注册工厂,就是建立一个小型“DB”,采用注册方式,对实例进行管理。
从实现上有两种方式,但本质是相同的,都是在管理一组对象。
为了讲述方便,先建立一组父子类,Subject是ConcreteSubject1和ConcreteSubject2的父类。
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第1种,可以认为是Singleton模式的一个扩展,所谓的“多件”,用简单工厂+单件实现:在单件的私有构造函数中,一次性把DB建立好,有多少元素,就new多少实例出来;在单件的一个方法中,用简单工厂根据不同参数返回不同的实例对象。这个方法的好处是避免了用反射根据参数动态生成新对象;由于在开始一次性就要建立好所有对象,所以需要创建的小对象很多时,不宜采用。代码如下:
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第2种,则是在注册工厂类中放一个私有HashTable,来模拟一个小型数据库。比如说要创建对象A,先在注册工厂的HashTable中根据键,看A是否存在,如果有就返回这个对象,否则就用发射原理new一个A对象,在Hashtable中注册并返回。UML静态类图及顺序图如下:
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补注:说到这里,一定要花些篇幅讲讲反射工厂,这是.NET专有的发射技术。