BeanPostProcessor原理学习

《Spring源码解析》笔记

1、自定义的BeanPostProcessor

@Component
public class MyBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("postProcessBeforeInitialization..."+beanName+"=>"+bean);
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        // TODO Auto-generated method stub
        System.out.println("postProcessAfterInitialization..."+beanName+"=>"+bean);
        return bean;
    }
}

2、其中postProcessBeforeInitialization函数的调用链如下：

在容器启动的时候：refresh()-->finishBeanFactoryInitialization(beanFactory)-->beanFactory.preInstantiateSingletons()->this.getBean(beanName)-->

this.doGetBean(name, (Class)null, (Object[])null, false)--this.getSingleton(beanName, new ObjectFactory<Object>()-->

singletonFactory.getObject()-->AbstractBeanFactory.this.createBean(beanName, mbd, args)-->this.doCreateBean(beanName, mbdToUse, args)

-->this.initializeBean(beanName, exposedObject, mbd)

在initializeBean()执行之前，首先执行的是populateBean()函数，该函数主要是为Bean的各个属性赋值。

　　　　　this.populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
             exposedObject = this.initializeBean(beanName, exposedObject, mbd);
        }

3、在initializeBean(beanName, exposedObject, mbd) 中观察调用的代码

　　　　if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = this.applyBeanPostProcessorsBeforeInitialization(bean, beanName);
        }
　　　　//初始化方法执行代码
        try {
            this.invokeInitMethods(beanName, wrappedBean, mbd);
        } catch (Throwable var6) {
            throw new BeanCreationException(mbd != null ? mbd.getResourceDescription() : null, beanName, "Invocation of init method failed", var6);
        }

        if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = this.applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

4、查看applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization函数代码

遍历得到容器中所有的BeanPostProcessor；挨个执行beforeInitialization，一但返回null，跳出for循环，不会执行后面的BeanPostProcessor.postProcessorsBeforeInitialization

对于容器中每一个Bean实例，只要符合步骤3中if语句条件，都会执行这两个函数，并不是实现BeanPostProcessor接口才会进入这两个函数。

@Override
    public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
            throws BeansException {

        Object result = existingBean;
        for (BeanPostProcessor processor : getBeanPostProcessors()) {
            result = processor.postProcessBeforeInitialization(result, beanName);
            if (result == null) {
                return result;
            }
        }
        return result;
    }

    @Override
    public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
            throws BeansException {

        Object result = existingBean;
        for (BeanPostProcessor processor : getBeanPostProcessors()) {
            result = processor.postProcessAfterInitialization(result, beanName);
            if (result == null) {
                return result;
            }
        }
        return result;
    }