# 2.1 Simple Factory

![](/files/-M523vAubowptzerEzlf)

* 關於簡單工廠, 大概有以下幾點特性:
  * 可用來創造介面、抽象類別或著是普通類別的實例
  * 通常把簡單工廠類別實作成一個util class,直接使用其靜態方法即可, 即簡單工廠的方法通常是靜態的, 故也稱作靜態工廠
  * 理論上簡單工廠可以用來構築任何物件, 故又稱萬能工廠
  * 關於簡單工廠建立之物件的scope, 建議控制在一個獨立的元件級別或著是一個模組的級別
* 簡單工廠的內部核心實作是要選擇合適的實作類別來建立實例物件

以下是簡單的範例:

Api介面的部分:

```java
package idv.design.pattern.simpleFactory.example2;

/**
 * Created by Carl on 2015/9/8.
 */
public interface Api {

    public void callApi(String s);

}
```

Api介面的不同實作1:

```java
package idv.design.pattern.simpleFactory.example2;

/**
 * Created by Carl on 2015/9/8.
 */
public class ApiImpl1 implements Api {

    public void callApi(String s) {
        System.out.println("The content from impl1 is: " + s);
    }

}
```

Api介面的不同實作2:

```java
package idv.design.pattern.simpleFactory.example2;

/**
 * Created by Carl on 2015/9/8.
 */
public class ApiImpl2 implements Api {

    public void callApi(String s) {
        System.out.println("The content from impl2 is: " + s);
    }

}
```

可以依據傳入的參數來選擇實作的Factory:

```java
package idv.design.pattern.simpleFactory.example2;

/**
 * Created by Carl on 2015/9/8.
 */
public class Factory {

    public static Api createApi(int condition) {

        Api api = null;
        if(condition == 1) {
            api = new ApiImpl1();
        } else if(condition == 2) {
            api = new ApiImpl2();
        }
        return api;

    }

}
```

Client的部分:

```java
package idv.design.pattern.simpleFactory.example2;

/**
 * Created by Carl on 2015/9/8.
 */
public abstract class Client {

    public static void main(String[] args) {
        Api api = Factory.createApi(1);
        api.callApi("With simple factory.");
    }

}
```

關於Simple Factory Pattern的範例, 可參考[這邊](https://github.com/yotsuba1022/design-pattern/tree/master/src/main/java/idv/design/pattern/simpleFactory)

* 可配置的簡單工廠: 使用反射(reflection)機制加上配置文件, 來實現添加新的實作類別後, 不需要修改程式, 就能把這個新的實作類別加入程式中
* 簡單工廠的優缺點
  * 幫助封裝
  * 解耦
  * 可能增加客戶端的複雜度
  * 不方便擴展子工廠
* **簡單工廠的本質: 選擇實作**
* 何時選用簡單工廠
  * 若想要完全封裝隔離具體實作, 讓外部只能通過介面來操作封裝體, 那就可以考慮此模式, 讓客戶通過工廠來獲取相應的介面而無需關心具體實作
  * 若想要把對外建立物件的職責集中管理和控制, 即可選擇簡單工廠, 一個簡單工廠可以建立很多的且不相關的物件, 可以把對外建立物件的職責集中到一個簡單工廠中, 進而實現集中管理與控制


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