Java设计模式(5) -- 外观

Facade

 

英文简单描述

Intent
Provide a unified interface to a set of interfaces in a subsystem.

Facade defines a higher-level interface that makes the subsystem easier to use.
You want to provide a simple interface to a complex subsystem.

How to
Facade
   delegates client requests to appropriate subsystem objects.
subsystem classes
   implement subsystem functionality.
   handle work assigned by the Facade object.
   have no knowledge of the facade; that is, they keep no references to it.

Clients communicate with the subsystem by sending requests to Facade, which forwards them to the appropriate subsystem objects.

Known cases
complier system

UML

 

代码:

public class Client {public void printByDirect() { // 必须了解子系统，依赖性增加SubSystemClassA a = new SubSystemClassA();SubSystemClassB b = new SubSystemClassB();a.print();b.print();}public void printByFacade() { // 只需要知道Facade便可，减少了依赖性Facade facade = Facade.getInstance();facade.print();}}public class Facade { // 一般是单例private static Facade unique = new Facade();private Facade() {}public static Facade getInstance() {return unique;}public void print() {SubSystemClassA a = new SubSystemClassA();SubSystemClassB b = new SubSystemClassB();a.print();b.print();}}public class SubSystemClassA {public void print() {System.out.println("SubSystemClassA");}}public class SubSystemClassB {public void print() {System.out.println("SubSystemClassB");}}

 

和Mediator区别：

Subsystem中其他类不知道Facade类的存在，但是Mediator中其他类都知道Mediator类的存在