维基百科

享元模式

享元模式(英语:Flyweight Pattern)是一种软件设计模式。它使用共享物件,用来尽可能减少内存使用量以及分享资讯给尽可能多的相似物件;它适合用于当大量物件只是重复因而导致无法令人接受的使用大量内存。通常物件中的部分状态是可以分享。常见做法是把它们放在外部数据结构,当需要使用时再将它们传递给享元。

典型的享元模式的例子为文书处理器中以图形结构来表示字符。一个做法是,每个字形有其字型外观, 字模 metrics, 和其它格式资讯,但这会使每个字符就耗用上千字节。取而代之的是,每个字符参照到一个共享字形物件,此物件会被其它有共同特质的字符所分享;只有每个字符(文件中或页面中)的位置才需要另外储存。

Java

以下程式用来解释上述的文字。这个例子用来解释享元模式利用只加载执行任务时所必需的最少资料,因而减少内存使用量。

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public enum FontEffect {
    BOLD, ITALIC, SUPERSCRIPT, SUBSCRIPT, STRIKETHROUGH
}

public final class FontData {
    /**
     * A weak hash map will drop unused references to FontData.
     * Values have to be wrapped in WeakReferences,
     * because value objects in weak hash map are held by strong references.
     */
    private static final WeakHashMap<FontData, WeakReference<FontData>> FLY_WEIGHT_DATA =
        new WeakHashMap<FontData, WeakReference<FontData>>();
    private final int pointSize;
    private final String fontFace;
    private final Color color;
    private final Set<FontEffect> effects;

    private FontData(int pointSize, String fontFace, Color color, EnumSet<FontEffect> effects) {
        this.pointSize = pointSize;
        this.fontFace = fontFace;
        this.color = color;
        this.effects = Collections.unmodifiableSet(effects);
    }

    public static FontData create(int pointSize, String fontFace, Color color,
        FontEffect... effects) {
        EnumSet<FontEffect> effectsSet = EnumSet.noneOf(FontEffect.class);
        for (FontEffect fontEffect : effects) {
            effectsSet.add(fontEffect);
        }
        // We are unconcerned with object creation cost, we are reducing overall memory consumption
        FontData data = new FontData(pointSize, fontFace, color, effectsSet);

        // Retrieve previously created instance with the given values if it (still) exists
        WeakReference<FontData> ref = FLY_WEIGHT_DATA.get(data);
        FontData result = (ref != null) ? ref.get() : null;

        // Store new font data instance if no matching instance exists
        if (result == null) {
            FLY_WEIGHT_DATA.put(data, new WeakReference<FontData> (data));
            result = data;
        }
        // return the single immutable copy with the given values
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof FontData) {
            if (obj == this) {
                return true;
            }
            FontData other = (FontData) obj;
            return other.pointSize == pointSize && other.fontFace.equals(fontFace)
                && other.color.equals(color) && other.effects.equals(effects);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return (pointSize * 37 + effects.hashCode() * 13) * fontFace.hashCode();
    }

    // Getters for the font data, but no setters. FontData is immutable.
}

例子

Flyweight

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public interface Flyweight {
    void action(int arg);
}

ConcreteFlyweight

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public class FlyweightImpl implements Flyweight {
    @Override
    public void action(int arg) {
        System.out.println("参数值:"+arg);
    }
}

FlyweightFactory

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import java.util.HashMap;
import java.util.Map;

public class FlyweightFactory {
    private static Map flyweights = new HashMap();

    public FlyweightFactory(String arg){
        flyweights.put(arg,new FlyweightImpl());
    }

    public static Flyweight getFlyweight(String key){
        if(flyweights.get(key)==null){
            flyweights.put(key,new FlyweightImpl());
        }
        return (Flyweight) flyweights.get(key);
    }

    public static int getSize(){
        return flyweights.size();
    }
}

Test

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public class Test {
    public static void main(String[] args) {
        Flyweight fly1 = FlyweightFactory.getFlyweight("a");
        fly1.action(1);

        Flyweight fly2 = FlyweightFactory.getFlyweight("a");
        System.out.println(fly1==fly2);

        Flyweight fly3 = FlyweightFactory.getFlyweight("b");
        fly3.action(2);

        Flyweight fly4 = FlyweightFactory.getFlyweight("c");
        fly4.action(3);

        Flyweight fly5 = FlyweightFactory.getFlyweight("d");
        fly5.action(4);

        System.out.println(FlyweightFactory.getSize());
    }
}

运行结果:

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参数值:1
true
参数值:2
参数值:3
参数值:4
4