外观模式精讲-优快云博客

外观模式（Facade Pattern）

为子系统中的一组接口提供一个一致的界面，外观模式定义了一个高层接口，这个接口使得这一子系统更加容易使用。

将一个系统划分成为若干个子系统有利于降低系统的复杂性。一个常见的设计目标是使子系统间的通信和相互依赖关系达到最小。达到该目标的途径之一就是引入一个外观对象，它为子系统中较为一般的设施提供了一个单一而简单的界面。

以下情况使用外观模式：

当你要为一个复杂的子系统提供一个简单的接口，子系统往往因为不断演化而变得越来越复杂。大多数模式使用时都会产生更多更小的类，这使得子系统更具可重用性，也更容易对子系统进行定制，但这也给那些不需要定制子系统的用户带来一些使用上的困难。
客户程序与抽象类的实现部分之间存在很大的依赖性。引入外观将这个子系统与客户以及其他子系统分离，可以提高子系统的独立性和可移植性。
当你需要构建一个层次结构的子系统时，使用外观模式定义子系统中每层的入口点。如果子系统之间是相互依赖的，你可以让它们仅通过外观进行通讯，从而简化了它们之间的依赖关系。

示例：

CPU.java

public class CPU {

    public void freeze() {

        System.out.println("CPU freeze");

    }



    public void jump(long position) {

        System.out.println("CPU jump");

    }



    public void execute() {

        System.out.println("CPU execute");

    }



}

HardDevice.java

public class HardDrive {

    public byte[] read(long lba, int size) {

        System.out.println("HardDevice read");

        return new byte[size];

    }

}

public class Memory {

    public static long BOOT_ADDRESS;

    public static long BOOT_SECTOR;

    public static int SECTOR_SIZE;

    public void load(long position, byte[] data) {

        System.out.println("Memory load");

    }

}

ComputerFacade.java

public class ComputerFacade {

    private CPU processor;

    private Memory ram;

    private HardDrive hd;



    public ComputerFacade() {

        this.processor = new CPU();

        this.ram = new Memory();

        this.hd = new HardDrive();

    }



    public void start() {

        processor.freeze();

        ram.load(Memory.BOOT_ADDRESS, hd.read(Memory.BOOT_SECTOR, Memory.SECTOR_SIZE));

        processor.jump(Memory.BOOT_ADDRESS);

        processor.execute();

    }

}

App.java

public class App {

    public static void main(String[] args) {

        ComputerFacade computer = new ComputerFacade();

        computer.start();

    }

}

示例2

DwarvenCartOperator.java

public class DwarvenCartOperator extends DwarvenMineWorker {



  @Override

  public void work() {

    System.out.println(name() + " moves gold chunks out of the mine.");

  }



  @Override

  public String name() {

    return "Dwarf cart operator";

  }

}

DwarvenGoldDigger.java

public class DwarvenGoldDigger extends DwarvenMineWorker {



  @Override

  public void work() {

    System.out.println(name() + "digs for gold.");

  }



  @Override

  public String name() {

    return "Dwarf gold digger";

  }

}

DwarvenMineWorker.java

public abstract class DwarvenMineWorker {



  public void goToSleep() {

    System.out.println(name() + "goes to sleep.");

  }



  public void wakeUp() {

    System.out.println(name() + " wakes up.");

  }



  public void goHome() {

    System.out.println(name() + " goes home.");

  }



  public void goToMine() {

    System.out.println(name() + " goes to the mine.");

  }



  private void action(Action action) {

    switch (action) {

      case GO_TO_SLEEP:

        goToSleep();

        break;

      case WAKE_UP:

        wakeUp();

        break;

      case GO_HOME:

        goHome();

        break;

      case GO_TO_MINE:

        goToMine();

        break;

      case WORK:

        work();

        break;

      default:

        System.out.println("Undefined action");

        break;

    }

  }



  /**

   * Perform actions

   */

  public void action(Action... actions) {

    for (Action action : actions) {

      action(action);

    }

  }



  public abstract void work();



  public abstract String name();



  static enum Action {

    GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK

  }

}

DwarvenTunelDigger.java

public class DwarvenTunnelDigger extends DwarvenMineWorker {



  @Override

  public void work() {

    System.out.println(name() + " creates another promising tunnel.");

  }



  @Override

  public String name() {

    return "Dwarven tunnel digger";

  }

}

DwarvenGoldmineFacade.java

public class DwarvenGoldmineFacade {



  private final List<DwarvenMineWorker> workers;



  /**

   * Constructor

   */

  public DwarvenGoldmineFacade() {

    workers = new ArrayList<>();

    workers.add(new DwarvenGoldDigger());

    workers.add(new DwarvenCartOperator());

    workers.add(new DwarvenTunnelDigger());

  }



  public void startNewDay() {

    makeActions(workers, DwarvenMineWorker.Action.WAKE_UP, DwarvenMineWorker.Action.GO_TO_MINE);

  }



  public void digOutGold() {

    makeActions(workers, DwarvenMineWorker.Action.WORK);

  }



  public void endDay() {

    makeActions(workers, DwarvenMineWorker.Action.GO_HOME, DwarvenMineWorker.Action.GO_TO_SLEEP);

  }



  private static void makeActions(Collection<DwarvenMineWorker> workers,

      DwarvenMineWorker.Action... actions) {

    for (DwarvenMineWorker worker : workers) {

      worker.action(actions);

    }

  }

}