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11. Spring源码篇之实例化前的后置处理器

news 2025/7/27 22:04:27

简介

spring在创建Bean的过程中，提供了很多个生命周期，实例化前就是比较早的一个生命周期，顾名思义就是在Bean被实例化之前的处理，这个时候还没实例化，只能拿到该Bean的Class对象，如果在这个时候直接返回一个对象，那么就不用spring给我们创建了

简单使用

实现 InstantiationAwareBeanPostProcessor 接口的 postProcessBeforeInstantiation方法

@Component
public class UserBean {public UserBean() {System.out.println("实例化UserBean");}
}@Component
public class BeforeInstantiation implements InstantiationAwareBeanPostProcessor {@Overridepublic Object postProcessBeforeInstantiation(@NotNull Class<?> beanClass, String beanName) throws BeansException {if (beanName.equals("userBean")) {return "1";}return null;}
}public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);System.out.println(context.getBean("userBean"));
}

输出结果 1

从上面的逻辑可以看出，如果beanName是userBean，那么直接返回一个
1，这个返回值是个object，任意类型都可以，如果直接返回有结果，那么这个结果就是该bean的结果，spring不会再给我们去实例化bean

源码分析

createBean源码

Object bean = resolveBeforeInstantiation(beanName, mbdToUse);// 如果拿到Bean直接返回，也就是不会再有后面的逻辑,后面的一些扩展点会不生效
if (bean != null) {return bean;
}

进入 resolveBeforeInstantiation

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {Object bean = null;if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {// 有没有 InstantiationAwareBeanPostProcessors 后置处理器if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {Class<?> targetType = determineTargetType(beanName, mbd);if (targetType != null) {// 如果有后置处理去执行初始化前逻辑，可能获得了一个Beanbean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);if (bean != null) {// 如果获得了Bean，由于外面是直接返回，但是Aop逻辑应该还是要有，而aop就是通过初始化后的后置处理实现的，所以需要执行bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);}}}mbd.beforeInstantiationResolved = (bean != null);}return bean;
}protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {// 遍历执行postProcessBeforeInstantiation方法，可能可以直接获得到Beanfor (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {Object result = bp.postProcessBeforeInstantiation(beanClass, beanName);if (result != null) {return result;}}return null;
}public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException {Object result = existingBean;// 遍历执行初始化后的逻辑postProcessAfterInitializationfor (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessAfterInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;
}