Spring注解--AOP原理(三):BeanPostProcessor创建与注册

1. 用途

Spring 中使用BeanPostProcessor 来拦截bean的创建，并在bean初始化前后，添加相应的操作。

2. 注册流程

1. 传入配置类，创建IOC容器
2. 注册配置类，调用refresh，初始化容器
3. 容器初始化的时候，注册BeanPostProcessor

// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);

• 找到容器中，需要创建的BeanPostProcessors

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

• 分离不同的BeanPostProcessors（implements PriorityOrdered，Ordered）

List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();

• 优先级顺序
  PriorityOrdered > Ordered > 普通的BeanPostProcessor
• 最后创建并注册BeanPostProcessor到beanFactory中

List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
for (String ppName : nonOrderedPostProcessorNames) {
    BeanPostProcessor pp = beanFactory.getBean(ppName,BeanPostProcessor.class);
    nonOrderedPostProcessors.add(pp);
    if (pp instanceof MergedBeanDefinitionPostProcessor) {              
        internalPostProcessors.add(pp);       
    }
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

private static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {
        for (BeanPostProcessor postProcessor : postProcessors) {
            beanFactory.addBeanPostProcessor(postProcessor);
        }
    }

3. 创建流程

• 创建BeanPostProcessors实例的流程和创建一般bean实例一样（具体debug过程不详细描述）（AbstractAutowireCapableBeanFactory）

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
    //创建bean实例
    instanceWrapper = createBeanInstance(beanName, mbd, args);
    ...
    //给bean的属性赋值
    populateBean(beanName, mbd, instanceWrapper);
    //初始化bean
    if (exposedObject != null) {
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
    ...
}

• bean初始化

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
  //处理Aware接口的方法回调
  invokeAwareMethods(beanName, bean);
  ...
  //调用PostBeanProcessor的postProcessBeforeInitialization
  wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName)
  ...
  //执行自定义的初始方法（implements InitializingBean）
  invokeInitMethods(beanName, wrappedBean, mbd);
  ...
  //最后调用PostBeanProcessor的postProcessAfterInitialization
  wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName)
}