} public void setPoingtsPaymentService(PoingtsPaymentService poingtsPaymentService) {
this.poingtsPaymentService = poingtsPaymentService;
} public ShippingService getShippingService() {
return shippingService;
} public void setShippingService(ShippingService shippingService) {
this.shippingService = shippingService;
}
}
/**
* @program: designModel
* @description:
* @author: YuKai Fan
* @create: 2018-12-17 10:21
**/
public class Test {
public static void main(String[] args) {
PointsGift pointsGift = new PointsGift("T shirt");
GiftExchangeService giftExchangeService = new GiftExchangeService(); // giftExchangeService.setQualifyService(new QualifyService());
// giftExchangeService.setPoingtsPaymentService(new PoingtsPaymentService());
// giftExchangeService.setShippingService(new ShippingService()); giftExchangeService.giftExchange(pointsGift);
}
}
UML类图：
上面的新类图，才是真正的外观模式的结构类图，支持了迪米特法则。
在开发过程中，要注意应用层到底有没有与子系统发生关系，要是又有了一个新的子系统，那就不符合开闭原则了。如果这个系统以后不需要扩展，或者扩展的子系统非常有限，那就用实体外观类就可以了，可以减少复杂度。但是如果需要经常新加子系统，那就需要使用抽象外观类。
六. 源码分析
springjdbc中的closeConnection()，closeStatement()，closeResultSet()等方法
mybatis中的Configuration配置类，使用的也是外观模式思想
tomcat源码中RequestFacade是一个request的一个外观类，实现了HttpServletRequest接口，
还有一个Request类也实现了HttpServletRequest接口，在request里声明了RequestFacade对象，而且具体的操作都是用的这个对象
tomcat源码中大量的使用了外观类