设计模式--结构型：享元模式

享元模式：运用共享技术有效的支持大量细粒度的对象。

享元模式 Flyweight

享元模式：将对象划分为内部状态和外部状态，共享内部状态，通过共享池来减少对象的一种模式。

内部状态/外部状态

• 内部状态
  享元对象内部不会随环境改变而改变的共享部分，称为享元对象的内部状态。也就是说，内部状态是不变的部分。
• 外部状态
  受环境影响改变，不可共享的部分，称为享元对象的外部状态。也就是说，外部状态是变化的。

比如一堆衣服中有男女两个款式，如果需要请模特过来拍照展示，其中性别是无法改变的部分，即内部状态；而衣服款式是多变不一样的，所以款式是外部状态。在这个例子中，通过创建共享池共享性别对象。

类图结构

结构解析

• Flyweight
  抽象类，享元类，表示可以共享的抽象类。
• ConcreteFlyweight
  实现类，具体共享类。
• UnsharedConcreteFlyweight
  实现类，具有共享能力，但是不强制该类共享。
• FlyweightFactory
  享元工厂，创建并管理享元对象，通过共享池保存。

示例

// 1. Flyweight
public abstract class Flyweight {
    public abstract void operation(String extrinsicState);
}

// 2. ConcreteFlyweight
public class ConcreteFlyweight extends Flyweight{
    @Override
    public void operation(String extrinsicState) {
        System.out.println("ConcreteFlyweight::operation::extrinsicState = "
                + extrinsicState);
    }
}

// 3. UnsharedConcreteFlyweight
public class UnsharedConcreteFlyweight extends Flyweight{
    @Override
    public void operation(String extrinsicState) {
      System.out.println("UnsharedConcreteFlyweight::operation::extrinsicState
                = " + extrinsicState);
    }
}

// 4. FlyweightFactory
public class FlyweightFactory {
    private static HashMap<String, Flyweight> flyweightMaps = new HashMap<>();

    public static Flyweight getFlyweight(String key){
        if (flyweightMaps.containsKey(key)){
            return flyweightMaps.get(key);
        }else {
            Flyweight flyweight = new ConcreteFlyweight();
            flyweightMaps.put(key, flyweight);
            return flyweight;
        }
    }
}

// 5. Test
public class TestFlyweight {
    public static void main(String[] args) {
        String extrinsicState = "A";
        Flyweight flyweight1 = FlyweightFactory.getFlyweight(extrinsicState);
        flyweight1.operation(extrinsicState);

        extrinsicState = "B";
        Flyweight flyweight2 = FlyweightFactory.getFlyweight(extrinsicState);
        flyweight2.operation(extrinsicState);

        extrinsicState = "C";
        Flyweight flyweight3 = new UnsharedConcreteFlyweight();
        flyweight3.operation(extrinsicState);
    }
}

// 6. Result
ConcreteFlyweight::operation::extrinsicState = A
ConcreteFlyweight::operation::extrinsicState = B
UnsharedConcreteFlyweight::operation::extrinsicState = C

总结

享元模式主要使用了缓存来减少创建重复对象，能大大降低内存占用率。

参考文档

• 大话设计模式
• Android 源码设计模式解析与实战