享元模式：String常量池，数据库连接池，缓冲池等等都是享元模式的应用。
每次创建字符创对象时候，都需要创建一个新的字符串对象的话，内存开销很大，所以如果第一次创建了字符串对象hunter，下次在创建相同的字符串hunter时，只是把它的引用再次指向hunter，这样就实现了hunter字符串内内存中的共享。
角色：
Flyweight：抽象享元类，所有具体享元类的超类or接口，
ConcreteFlyweight：具体享元类，指定内部状态，为内部状态增加存储空间
UnsharedConcreteFlywight: 非共享具体享元类。支出那些不需要共享的Flyweight子类
FlyweightFactory:享元工厂类，用来创建并管理Flyweight对象，它主要用来确保合理地共享Flyweight，当用户请求一个Flyweight时，工厂会提供一个已经创建的对象or新建一个（如果不存在）
代码实现：
Flyweight
public abstract class Flyweight {//内部状态public String intrinsic;//外部状态protected final String extrinsic;//要求享元角色必须接受外部状态public Flyweight(String extrinsic) {this.extrinsic = extrinsic;}//定义业务操作public abstract void operate(int extrinsic);public String getIntrinsic() {return intrinsic;}public void setIntrinsic(String intrinsic) {this.intrinsic = intrinsic;}}
ConcreteFlywight
public class ConcreteFlyweight extends Flyweight {//接受外部状态public ConcreteFlyweight(String extrinsic) {super(extrinsic);}//根据外部状态进行逻辑处理@Overridepublic void operate(int extrinsic) {System.out.println("具体Flyweight:" + extrinsic);}}
UnsharedConcreteFlyweight
public class UnsharedConcreteFlyweight extends Flyweight {public UnsharedConcreteFlyweight(String extrinsic) {super(extrinsic);}@Overridepublic void operate(int extrinsic) {System.out.println("不共享的具体Flyweight:" + extrinsic);}}
FlyweightFactory
public class FlyweightFactory {//定义一个池容器private static HashMap<String, Flyweight> pool = new HashMap<>();//享元工厂public static Flyweight getFlyweight(String extrinsic) {Flyweight flyweight = null;if(pool.containsKey(extrinsic)) { //池中有该对象flyweight = pool.get(extrinsic);System.out.print("已有 " + extrinsic + " 直接从池中取---->");} else {//根据外部状态创建享元对象flyweight = new ConcreteFlyweight(extrinsic);//放入池中pool.put(extrinsic, flyweight);System.out.print("创建 " + extrinsic + " 并从池中取出---->");}return flyweight;}
}
测试程序：
public class Client {public static void main(String[] args) {int extrinsic = 22;Flyweight flyweightX = FlyweightFactory.getFlyweight("X");flyweightX.operate(++ extrinsic);Flyweight flyweightY = FlyweightFactory.getFlyweight("Y");flyweightY.operate(++ extrinsic);Flyweight flyweightZ = FlyweightFactory.getFlyweight("Z");flyweightZ.operate(++ extrinsic);Flyweight flyweightReX = FlyweightFactory.getFlyweight("X");flyweightReX.operate(++ extrinsic);Flyweight unsharedFlyweight = new UnsharedConcreteFlyweight("X");unsharedFlyweight.operate(++ extrinsic);}}
第一次创建X、Y、Z时，都是先创建再从池中取出，而第二次创建X时，因为池中已经存在了，所以直接从池中取出
应用场景：
系统中存在大量相似对象
需要缓冲池的场景 比如：String常量池，数据库连接池
方法：
用唯一标识码判断，如果在内存中，则返回这个唯一标识码所标识的对象，用HashMap/HashTable存储
优点：
大大减少了对象的创建，降低了程序内存的占用，提高效率
缺点：
提高了系统的复杂度。需要分离出内部状态和外部状态，而外部状态具有固化性。不应该随着内部状态的改变而改变。