定义
享元模式(Flyweight Pattern):运用共享技术有效地支持大量细粒度对象的复用。系统只使用少量的对象,而这些对象都很相似,状态变化很小,可以实现对象的多次复用。由于享元模式要求能够共享的对象必须是细粒度对象,因此它又称为轻量级模式,它是一种对象结构型模式。
UML图描述
享元模式主要包含三种角色:
- 抽象享元类:被共享对象的抽象父类,如下图的Flyweight
- 具体享元类:享元对象的具体实现,如下图的ConcreteFlyweight1,ConcreteFlyweight2
- 享元工厂类:生产享元对象的工厂类,如下图逇FlyweightFactory
代码实现
/** * 共享对象的抽象父类 */ public abstract class Flyweight { public abstract void behavior(); }
/** * 具体享元对象1 */ public class ConcreteFlyweight1 extends Flyweight { @Override public void behavior() { System.out.println("这是具体享元对象1的行为"); } }
/** * 具体享元对象2 */ public class ConcreteFlyweight2 extends Flyweight { @Override public void behavior() { System.out.println("这是具体享元对象2的行为"); } }
/** * 享元对象工厂类 */ public class FlyweightFactory { private static Map<String,Flyweight> flyweights; static { flyweights = new HashMap<>(); } public static Flyweight getConcreteFlyweight(String name){ switch (name){ case "flyweight1": if(flyweights.containsKey("flyweight1")){ System.out.println("正在共享flyweight1对象"); }else{ Flyweight flyweight = new ConcreteFlyweight1(); flyweights.put("flyweight1",flyweight); } break; case "flyweight2": if(flyweights.containsKey("flyweight2")){ System.out.println("正在共享flyweight1对象"); }else{ Flyweight flyweight = new ConcreteFlyweight2(); flyweights.put("flyweight2",flyweight); } break; default: System.out.println("共享池参数有误"); return null; } return flyweights.get(name); } }
/** * 测试类 */ public class Test { public static void main(String[] args) { Flyweight flyweight1 = FlyweightFactory.getConcreteFlyweight("flyweight1"); flyweight1.behavior(); Flyweight flyweight1_1 = FlyweightFactory.getConcreteFlyweight("flyweight1"); flyweight1_1.behavior(); Flyweight flyweight2 = FlyweightFactory.getConcreteFlyweight("flyweight2"); flyweight2.behavior(); FlyweightFactory.getConcreteFlyweight("flyweight3"); } }
适用场景
1.一个系统有大量相同或者相似的对象,由于这类对象的大量使用,造成内存的大量耗费。
2.对象的大部分状态都可以外部化,可以将这些外部状态传入对象中。
3.使用享元模式需要维护一个存储享元对象的享元池,而这需要耗费资源,因此,应当在多次重复使用享元对象时才值得使用享元模式。
享元模式的优点
1.享元模式的优点在于它可以极大减少内存中对象的数量,使得相同对象或相似对象在内存中只保存一份。
2.享元模式的外部状态相对独立,而且不会影响其内部状态,从而使得享元对象可以在不同的环境中被共享。
享元模式的缺点
1.享元模式使得系统更加复杂,需要分离出内部状态和外部状态,这使得程序的逻辑复杂化。
2.为了使对象可以共享,享元模式需要将享元对象的状态外部化,而读取外部状态使得运行时间变长。