设计模式-策略工厂

在介绍策略工厂的实现前，我们要先了解spring中beanFactory：

BeanFactory

作为IOC服务的提供者，通过XmlBeanFactory来得到实例

生产Bean的工厂，采用延迟初始化策略（只有在需要某个对象时，才进行初始化和依赖注入）；通常会用xml文件来注册并管理各个业务对象之间的依赖关系

DefaultListableBeanFactory实现了BeanFactory接口和BeanDefinitionRegistry接口（负责Bean的注册管理）

属性键值对注入PropertyValue

这里的key：group，value：name

位于beans包下，是bean属性键值对的封装，缓存了对key-value解析相关的信息，避免重复解析

• AttributeAccessor 可以访问对象的属性或将属性附加到对象上。

public interface AttributeAccessor {
	//增删改查
	void setAttribute(String name, @Nullable Object value);
	@Nullable
	Object getAttribute(String name);
	@SuppressWarnings("unchecked")
	default <T> T computeAttribute(String name, Function<String, T> computeFunction) {
		Assert.notNull(name, "Name must not be null");
		Assert.notNull(computeFunction, "Compute function must not be null");
		Object value = getAttribute(name);
		if (value == null) {
			value = computeFunction.apply(name);
			Assert.state(value != null,
					() -> String.format("Compute function must not return null for attribute named '%s'", name));
			setAttribute(name, value);
		}
		return (T) value;
	}
	@Nullable
	Object removeAttribute(String name);
	boolean hasAttribute(String name);
	String[] attributeNames();
}

• BeanMetadataElement 持有属性信息的对象。

public interface BeanMetadataElement {

	@Nullable
	default Object getSource() {
		return null;
	}

}

• BeanMetadataAttributeAccessor 实现了 AttributeAccessor和 BeanMetadataElement 两个接口，属性为 BeanMetadataAttribute 对象

public class BeanMetadataAttributeAccessor extends AttributeAccessorSupport implements BeanMetadataElement {

	@Nullable
	private Object source;

	public void setSource(@Nullable Object source) {
		this.source = source;
	}

	@Override
	@Nullable
	public Object getSource() {
		return this.source;
	}
}

• PropertyValue 属性键值对

public class PropertyValue extends BeanMetadataAttributeAccessor implements Serializable {
	//属性名称
	private final String name;
	//属性值
	@Nullable
	private final Object value;
	//属性值是否是optional
	private boolean optional = false;
	//属性值是否进行了属性转换
	private boolean converted = false;
	//类型转换后的属性值
	@Nullable
	private Object convertedValue;
	//属性值是否需要进行转换
	@Nullable
	volatile Boolean conversionNecessary;
	//缓存解析后的属性名称
	@Nullable
	transient volatile Object resolvedTokens;

MutablePropertyValues

PropertyValues的默认实现类是MutablePropertyValues

MutablePropertyValues：

• 内部维护了一个List propertyValueList，将属性值转换为PropertyValue进行存储
  Bean的结构图：
  
  每一个受管的对象，在容器中都会有一个BeanDefinition的实例与之对应，BeanDefinition存储对象的信息，包括：

• 对应的对象的class类型，是否抽象类，构造方法参数及其他信息

• beanName就是实例名

实现策略工厂：

注解StrategyAnnotation

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface StrategyAnnotation {
    /**
     * 策略类型名称，可以支持多个
     *
     * @return
     */
    String[] name() default "";

    /**
     * 默认组为: defaultGroup
     *
     * @return
     */
    String group() default "defaultGroup";

    /**
     * 是否默认类型
     *
     * @return
     */
    boolean isDefault() default false;
}

创建策略工厂实现类

管理所有的方法

/**
 * 策略工厂
 * 当你使用注解 StrategyAnnotation 在类上面
 * 那么会通过 RegisterMapDefinition 去自动创建策略工厂。
 *
 * 策略工厂的名称就是 StrategyAnnotation 定义的group.
 *
 * 支持的state 参数，就是 StrategyAnnotation 定义的 name
 *
 */
public class StrategyFactory<T> {
    //key是策略类型，value是具体策略
    Map<String, T> localMap;

    public T getObjectByState(String state) {
        return localMap.get(state);
    }

    public T defaultObject(String state) {
        T t = localMap.get(state);
        if (t == null) {
            t = localMap.get(null);
        }
        return t;
    }

    public T getObjectThrowException(String state) {
        T t = localMap.get(state);
        if (t == null) {
            throw new IllegalStateException("获取对象失败,状态:" + state);
        }
        return t;
    }

    public void setLocalMap(Map<String, T> localMap) {
        this.localMap = localMap;
    }

    public Map<String, T> getLocalMap() {
        return Collections.unmodifiableMap(this.localMap);
    }
}

RegisterMapDefinition

自动创建策略工厂

1. 获取注解的group
2. 首次registry这个groupBean
3. 获取这个groupBean的BeanDefinition
4. 遍历注解的name
5. 通过定义的key（localMap）获得PropertyValue属性值
6. 首次属性值为空，创建一个PropertyValue（以localMap为key，ManagedMap为value）
7. 将localMap这个PropertyValue属性添加到propertyValues中即可

/**
 * 策略配置加载器
 */
@Component
public class RegisterMapDefinition implements BeanDefinitionRegistryPostProcessor {

    public static final String LOCAL_MAP_PROPERTY = "localMap";
    Logger logger = LoggerFactory.getLogger(getClass());

    /**
     * 本文加载有注解的bean为map 作为策略映射，
     *
     * @param beanFactory
     * @throws BeansException
     * @see StrategyAnnotation
     */
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    }

    private void registerGroupMap(BeanDefinitionRegistry registry, StrategyAnnotation strategyAnnotation, String beanName, BeanDefinition beanDefinition) {
        String groupBean = strategyAnnotation.group();
        if (!registry.containsBeanDefinition(groupBean)) {
            RootBeanDefinition strategyFactoryDefinition = new RootBeanDefinition();
            strategyFactoryDefinition.setBeanClass(StrategyFactory.class);
            registry.registerBeanDefinition(groupBean, strategyFactoryDefinition);
        }
        BeanDefinition groupBeanDefinition = registry.getBeanDefinition(groupBean);

        String[] names = strategyAnnotation.name();
        for (String name : names) {
            MutablePropertyValues propertyValues = groupBeanDefinition.getPropertyValues();
            PropertyValue localMap = propertyValues.getPropertyValue(LOCAL_MAP_PROPERTY);
            if (localMap == null) {
                localMap = new PropertyValue(LOCAL_MAP_PROPERTY, new ManagedMap<>());
                propertyValues.addPropertyValue(localMap);
            }
            ManagedMap<TypedStringValue, BeanReference> stringObjectManagedMap =
                    (ManagedMap<TypedStringValue, BeanReference>) localMap.getValue();
            stringObjectManagedMap.put(new TypedStringValue(name), new RuntimeBeanReference(beanName));
            if (strategyAnnotation.isDefault()) {
                stringObjectManagedMap.put(null, new RuntimeBeanReference(beanName));
            }
            propertyValues.addPropertyValue(localMap);
        }
    }


    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        String op = "RegisterMapDefinition.postProcessBeanDefinitionRegistry";
        logger.info(Logs.op(op).msg("批量创建策略实例").toString());
        //获取所有的beanName
        String[] strategyAnnotationsArray = registry.getBeanDefinitionNames();
        //遍历每个beanName
        for (String beanName : strategyAnnotationsArray) {
            BeanDefinition beanDefinition = registry.getBeanDefinition(beanName);
            try {
                //获得bean的类名
                String beanClassName = beanDefinition.getBeanClassName();
                if (beanClassName == null) {
                    continue;
                }
				//找到类对象
                Class<?> clazz = Class.forName(beanClassName);
                //获取类对象上的StrategyAnnotations注解
                StrategyAnnotations annotation = AnnotationUtils.findAnnotation(clazz, StrategyAnnotations.class);
                //有StrategyAnnotations注解的进行注册
                if (annotation != null) {
                    for (StrategyAnnotation strategyAnnotation : annotation.value()) {
                        registerGroupMap(registry, strategyAnnotation, beanName, beanDefinition);
                    }
                }

                StrategyAnnotation strategyAnnotation = AnnotationUtils.findAnnotation(clazz, StrategyAnnotation.class);
                if(strategyAnnotation!=null){
                    registerGroupMap(registry, strategyAnnotation, beanName, beanDefinition);
                }

            } catch (ClassNotFoundException e) {
                logger.debug(Logs.op(op).msg("策略加载器加载指定类不存在").toString(), e);
            }

        }
    }
}