设计模式-工厂方法（Factory Method）

Posted 2020-10-21

2018-1-20 by Atlas

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• 应用场景

Template Method Pattern是在父类建立处理逻辑的大纲骨架，而在子类补充具体的处理内容。把Template Method Pattern应用在生成对象实例方面，以Template Method Pattern架构获取产生对象实例的工厂，就是Factory Method Pattern。

• UML 类图

• Product（产品）参与者
  框架的部分。这个抽象类规定此Pattern所产生的对象实例应有的接口，具体内容则由子类的ConcreteProduct参与者规定。
• Creator（生产者）参与者
  框架的部分。这是产生Product参与者的抽象类。具体的内容由子类的ConcreteCreator参与者决定。
  Creator参与者对于实际产生的ConcreteProduct参与者完全一无所知。Creator参与者唯一直到的是只要调用Product参与者和产生对象的方法，就能产生Product。
• ConcreteProduct（具体产品）参与者
  实际处理内容的部分。规定具体的产品样式。
• ConcreteCreator（具体生产者）参与者
  实际处理内容的部分。规定制造实际产品的类。

• 标准示例

public abstract class Product {
    public abstract void use();
}

public abstract class Factory {
    public final Product create(String owner) {
        Product p = createProduct(owner);
        registerProduct(p);
        return p;
    }
    protected abstract Product createProduct(String owner);
    protected abstract void  registerProduct(Product product);
}

public class IDCard extends Product {
    private String owner;
    public IDCard(String owner){
        System.out.println("创建" + owner + "的卡。");
        this.owner = owner;
    }
    public void use() {
        System.out.println("使用" + owner + "的卡。");
    }
    public String getOwner() {
        return owner;
    }
}

public class IDCardFactory extends Factory {
    private Vector<String> owners = new Vector<String>();
    protected Product createProduct(String owner) {
        return new IDCard(owner);
    }
    protected void registerProduct(Product product) {
        IDCard card = (IDCard) product;
        owners.add(card.getOwner());
    }
    public Vector<String> getOwners() {
        return owners;
    }
}

Product定义产品骨架。
Factory定义工厂生产过程骨架。
IDCard具体实现产品的内容。
IDCardFactory具体实现工厂生产IDCard对象实例。

• 案例鉴赏

public class ContextLoader {
        protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
        // ...
        wac.refresh();
        // ...
        }
}

public interface ConfigurableApplicationContext extends ApplicationContext, Lifecycle, Closeable {
        // ...
        void refresh() throws BeansException, IllegalStateException;
        ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;
        // ...
}

public abstract class AbstractApplicationContext extends DefaultResourceLoader
        implements ConfigurableApplicationContext, DisposableBean {
        // ...
        public void refresh() throws BeansException, IllegalStateException {
                // ...
                ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        }
        protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
                refreshBeanFactory();
                ConfigurableListableBeanFactory beanFactory = getBeanFactory();
                // ...
                return beanFactory;
        }
        public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;
        // ...
}

public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext {
        // ...
        protected final void refreshBeanFactory() throws BeansException {
                // ...
                DefaultListableBeanFactory beanFactory = createBeanFactory();
                // ...
        }
        protected DefaultListableBeanFactory createBeanFactory() {
                return new DefaultListableBeanFactory(getInternalParentBeanFactory());
        }
        public final ConfigurableListableBeanFactory getBeanFactory() {
            // ...
            return this.beanFactory;
        }
        // ...
}

spring框架初始化ApplicationContext创建BeanFactory过程删减后的骨架。