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InstantiationAwareBeanPostProcessor 是 Spring 的一个扩展点，他是 BeanPostProcessor 的子类，扩展了 BeanPostProcessor ，而外提供了 3 个方法：
1、Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName)
2、boolean postProcessAfterInstantiation(Object bean, String beanName) 3、PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName)本篇我们来探究 postProcessBeforeInstantiation 这个方法的作用，这个方法会返回一个 Object 类型的对象，在源码里面，如果有一个类实现了 InstantiationAwareBeanPostProcessor 这个后置处理器，在 postProcessBeforeInstantiation 方法里面返回了一个对象，那么直接返回，后续只会实现 BeanPostProcessor 的 postProcessAfterInitialization 方法，否则，按照正常流程走。我们可以看看源码：
Object bean = resolveBeforeInstantiation(beanName, mbdToUse); //如果bean不为空直接返回 if (bean != null) { return bean; } @Nullable protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) { Object bean = null; if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) { // Make sure bean class is actually resolved at this point. if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { Class targetType = determineTargetType(beanName, mbd); if (targetType != null) { //1、调用 InstantiationAwareBeanPostProcessor 的 postProcessBeforeInstantiation 方法 bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName); if (bean != null) { //2、调用 BeanPostProcessor 的 postProcessAfterInitialization 方法 bean = applyBeanPostProcessorsAfterInitialization(bean, beanName); } } } mbd.beforeInstantiationResolved = (bean != null); } return bean; } 下面的方法里面实现的是 postProcessBeforeInstantiation 这个方法
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
@Nullable protected Object applyBeanPostProcessorsBeforeInstantiation(Class beanClass, String beanName) { //拿到实现了BeanPostProcessors的接口 for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName); if (result != null) { return result; } } } return null; } 默认情况上面的方法返回的是空,如果不为空, 调用 BeanPostProcessor 的 postProcessAfterInitialization 方法
if (bean != null) {//2、调用 BeanPostProcessor 的 postProcessAfterInitialization 方法 bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);} 之后便是直接返回得到 Bean
源码比较复杂，可能文字无法表达清楚，下面通过具体代码演示结果
@ComponentScan("com.javahly.spring59")@Configurationpublic class Appconfig {} @Componentpublic class IndexService {} @Componentpublic class OrderService {} @Componentpublic class UserService {} @Componentpublic class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor { @Override public Object postProcessBeforeInstantiation(Class beanClass, String beanName) throws BeansException { if (beanName.endsWith("indexService")) return new UserService(); return null; }} public class Test { public static void main(String[] args){ AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(); applicationContext.register(Appconfig.class); applicationContext.refresh(); System.out.println("orderService："+applicationContext.getBean("orderService")); System.out.println("userService："+applicationContext.getBean("userService")); System.out.println("indexService："+applicationContext.getBean("indexService")); System.out.println("indexService："+applicationContext.getBean("indexService")); }} 这里我们把三个对象使用 @Component 标注，依旧是说我们想把这三个对象注入容器，不出意外，我们在运行 main 方法的时候会打印出这三个对象，但是，我们自定义了一个 MyInstantiationAwareBeanPostProcessor 扩展了InstantiationAwareBeanPostProcessor，住了一个偷梁换柱，把原本需要放入容器的 indexService 换成了 userService，打印结果如下：
orderService：com.javahly.spring59.service.OrderService@d7b1517
userService：com.javahly.spring59.service.UserService@16c0663d indexService：com.javahly.spring59.service.UserService@23223dd8 indexService：com.javahly.spring59.service.UserService@23223dd8—— 完
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