JAVA Facade模式，正确实现？

Question

我有一个像构造函数一样的客户端，在参数列表方面相当冗长，例如，

Class Client {
   private ServiceA _serviceA;
   private ServiceB _serviceB;
   ....
   private ServiceE _serviceE;

   public Client(ServiceA serviceA, ServiceB serviceB,...ServiceE service E) { ... }

   public doTask1(TypeA typeA) { //only serviceA, serviceB service being used... }

   public doTask2(TypeB typeB) { //only serviceD, serviceE being used ... }
}

我想在这里使用服务门面来修剪构造函数参数列表。但是，我很困惑作为Facade实现的核心职责。所以我用服务作为类变量和它们的getter写下了一个外观，如下所示：

Class Facade {
   private ServiceA _serviceA;
   private ServiceB _serviceB;
   ....
   private ServiceE _serviceE;

   getters () ...
}

在这种情况下，这是抽象立面的正确方法吗？如果没有，那么重构Client类的正确方法是什么？

Answer 1

Facedes有一个完全不同的意图：创建它们是为了封装和隐藏类的底层结构和行为。举一个汽车的例子。它由许多组件组成：车载电脑，燃油泵，发动机等。如果你想启动它，只需按下开始按钮：

class FuelPump {
  private boolean pumpTurnedOn;
  public FuelPump() {
    pumpTunrnedOn=false;
  }
  public boolean isPumpTunredOn() {
    return pumpTurnedOn;
  }
  public void setPumpTurnedOn (boolean newState) {
    pumpTurndeOn=newState;
    if (newState) {
      System.out.println ("fuel pump now is on");
    } else {
      System.out.println ("fuel pump now is turned off");
    }
  }
}

---

class Engine {
  private boolean engineStarted;
  public Engine() {
    engineStarted=false;
  }
  public boolean isEngineStarted() {
    return engineStarted;
  }
  public void setEngineStarted (boolean newState) {
    engineStarted=newState;
    if (newState) {
      System.out.println("engine now is on");
    } else {
      System.out.println("engine now is turned off");
    }
  }
}

---

// this is the Car facade:
class Car {
  private FuelPump fuelPump;
  private Engine engine;
  // + other components of Car
  public Car () {
    fuelPump = new FuelPump();
    engine = new Engine();
  }
  public void startCar() {
    fuelPump.setPumpTurnedOn(true);
    engine.setEngineStarted(true);
    // + other methods of start procedures with other components
    System.out.println("Your car has been startded");
  }
  public void stopCar() {
    engine.setEngineStarted(false);
    fuelPump.setPumpTurnedOn(false);
    // + other methods on other components for shutting down car
  }
}

---

客户端代码段：

Car car=new Car();
car.startCar();
// later on
car.stopCar();

您可能会看到客户端对启动汽车的基础组件一无所知。它只需要使用startCar（）方法，而Car facade将完成剩下的工作。立面是一种结构模式。如果你有许多构造函数参数并想要重新使用它们，请使用其中一个创建模式。如果你有强制性和非强制性字段，我建议使用构建器模式。例如，您的必需构造函数参数是Service_A和Service_B，而Service_C不需要Service_C。那么你的ClientBuilder类应该是这样的：

class ClientBuilder{
  private static Service_A serviceA;  // required
  private static Service_B serviceB;  // required
  private static Service_C serviceC;
  private static Service_D serviceD;
  private static Service_E serviceE;

  // since this builder is singleton
  private static ClientBuilder builderInstance = new ClientBuilder();
  private ClientBuilder () {};

  public static ClientBuilder getBuilderInstance (Service_A service_A, Service_B service_B){
    serviceA = service_A;
    serviceB = service_B;
    serviceC = null; 
    serviceD = null;
    serviceE = null;
    return builderInstance;
  }

  public static ClientBuilder addServiceC (Service_C service_C) {
    serviceC = service_C;
    return builderInstance; 
  }

  public static ClientBuilder addServiceD (Service_D service_D) {
    serviceC = service_D;
    return builderInstance; 
  }

  public static ClientBuilder addServiceE (Service_E service_E) {
    serviceE = service_E;
    return builderInstance; 
  }

  public static Client build(){
    return new Client (serviceA, ServiceB, ServiceC, ServiceD, ServiceE);
  }

在这种情况下，您只能使用必需参数即时连接Client类。最好的事情是不需要的参数'顺序是可以互换的：

  Client aClient = ClientBuilder.getBuilderInstance(aServiceA, aServiceB)
        .addServiceE(aServiceE)
        .addServiceC(aServiceC)
        .build();

现在已经使用服务A，B，C，E创建了aClient，并且serviceD保持为null。稍后您可以通过适当的setter进行设置。 getter和setter必须位于Client类中。简而言之，使用构建器类，您可以减少强制的构造函数参数的数量，稍后使用setter设置可选字段。您可以在Gang of Four book中阅读更多详细信息，或者如果您是一个严肃的Java乐趣，我建议您使用Head First's Design Patterns。希望我能帮助你，再见！