享元模式

定义：使用共享对象可以有效的支持大量的细粒度的对象

FlyWeight抽象享元类：通常是一个接口或抽象类，声明公共方法，这些方法可以向外界提供对象的内部状态，设置外部状态。
UnsharedConcreteFlyWeight非共享享元类：不能被共享的子类可以设计为非共享享元类。
ConcreteFlyWeight具体享元类：为内部状态提供成员变量进行存储。
FlyWeightFactory享元工厂类：创建并管理享元对象，享元池一般设计为键值对。

//抽象享元类
abstract class Flyweight {
    private String intrinsic;
    protected final String Extrinsic;

    public Flyweight(String Extrinsic){
        this.Extrinsic = Extrinsic;
    }

    public abstract void operate();

    public String getIntrinsic(){
        return intrinsic;
    }
    public void setIntrinsic(String intrinsic){
        this.intrinsic = intrinsic;
    }
}
//具体享元类
class ConcreteFlyweight1 extends Flyweight{
    public ConcreteFlyweight1(String Extrinsic) {
        super(Extrinsic);
    }

    @Override
    public void operate() {
    }
}
//享元工厂
class FlyweightFactory {
    //对象池
    private static HashMap<String,Flyweight> pool = new HashMap<>();

    public static Flyweight getFlyweight(String Extrinsic){
        Flyweight flyweight = null;
        if(pool.containsKey(Extrinsic))
            flyweight = pool.get(Extrinsic);
        else {
            flyweight = new ConcreteFlyweight1(Extrinsic);
            pool.put(Extrinsic, flyweight);
        }
        return flyweight;
    }
}

优缺点：

减少对象的创建，减少内存占用
增加系统复杂度

使用场景

系统中存在大量相似对象
缓冲对象池

注意事项

多线程问题