享元（Flyweight）

1. 意图（Intent）

利用共享的方式来支持大量细粒度的对象，这些对象一部分内部状态是相同的。

2. 类图（Class Diagram）

• Flyweight：享元对象
• IntrinsicState：内部状态，享元对象共享内部状态
• ExtrinsicState：外部状态，每个享元对象的外部状态不同

  3. 实现（Implementation）

英文是 Flyweight Pattern，不知道是谁最先翻译的这个词，感觉这翻译真的不好理解，我们试着强行关联起来吧。Flyweight 是轻量级的意思，享元分开来说就是 共享 元器件，也就是复用已经生成的对象，这种做法当然也就是轻量级的了。

复用对象最简单的方式是，用一个 HashMap 来存放每次新生成的对象。每次需要一个对象的时候，先到 HashMap 中看看有没有，如果没有，再生成新的对象，然后将这个对象放入 HashMap 中。

public interface Flyweight {
    void doOperation(String extrinsicState); //外在状态(extrinsicState)
}

public class ConcreteFlyweight implements Flyweight {
    private String intrinsicState; // 内在状态(intrinsicState)
    public ConcreteFlyweight(String intrinsicState) {
        this.intrinsicState = intrinsicState;
    }
    @Override
    public void doOperation(String extrinsicState) {
        System.out.println("Object address: " + System.identityHashCode(this));
        System.out.println("IntrinsicState: " + intrinsicState);
        System.out.println("ExtrinsicState: " + extrinsicState);
    }
}

public class FlyweightFactory {
    private HashMap<String, Flyweight> flyweights = new HashMap<>();
    Flyweight getFlyweight(String intrinsicState) {
        if (!flyweights.containsKey(intrinsicState)) {
            Flyweight flyweight = new ConcreteFlyweight(intrinsicState);
            flyweights.put(intrinsicState, flyweight);
        }
        return flyweights.get(intrinsicState);
    }
}

客户端

public class Client {
    public static void main(String[] args) {
        FlyweightFactory factory = new FlyweightFactory();
        Flyweight flyweight1 = factory.getFlyweight("aa");
        Flyweight flyweight2 = factory.getFlyweight("aa");
        flyweight1.doOperation("x");
        flyweight2.doOperation("y");
    }
}

输出

Object address: 1163157884
IntrinsicState: aa
ExtrinsicState: x
Object address: 1163157884
IntrinsicState: aa
ExtrinsicState: y