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设计模式 - 享元.md

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享元(Flyweight)

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。

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

Implementation

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");
    }
}
Object address: 1163157884
IntrinsicState: aa
ExtrinsicState: x
Object address: 1163157884
IntrinsicState: aa
ExtrinsicState: y

JDK

Java 利用缓存来加速大量小对象的访问时间。

  • java.lang.Integer#valueOf(int)
  • java.lang.Boolean#valueOf(boolean)
  • java.lang.Byte#valueOf(byte)
  • java.lang.Character#valueOf(char)