Flyweight概述

玩具代码案例 - 字体享元管理器

享元对象

同一种类型的字体没有必要频繁创建，完全可以重复使用

package online.javabook.gof.structural.patterns6.flyweight.font;
import java.util.Objects;
public class Font {
    final String name;
    final int style;
    final int size;
    public Font(String name, int style, int size) {
        this.name = name;
        this.style = style;
        this.size = size;
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Font font = (Font) o;
        return style == font.style &&
                size == font.size &&
                Objects.equals(name, font.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, style, size);
    }
}

package online.javabook.gof.structural.patterns6.flyweight.font;
public interface Style {
    final static int PLAIN = 1;
    final static int BOLD = 2;
    final static int ITALIC = 3;
}

享元对象管理器

package online.javabook.design.gof.structural6.flyweight.font;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
public class FontFlyweight {
    private static ConcurrentMap<String, Font> flyweights = new ConcurrentHashMap<>();
    public static Font create(String name, int style, int size) {
        Font font = null;
        if(!flyweights.containsKey(name)) {
            font = flyweights.putIfAbsent(name, new Font(name, style, size));
        }
        return font;
    }
}

不基于享元模式的实现

Main

package online.javabook.gof.structural.patterns6.flyweight.font.app.bad;
import online.javabook.gof.structural.patterns6.flyweight.font.Font;
import online.javabook.gof.structural.patterns6.flyweight.font.Style;
public class Main {
    public static void main(String[] args) {
        Font font1 = new Font("宋体", Style.BOLD, 12);
        Font font2 = new Font("宋体", Style.BOLD, 12);
        System.out.println("font1 == font2 ? " + (font1 == font2));
    }
}

Console

font1 == font2 ? false

基于享元模式的实现

Main

package online.javabook.gof.structural.patterns6.flyweight.font.app.good;

import online.javabook.gof.structural.patterns6.flyweight.font.Font;
import online.javabook.gof.structural.patterns6.flyweight.font.FontFlyweight;
import online.javabook.gof.structural.patterns6.flyweight.font.Style;

public class Main {
    public static void main(String[] args) {

        Font font1 = FontFlyweight.create("宋体", Style.BOLD, 12);
        Font font2 = FontFlyweight.create("宋体", Style.BOLD, 12);

        System.out.println("font1 == font2 ? " + (font1 == font2));
    }
}

Console

font1 == font2 ? true

现实世界中的享元模式

字符集对象

没有必要每次都生成一个字符集对象

package online.javabook.io.bio.charset;

import java.nio.charset.Charset;

public class CharsetMain {
    public static void main(String[] args) {

        //返回指定的字符集CharSet
        Charset utf81 = Charset.forName("utf8");

        //返回指定的字符集CharSet
        Charset utf82 = Charset.forName("utf8");

        System.out.println(utf81 == utf82);
    }
}

Console

true

字符串对象

Java字符串对象的常量池我认为也是一种享元模式。字符串在开发者被频繁创建，相同的字符串如果能够重用，自然是能够降低内存的使用。

只是大家关注的都是string创建了几个对象，引用比较，放在永生区还是堆中等等这些问题上。而不是真正学习设计的本身，本末倒置。

    /**
     * Returns a canonical representation for the string object.
     * <p>
     * A pool of strings, initially empty, is maintained privately by the
     * class {@code String}.
     * <p>
     * When the intern method is invoked, if the pool already contains a
     * string equal to this {@code String} object as determined by
     * the {@link #equals(Object)} method, then the string from the pool is
     * returned. Otherwise, this {@code String} object is added to the
     * pool and a reference to this {@code String} object is returned.
     * <p>
     * It follows that for any two strings {@code s} and {@code t},
     * {@code s.intern() == t.intern()} is {@code true}
     * if and only if {@code s.equals(t)} is {@code true}.
     * <p>
     * All literal strings and string-valued constant expressions are
     * interned. String literals are defined in section 3.10.5 of the
     * <cite>The Java&trade; Language Specification</cite>.
     *
     * @return  a string that has the same contents as this string, but is
     *          guaranteed to be from a pool of unique strings.
     */
    public native String intern();