Springboot 之 ObjectPostProcessor

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一、ObjectPostProcessor 是什么？

ObjectPostProcessor 能对我们没有被扫描的类执行初始化, 注意是初始化 (initializeBean)。

二、代码示例

ObjectPostProcessor 有两个实现类，这里只研究 AutowireBeanFactoryObjectPostProcessor

1. 创建一个普通的 Service

   @Service
   public class ComponentService {
   }
   

2. 创建一个组件，但是不放入到 IOC 中管理

   // 实现了 ApplicationContextAware，但是没有放入到 ioc 中
   public class ComponentDemo implements ApplicationContextAware {
   
       @Autowired
       private ComponentService componentService;
   
       public void printComponentService() {
           System.out.println(componentService);
       }
   
       @Override
       public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
           System.out.println("ComponentDemo - setApplicationContext 执行了");
       }
   }
   

   // 配置类
   @Configuration
   public class ObjectPostProcessorConfig {
   
       private ComponentDemo componentDemo;
   
   // objectPostProcessor 存在的话 @Autowired 标注的方法会自动执行
   // 如果不存在可以自行注入一个 objectPostProcessor
       @Autowired(required = false)
       public void autowiredObject(ObjectPostProcessor<Object> objectPostProcessor) {
       // 使用 objectPostProcessor 自动处理 ComponentDemo
           componentDemo = objectPostProcessor.postProcess(new ComponentDemo());
           componentDemo.printComponentService();
       }
   }
   

3. 观察输出结果

   ComponentDemo 的 printComponentService() 和 setApplicationContext() 都会被执行，而且 ComponentDemo 的 componentService 就是我们注入到 IOC 中的 componentService

4. 附录：注入 AutowireBeanFactoryObjectPostProcessor

   @Bean
   @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
   public ObjectPostProcessor<Object> objectPostProcessor(AutowireCapableBeanFactory beanFactory) {
   	return new AutowireBeanFactoryObjectPostProcessor(beanFactory);
   }
   

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三、AutowireBeanFactoryObjectPostProcessor

实现原理

@Override
public <T> T postProcess(T object) {
	if (object == null) {
		return null;
	}
	T result = null;
	try {
		// 初始化 Bean 如果需要的话
		result = initializeBeanIfNeeded(object);
	}
	...
	// autowireBean
	this.autowireBeanFactory.autowireBean(object);
	...
	return result;
}

// AutowireBeanFactoryObjectPostProcessor#initializeBeanIfNeeded
private <T> T initializeBeanIfNeeded(T object) {
	if (!NativeDetector.inNativeImage() || !AopUtils.isCglibProxy(object)) {
	// initializeBean 你看到了什么，这不正是 springboot 创建 bean 的最后一步初始化 Bean 吗，这里面会干啥，执行 Aware 方法，执行初始化方法
		return (T) this.autowireBeanFactory.initializeBean(object, object.toString());
	}
	ObjectProvider<?> provider = this.autowireBeanFactory.getBeanProvider(object.getClass());
	Object bean = provider.getIfUnique();
	if (bean == null) {
		String msg = """
				Failed to resolve an unique bean (single or primary) of type [%s] from the BeanFactory.
				Because the object is a CGLIB Proxy, a raw bean cannot be initialized during runtime in a native image.
				"""
			.formatted(object.getClass());
		throw new IllegalStateException(msg);
	}
	return (T) bean;
}