CHARLIE SAYS

意图

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。它让某个类的一个实例能用来提供许多”虚拟实例”。

类图

• 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

classDiagram
    class Flyweight {
        <<interface>>
        +doOperation(extrinsicState String) void
    }
    class ConcreteFlyweight {
        -intrinsicState String
        +doOperation(extrinsicState String) void
    }
    class FlyweightFactory {
        -flyweights HashMap~String,Flyweight~
        +getFlyweight(intrinsicState String) Flyweight
    }
    class Client
    Flyweight <|.. ConcreteFlyweight
    FlyweightFactory o-- Flyweight : 缓存共享
    Client ..> FlyweightFactory

关键在 FlyweightFactory：相同内部状态的对象只在第一次创建，之后全部复用同一个实例。

实现

public interface Flyweight {
    void doOperation(String extrinsicState);
}

public class ConcreteFlyweight implements Flyweight {
    private String 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);
    }
}

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——注意两次获取的对象地址相同（同一实例复用），但外部状态不同：

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

JDK 中的应用

Java 利用缓存来加速大量小对象的访问时间，这就是享元模式：

• java.lang.Integer#valueOf(int)——Integer 缓存池（-128 ~ 127，详见 Java 基础 - 知识点中的常量池分析）
• java.lang.Boolean#valueOf(boolean)
• java.lang.Byte#valueOf(byte)
• java.lang.Character#valueOf(char)
• String 字符串常量池

系列导航

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