【图解设计模式】Flyweight模式

Posted on | In

通过尽量共享实例来避免new出实例。

示例

将许多普通字符组合成为“大型字符”。

类图

mTBO1O.png

BigChar类

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import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class BigChar {
    private char charname;
    private String fontdata;

    public BigChar(char charname) {
        this.charname = charname;

        try {
            BufferedReader reader = new BufferedReader(new FileReader("big" + charname + ".txt"));
            String line;
            StringBuffer buf = new StringBuffer();

            while ((line = reader.readLine()) != null) {
                buf.append(line);
                buf.append("\n");
            }

            reader.close();
            this.fontdata = buf.toString();
        } catch (IOException e) {
            this.fontdata = charname + "?";
        }
    }

    public void print() {
        System.out.print(fontdata);
    }
}

BigCharFactory类

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import java.util.HashMap;

public class BigCharFactory {
    private HashMap pool = new HashMap();
    private static BigCharFactory singleton = new BigCharFactory();

    private BigCharFactory() {}

    public static BigCharFactory getInstance() {
        return singleton;
    }

    public synchronized BigChar getBigChar(char charname) {
        BigChar bc = (BigChar) pool.get(charname);

        if (bc == null) {
            bc = new BigChar(charname);
            pool.put("" + charname, bc);
        }

        return bc;
    }
}

BigString类

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public class BigString {
    private BigChar[] bigChars;

    public BigString(String string) {
        bigChars = new BigChar[string.length()];
        BigCharFactory factory = BigCharFactory.getInstance();

        for (int i = 0; i < string.length(); i++)
            bigChars[i] = factory.getBigChar(string.charAt(i));
    }

    public void print() {
        for (int i = 0; i < bigChars.length; i++)
            bigChars[i].print();
    }
}

Main类

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public class Main {

    public static void main(String[] args) {
        if (args.length == 0) {
            System.out.println("Usage: java Main digits");
            System.out.println("Example: java Main 1212123");
            System.exit(0);
        }

        BigString bs = new BigString(args[0]);
        bs.print();
    }
}

运行结果

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......##........
..######........
......##........
......##........
......##........
......##........
..##########....
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....######......
..##......##....
..........##....
......####......
....##..........
..##............
..##########....
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..######........
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......##........
..##########....
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....######......
..##......##....
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......####......
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..##............
..##########....
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..######........
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..##########....
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..##......##....
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..##########....
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..##......##....
....######......
................

登场角色

Flyweight（轻量级）

按照通常方式编写程序会导致程序变重，所以如果能够共享实例会比较好，而Flyweight角色表示的就是那些实例会被共享的类。在示例程序中，由BigChar类扮演此角色。

FlyweightFactory（轻量级工厂）

FlyweightFactory角色是生成Flyweight角色的工厂。在工厂中生成Flyweight角色可以实现共享实例。在示例程序中，由BigCharFactory类扮演此角色。

Client（请求者）

Client角色使用FlyweightFactory角色来生成Flyweight角色。在示例程序中，由BigString类扮演此角色。

类图

m74D1J.png