享元模式

享元模式定义
英文定义：A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects
中文定义：享元模式运用共享技术有效地支持大量细粒度的对象。

享元模式UML图（来自Google图片）




抽象享元（Flyweight）角色：是给实现享元提供的接口。
具体享元（ConcreteFlyweight）角色：实现抽象角色，此对象必须是共享的，所含的状态必须是内部状态。
不共享享元（UnSharedConcreteFlyweight）角色：此对象不可共享，不是所有实现抽象享元接口的的对象都要共享，此对象通常将ConcreteFlyweight作为组成元素。
享元模式使用场景（摘自《大话设计模式》）
在襄垣对象内部并且不会随环境改变而改变的共享部分，可以称为是享元对象的内部状态，随环境改变而改变的、不可以共享的状态就是外部状态了。事实上，享元模式可以避免大量非常相似类的开销。在程序设计中，有时需要生成大量细粒度的类实例来表示数据。如果能发现这些实例除了几个参数外基本上都是相同的，有时就能够大幅度地减少需要实例化的类的数量。如果能把那些参数移到类实例的外面，在方法调用时将它们传递进来，就可以通过共享大幅度地减少单个实例的数目。也就是说，享元模式Flyweight执行时所需要的状态是有内部的也可能有外部的，内部状态存储于ConcreteFlyweight对象之中，而外部对象则应该考虑由客户端对象存储或计算，当调用Flyweight对象的操作时，将该状态传递给它。
如果一个应用程序使用了大量的对象，而大量的这些对象造成了很大的存储开销时就应该考虑使用；还有就是对象的大多数状态可以外部状态，如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象，此时可以考虑使用享元模式。

在Java中，String类型就是使用了享元模式。String对象是final类型，对象一旦创建就不可改变。在Java中字符串常量都是存在常量池中的，Java会确保一个字符串常量在常量池中只有一个拷贝。

享元模式源码示例

享元模式原型

public abstract class FlyWeight {
    public abstract void operation(int state);
}

public class ConcreteFlyWeight extends FlyWeight {

    @Override
    public void operation(int state) {
        System.out.println("Concrete Flyweight Information: " + state);
    }

}

public class FlyWeightFactory {
    private Map<String, FlyWeight> flyweightMap = new HashMap<String, FlyWeight>();


    public FlyWeightFactory() {
        // 这里我们初始化三个对象
        flyweightMap.put("X", new ConcreteFlyWeight());
        flyweightMap.put("X", new ConcreteFlyWeight());
        flyweightMap.put("X", new ConcreteFlyWeight());
    }

    public FlyWeight getFlyWeight(String key) {
        //当客户端要求生成一个对象时，工厂会检测是否存在此对象的实例，如果存在那么直接返回此对象实例，如果不存在就创建一个并保存起来，这点有些单例模式的意思。
        FlyWeight flyWeight = flyweightMap.get(key);
        if (null == flyWeight) {
            flyWeight = new ConcreteFlyWeight();
            flyweightMap.put(key, flyWeight);
        }
        return flyWeight;
    }
}

public class Client {
    public static void main(String[] args) {
        FlyWeightFactory factory = new FlyWeightFactory();
        FlyWeight flyWeightX = factory.getFlyWeight("X");
        flyWeightX.operation(2);

        FlyWeight flyWeightA = factory.getFlyWeight("A");
        flyWeightA.operation(1);
    }
}

一个实例：

import java.util.concurrent.CopyOnWriteArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// Instances of CoffeeFlavour will be the Flyweights
class CoffeeFlavour {
  private final String name;

  CoffeeFlavour(String newFlavor) {
    this.name = newFlavor;
  }

  @Override
  public String toString() {
    return name;
  }
}

// Menu acts as a factory and cache for CoffeeFlavour flyweight objects
class Menu {
  private Map<String, CoffeeFlavour> flavours = new HashMap<String, CoffeeFlavour>();

  CoffeeFlavour lookup(String flavorName) {
    if (!flavours.containsKey(flavorName))
      flavours.put(flavorName, new CoffeeFlavour(flavorName));
    return flavours.get(flavorName);
  }

  int totalCoffeeFlavoursMade() {
    return flavours.size();
  }
}

class Order {
  private final int tableNumber;
  private final CoffeeFlavour flavour;

  Order(int tableNumber, CoffeeFlavour flavor) {
    this.tableNumber = tableNumber;
    this.flavour = flavor;
  }

  void serve() {
    System.out.println("Serving " + flavour + " to table " + tableNumber);
  }
}

public class CoffeeShop {
  private final List<Order> orders = new CopyOnWriteArrayList<Order>();
  private final Menu menu = new Menu();

  void takeOrder(String flavourName, int table) {
    CoffeeFlavour flavour = menu.lookup(flavourName);
    Order order = new Order(table, flavour);
    orders.add(order);
  }

  void service() {
    for (Order order : orders) {
      order.serve();
      orders.remove(order);    
    }
  }

  String report() {
    return "\ntotal CoffeeFlavour objects made: "
        + menu.totalCoffeeFlavoursMade();
  }

  public static void main(String[] args) {
    CoffeeShop shop = new CoffeeShop();

    shop.takeOrder("Cappuccino", 2);
    shop.takeOrder("Frappe", 1);
    shop.takeOrder("Espresso", 1);
    shop.takeOrder("Frappe", 897);
    shop.takeOrder("Cappuccino", 97);
    shop.takeOrder("Frappe", 3);
    shop.takeOrder("Espresso", 3);
    shop.takeOrder("Cappuccino", 3);
    shop.takeOrder("Espresso", 96);
    shop.takeOrder("Frappe", 552);
    shop.takeOrder("Cappuccino", 121);
    shop.takeOrder("Espresso", 121);

    shop.service();
    System.out.println(shop.report());
  }
}