精通 Spring 源码 | InstantiationAwareBeanPostProcessor(1)

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一、简介

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)

二、源码分析

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;}

	@Nullableprotected 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);

	@Nullableprotected 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

三、示例

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")
@Configuration
public class Appconfig {
}

@Component
public class IndexService {}

@Component
public class OrderService {}

@Component
public class UserService {}

@Component
public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor {@Overridepublic 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