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名称
Flyweight享元模式
概念
内部状态：在享元对象内部不随外界环境改变而改变的共享部分。
外部状态：随着环境的改变而改变，不能够共享的状态就是外部状态。
意图
运用共享技术有效地支持大量的细粒度对象
动机
- 一个应用程序使用了大量的对象
- 完全由于使用大量的对象，造成很大的存储开销
- 对象的大部分状态都可以变为外部状态
- 如果删除对象的外部状态，那么可以使用相对较少的共享对象取代很多组对象
- 应用程序不依赖于对象标识。由于Flyweight对象可以被共享，对于概念上有明显区别的对象，标识测试将返回真值。
结构
参与者
- Flyweight（抽象享元类）
- 描述一个接口，通过这个接口flyweight可以接受并且作用于外部状态 - ConcreteFlyweight（具体享元类）
- 实现Flyweight接口，并且为内部状态增加存储空间。ConcreteFlyweight对象必须是可以共享的。它所存储的状态必须是内部的，即它必须独立于ConcreteFlyweight对象的场景。 - UnsharedConcreteFlyweight（非共享具体享元类）
- 在Flyweight对象的某些层次，UnsharedConcreteFlyweight对象通常将ConcreteFlyweight对象作为子节点。 - FlyweightFactory（享元工厂类）
- 创建并且管理flyweight对象
- 确保合理地共享flyweight，当客户请求一个flyweight时，FlyweightFactory对象提供一个已经创建地地实例或者创建一个。 Client
- 维持一个对flywiehgt地引用
- 计算或者存储一个（或者多个）flyweight的外部状态。示例代码
import java.util.HashMap;
interface Shape {//Flyweight（抽象享元类）
void draw();
}
class Circle implements Shape {//ConcreteFlyweight（具体享元类）
private String color;
private int x;
private int y;
private int radius;
public Circle(String color){
this.color = color;
}
public void setX(int x) {
this.x = x;
}
public void setY(int y) {
this.y = y;
}
public void setRadius(int radius) {
this.radius = radius;
}
@Override
public void draw() {
System.out.println("Circle: Draw() [Color : " + color
+", x : " + x +", y :" + y +", radius :" + radius);
}
}
class ShapeFactory {//FlyweightFactory（享元工厂类）
private static final HashMap<String, Shape> circleMap = new HashMap<>();//UnsharedConcreteFlyweight（非共享具体享元类）
public static Shape getCircle(String color) {
Circle circle = (Circle)circleMap.get(color);
if(circle == null) {\\存在则返回，不存在则创建并且返回
circle = new Circle(color);
circleMap.put(color, circle);
System.out.println("Creating circle of color : " + color);
}
return circle;
}
}
public class Client{
private static final String colors[] =
{ "Red", "Green", "Blue", "White", "Black" };
public static void main(String[] args) {
for(int i=0; i < 50; ++i) {
Circle circle = (Circle)ShapeFactory.getCircle(getRandomColor());
circle.setX(getRandomX());
circle.setY(getRandomY());
circle.setRadius(getRandomR());
circle.draw();
}
}
private static String getRandomColor() {
return colors[(int)(Math.random()*colors.length)];
}
private static int getRandomX() {
return (int)(Math.random()*100 );
}
private static int getRandomY() {
return (int)(Math.random()*100);
}
private static int getRandomR() {
return (int)(Math.random()*100);
}
}输出结果
Creating circle of color : Red
Circle: Draw() [Color : Red, x : 36, y :58, radius :51
Creating circle of color : Black
Circle: Draw() [Color : Black, x : 67, y :71, radius :30
Creating circle of color : White
Circle: Draw() [Color : White, x : 57, y :29, radius :92
Circle: Draw() [Color : Black, x : 31, y :26, radius :26
Circle: Draw() [Color : White, x : 96, y :35, radius :56
Circle: Draw() [Color : White, x : 33, y :45, radius :46
Circle: Draw() [Color : Black, x : 55, y :66, radius :25
Creating circle of color : Blue
Circle: Draw() [Color : Blue, x : 73, y :16, radius :19
Circle: Draw() [Color : Black, x : 94, y :28, radius :43
Creating circle of color : Green
Circle: Draw() [Color : Green, x : 73, y :97, radius :19
Circle: Draw() [Color : White, x : 12, y :34, radius :65
Circle: Draw() [Color : White, x : 65, y :33, radius :84
Circle: Draw() [Color : Black, x : 48, y :4, radius :15
Circle: Draw() [Color : White, x : 54, y :6, radius :11
Circle: Draw() [Color : Green, x : 23, y :10, radius :1
Circle: Draw() [Color : Black, x : 43, y :2, radius :89
Circle: Draw() [Color : Green, x : 2, y :93, radius :4
Circle: Draw() [Color : Blue, x : 51, y :95, radius :96
Circle: Draw() [Color : Black, x : 56, y :17, radius :28
Circle: Draw() [Color : Black, x : 64, y :68, radius :66
Circle: Draw() [Color : Black, x : 32, y :52, radius :89
Circle: Draw() [Color : Green, x : 12, y :37, radius :9
Circle: Draw() [Color : White, x : 69, y :0, radius :70
Circle: Draw() [Color : Blue, x : 42, y :45, radius :39
Circle: Draw() [Color : White, x : 23, y :8, radius :83
Circle: Draw() [Color : White, x : 40, y :25, radius :31
Circle: Draw() [Color : Red, x : 88, y :21, radius :1
Circle: Draw() [Color : Blue, x : 28, y :69, radius :31
Circle: Draw() [Color : Green, x : 21, y :72, radius :55
Circle: Draw() [Color : Black, x : 26, y :49, radius :69
Circle: Draw() [Color : Blue, x : 10, y :31, radius :21
Circle: Draw() [Color : Red, x : 67, y :66, radius :73
Circle: Draw() [Color : Blue, x : 43, y :33, radius :23
Circle: Draw() [Color : Red, x : 94, y :36, radius :66
Circle: Draw() [Color : White, x : 74, y :68, radius :36
Circle: Draw() [Color : Green, x : 90, y :25, radius :90
Circle: Draw() [Color : Green, x : 79, y :13, radius :75
Circle: Draw() [Color : Red, x : 5, y :6, radius :12
Circle: Draw() [Color : Black, x : 59, y :20, radius :83
Circle: Draw() [Color : Red, x : 91, y :94, radius :88
Circle: Draw() [Color : Black, x : 38, y :11, radius :71
Circle: Draw() [Color : Black, x : 63, y :79, radius :87
Circle: Draw() [Color : White, x : 44, y :22, radius :65
Circle: Draw() [Color : Blue, x : 33, y :70, radius :17
Circle: Draw() [Color : Blue, x : 91, y :10, radius :68
Circle: Draw() [Color : Green, x : 92, y :6, radius :93
Circle: Draw() [Color : Blue, x : 92, y :20, radius :33
Circle: Draw() [Color : Black, x : 43, y :22, radius :41
Circle: Draw() [Color : Black, x : 63, y :64, radius :20
Circle: Draw() [Color : Black, x : 4, y :50, radius :32
可以看到只创建了5次对象，其他时候都是已经存在的对象的复用。