Spring源码解析(三) -- registerBeanPostProcessors(beanFactory)

　　本节分析下 refresh()中的 registerBeanPostProcessors(beanFactory);方法

　　

public interface BeanPostProcessor {

	/**
	 * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
	 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
	 * or a custom init-method). The bean will already be populated with property values.
	 * The returned bean instance may be a wrapper around the original.
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return the bean instance to use, either the original or a wrapped one;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 */
	Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

	/**
	 * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
	 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
	 * or a custom init-method). The bean will already be populated with property values.
	 * The returned bean instance may be a wrapper around the original.
	 * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
	 * instance and the objects created by the FactoryBean (as of Spring 2.0). The
	 * post-processor can decide whether to apply to either the FactoryBean or created
	 * objects or both through corresponding {@code bean instanceof FactoryBean} checks.
	 * <p>This callback will also be invoked after a short-circuiting triggered by a
	 * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
	 * in contrast to all other BeanPostProcessor callbacks.
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return the bean instance to use, either the original or a wrapped one;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 * @see org.springframework.beans.factory.FactoryBean
	 */
	Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

　　BeanPostProcessor的作用是一个 在BeanDefinition实例化成一个真正的对象后，在该对象的初始化前和初始化后做一些工作。它和BeanFactoryPostProcess不同， BeanFactoryPostProcess一般是改变下某个BeanDefinition，而不是真的对象。

　　跟着代码，发现实际调用的是 PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);

　　

String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

		// Register BeanPostProcessorChecker that logs an info message when
		// a bean is created during BeanPostProcessor instantiation, i.e. when
		// a bean is not eligible for getting processed by all BeanPostProcessors.
		int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
		beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

		// Separate between BeanPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
		List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
		List<String> orderedPostProcessorNames = new ArrayList<String>();
		List<String> nonOrderedPostProcessorNames = new ArrayList<String>();

　　其实跟上一节的beanFactoryPostProcessor过程类似，在查找 BeanPostProcessor的Bean 名字时，都是按照 PriorityOrdered， Ordered， 和没有实现这两个接口的普通类的顺序，依次实例化和初始化生成这些bean，并且注册到beanFactory中

　　AbstractBeanFactory中的方法  addBeanPostProcessor，注意是 先 remove再 add

　　　　 @Override
	public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
		Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
		this.beanPostProcessors.remove(beanPostProcessor);
		this.beanPostProcessors.add(beanPostProcessor);
		if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
			this.hasInstantiationAwareBeanPostProcessors = true;
		}
		if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
			this.hasDestructionAwareBeanPostProcessors = true;
		}
	}

　　 InstantiationAwareBeanPostProcessor 这是一个值得注意的BeanPostProcessor，它的接口定义如下

　　

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor { 

    Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;
    
    boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;//动态代理就是用的这个
    
    PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException;//填充属性的方法，比如@AutoWired @Resources @Value
}

　　

　　他的实现类有哪些呢？

　　1  AnnotationAwareAspectJAutoProxyCreator  动态代理

　　2  AutowiredAnnotationBeanPostProcessor      解决@AutoWired注解的

　　3  CommonAnnotationBeanPostProcessor             

　　 

　　回到registerBeanPostProcessors方法中 里面有一个特别需要注意的地方

if (pp instanceof MergedBeanDefinitionPostProcessor) {
				internalPostProcessors.add(pp);
			}

　　如果某一个BeanPostProcessor实现了MergedBeanDefinitionPostProcessor这个接口，把它单独的放到 internalPostProcessors这个list中保存起来，并且在实例化，并注册到了beanFactory之后，最后再做一次把这些MergedBeanDefinitionPostProcessor注册进去。

　　 MergedBeanDefinitionPostProcessor 的作用就是负责把一个Bean的@AutoWired或者@Resource等注解的信息缓存起来留着后来使用

// Finally, re-register all internal BeanPostProcessors.
        sortPostProcessors(beanFactory, internalPostProcessors);
        registerBeanPostProcessors(beanFactory, internalPostProcessors);

所以，我们之前说注册时先remove再add，这样就能保证这些 internal BeanPostProcessors肯定是在AbstractBeanFactory中保存所有beanPostProcessor的最后。

　　MergedBeanDefinitionPostProcessor的实现类有哪些呢？

　　比较常见的有两个 AutowiredAnnotationBeanPostProcessor， CommonAnnotationBeanPostProcessor

　　AutowiredAnnotationBeanPostProcessor就是用来处理一个spring bean中autowired注解的，而 CommonAnnotationBeanPostProcessor是处理resource注解的。resource注解是java的不是spring的。

　　下一节，会分析beanFactory实例化的代码，就能够看到beanPostProcessor是怎么使用的了。

总结

　　BeanPostProcesssor最值得记忆的两个实现类

　　1 InstantiationAwareBeanPostProcessor 既能处理属性值填充，也能处理动态代理

　　2  MergedBeanDefinitionPostProcessor 专门收集自动注入等注解，并缓存起来