Spring源码 – 核心接口FactoryBean「建议收藏」

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Spring源码 – 核心接口FactoryBean

Spring版本：Spring 5.3.13-release

一般情况下，Spring是通过反射机制，利用在Bean进行声明时的class属性指定的实现类来进行Bean的实例化操作。但是在某些场景下，Bean的实例化操作时非常复杂繁琐的，如果按照传统的方式，则需要在<bean></bean>中进行大量配置信息的声明，这种配置方式的灵活性会受到非常大的局限性的。而如果采用编码的方式进行声明可能会变得非常简单。org.springframework.beans.factory.FactoryBean<T>接口应运而生，用户可以实现该接口来定制实例化Bean的逻辑。

# 1、核心接口FactoryBean

FactoryBean<T>在Spring占据非常重要的地位。很多第三方中间件都是通过实现FactoryBean<T>接口进行实例化。如open-feign。

FactoryBean<T>接口中一共定义了三个方法：

public interface FactoryBean<T> { 
   

	/** * The name of an attribute that can be * {@link org.springframework.core.AttributeAccessor#setAttribute set} on a * {@link org.springframework.beans.factory.config.BeanDefinition} so that * factory beans can signal their object type when it can't be deduced from * the factory bean class. * @since 5.2 */
	String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";


	/** * Return an instance (possibly shared or independent) of the object * managed by this factory. * <p>As with a {@link BeanFactory}, this allows support for both the * Singleton and Prototype design pattern. * <p>If this FactoryBean is not fully initialized yet at the time of * the call (for example because it is involved in a circular reference), * throw a corresponding {@link FactoryBeanNotInitializedException}. * <p>As of Spring 2.0, FactoryBeans are allowed to return {@code null} * objects. The factory will consider this as normal value to be used; it * will not throw a FactoryBeanNotInitializedException in this case anymore. * FactoryBean implementations are encouraged to throw * FactoryBeanNotInitializedException themselves now, as appropriate. * @return an instance of the bean (can be {@code null}) * @throws Exception in case of creation errors * @see FactoryBeanNotInitializedException * @see FactoryBean#isSingleton() * @see org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#singletonObjects */
	@Nullable
	T getObject() throws Exception;

	/** * Return the type of object that this FactoryBean creates, * or {@code null} if not known in advance. * <p>This allows one to check for specific types of beans without * instantiating objects, for example on autowiring. * <p>In the case of implementations that are creating a singleton object, * this method should try to avoid singleton creation as far as possible; * it should rather estimate the type in advance. * For prototypes, returning a meaningful type here is advisable too. * <p>This method can be called <i>before</i> this FactoryBean has * been fully initialized. It must not rely on state created during * initialization; of course, it can still use such state if available. * <p><b>NOTE:</b> Autowiring will simply ignore FactoryBeans that return * {@code null} here. Therefore it is highly recommended to implement * this method properly, using the current state of the FactoryBean. * @return the type of object that this FactoryBean creates, * or {@code null} if not known at the time of the call * @see ListableBeanFactory#getBeansOfType */
	@Nullable
	Class<?> getObjectType();

	/** * Is the object managed by this factory a singleton? That is, * will {@link #getObject()} always return the same object * (a reference that can be cached)? * <p><b>NOTE:</b> If a FactoryBean indicates to hold a singleton object, * the object returned from {@code getObject()} might get cached * by the owning BeanFactory. Hence, do not return {@code true} * unless the FactoryBean always exposes the same reference. * <p>The singleton status of the FactoryBean itself will generally * be provided by the owning BeanFactory; usually, it has to be * defined as singleton there. * <p><b>NOTE:</b> This method returning {@code false} does not * necessarily indicate that returned objects are independent instances. * An implementation of the extended {@link SmartFactoryBean} interface * may explicitly indicate independent instances through its * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean} * implementations which do not implement this extended interface are * simply assumed to always return independent instances if the * {@code isSingleton()} implementation returns {@code false}. * <p>The default implementation returns {@code true}, since a * {@code FactoryBean} typically manages a singleton instance. * @return whether the exposed object is a singleton * @see #getObject() * @see SmartFactoryBean#isPrototype() * * 返回由 FactoryBean 创建的 Bean 实例的作用域是单例还是原型 * true -> singleton / false -> prototype */
	default boolean isSingleton() { 
   
		return true;
	}

}

• getObject()：返回由FactoryBean创建的Bean实例，如果isSingleton()返回true，则该Bean实例会放入Spring的一级缓存中。
• getObjectType()：返回FactoryBean创建的Bean类型。
• isSingleton()：返回由FactoryBean创建的Bean实例的作用域是单例还是原型。

当声明Bean时指定的class属性为FactoryBean<T>接口的实现类时，Spring容器通过getBean()方法获取的Bean实例为T，即为getObject()方法返回的实例对象。也就是说此时FactoryBean<T>#getObject()方法代理了getBean()方法。如果需要获取此FactoryBean<T>实例，则需要通过&+BeanName进行获取。

# 2、BeanFactory与FactoryBean<T>

需要注意的是，BeanFactory与FactoryBean<T>是两个完全不同概念的接口。BeanFactory根据命名可以看出，它是一个工厂。而FactoryBean<T>根据命名也可以很清晰的看出，它是一个Bean。BeanFactory是Spring对于IOC思想实现的最基本方式，它提供了对Bean的管理。而FactoryBean<T>是Spring针对复杂类型Bean的实例化，允许开发者对此类型的Bean进行的高级定制处理。

GitHub源码地址：https://github.com/kapbc/kapcb-spring-source/tree/master/Spring-Framework-v5.3.13

备注：此文为笔者学习Spring源码的笔记，鉴于本人技术有限，文中难免出现一些错误，感谢大家批评指正。