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享元

Java 享元模式讲解和代码示例

享元是一种结构型设计模式 它允许你在消耗少量内存的情况下支持大量对象

模式通过共享多个对象的部分状态来实现上述功能 换句话来说 享元会将不同对象的相同数据进行缓存以节省内存

复杂度

流行度

使用示例 享元模式只有一个目的 将内存消耗最小化 如果你的程序没有遇到内存容量不足的问题 则可以暂时忽略该模式

享元模式在核心 Java 程序库中的示例

识别方法 享元可以通过构建方法来识别 它会返回缓存对象而不是创建新的对象

渲染一片森林

本例中 我们将渲染一片森林 1,000,000 棵树 每棵树都由包含一些状态的对象来表示 坐标和纹理等 尽管程序能够完成其主要工作 但很显然它需要消耗大量内存

原因很简单 太多树对象包含重复数据 名称 纹理和颜色 因此我们可用享元模式来将这些数值存储在单独的享元对象中 Tree­Type 现在我们不再将相同数据存储在数千个 Tree对象中 而是使用一组特殊的数值来引用其中一个享元对象

客户端代码不会知道任何事情 因为重用享元对象的复杂机制隐藏在了享元工厂中

trees

trees/Tree.java: 包含每棵树的独特状态

package refactoring_guru.flyweight.example.trees;

import java.awt.*;

public class Tree {
    private int x;
    private int y;
    private TreeType type;

    public Tree(int x, int y, TreeType type) {
        this.x = x;
        this.y = y;
        this.type = type;
    }

    public void draw(Graphics g) {
        type.draw(g, x, y);
    }
}

trees/TreeType.java: 包含多棵树共享的状态

package refactoring_guru.flyweight.example.trees;

import java.awt.*;

public class TreeType {
    private String name;
    private Color color;
    private String otherTreeData;

    public TreeType(String name, Color color, String otherTreeData) {
        this.name = name;
        this.color = color;
        this.otherTreeData = otherTreeData;
    }

    public void draw(Graphics g, int x, int y) {
        g.setColor(Color.BLACK);
        g.fillRect(x - 1, y, 3, 5);
        g.setColor(color);
        g.fillOval(x - 5, y - 10, 10, 10);
    }
}

trees/TreeFactory.java: 封装创建享元的复杂机制

package refactoring_guru.flyweight.example.trees;

import java.awt.*;
import java.util.HashMap;
import java.util.Map;

public class TreeFactory {
    static Map<String, TreeType> treeTypes = new HashMap<>();

    public static TreeType getTreeType(String name, Color color, String otherTreeData) {
        TreeType result = treeTypes.get(name);
        if (result == null) {
            result = new TreeType(name, color, otherTreeData);
            treeTypes.put(name, result);
        }
        return result;
    }
}

forest

forest/Forest.java: 我们绘制的森林

package refactoring_guru.flyweight.example.forest;

import refactoring_guru.flyweight.example.trees.Tree;
import refactoring_guru.flyweight.example.trees.TreeFactory;
import refactoring_guru.flyweight.example.trees.TreeType;

import javax.swing.*;
import java.awt.*;
import java.util.ArrayList;
import java.util.List;

public class Forest extends JFrame {
    private List<Tree> trees = new ArrayList<>();

    public void plantTree(int x, int y, String name, Color color, String otherTreeData) {
        TreeType type = TreeFactory.getTreeType(name, color, otherTreeData);
        Tree tree = new Tree(x, y, type);
        trees.add(tree);
    }

    @Override
    public void paint(Graphics graphics) {
        for (Tree tree : trees) {
            tree.draw(graphics);
        }
    }
}

Demo.java: 客户端代码

package refactoring_guru.flyweight.example;

import refactoring_guru.flyweight.example.forest.Forest;

import java.awt.*;

public class Demo {
    static int CANVAS_SIZE = 500;
    static int TREES_TO_DRAW = 1000000;
    static int TREE_TYPES = 2;

    public static void main(String[] args) {
        Forest forest = new Forest();
        for (int i = 0; i < Math.floor(TREES_TO_DRAW / TREE_TYPES); i++) {
            forest.plantTree(random(0, CANVAS_SIZE), random(0, CANVAS_SIZE),
                    "Summer Oak", Color.GREEN, "Oak texture stub");
            forest.plantTree(random(0, CANVAS_SIZE), random(0, CANVAS_SIZE),
                    "Autumn Oak", Color.ORANGE, "Autumn Oak texture stub");
        }
        forest.setSize(CANVAS_SIZE, CANVAS_SIZE);
        forest.setVisible(true);

        System.out.println(TREES_TO_DRAW + " trees drawn");
        System.out.println("---------------------");
        System.out.println("Memory usage:");
        System.out.println("Tree size (8 bytes) * " + TREES_TO_DRAW);
        System.out.println("+ TreeTypes size (~30 bytes) * " + TREE_TYPES + "");
        System.out.println("---------------------");
        System.out.println("Total: " + ((TREES_TO_DRAW * 8 + TREE_TYPES * 30) / 1024 / 1024) +
                "MB (instead of " + ((TREES_TO_DRAW * 38) / 1024 / 1024) + "MB)");
    }

    private static int random(int min, int max) {
        return min + (int) (Math.random() * ((max - min) + 1));
    }
}

OutputDemo.png: 屏幕截图

OutputDemo.txt: 内存使用统计

1000000 trees drawn
---------------------
Memory usage:
Tree size (8 bytes) * 1000000
+ TreeTypes size (~30 bytes) * 2
---------------------
Total: 7MB (instead of 36MB)

享元在其他编程语言中的实现

C# 享元模式讲解和代码示例 C++ 享元模式讲解和代码示例 Go 享元模式讲解和代码示例 PHP 享元模式讲解和代码示例 Python 享元模式讲解和代码示例 Ruby 享元模式讲解和代码示例 Rust 享元模式讲解和代码示例 Swift 享元模式讲解和代码示例 TypeScript 享元模式讲解和代码示例