Spring 中BeanDefinitionRegistryPostProcessor的作用

BeanDefinitionRegistryPostProcessor功能如下：

1、因为实现了BeanFactoryPostProcessor接口，所以可以修改bean定义中属性，具体操作可以参照https://blog.csdn.net/lsq_401/article/details/105508535

2、还可以动态的添加bean到spring容器中

而且它的执行会优先其他自定义的实现了该接口的BeanFactoryPostProcessor类，具体参考PostProcessorRegistrationDelegate

的invokeBeanFactoryPostProcessors方法，只保留部分代码：

public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set processedBeans = new HashSet<>();

		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			List regularPostProcessors = new ArrayList<>();
			List registryProcessors = new ArrayList<>();

			
			List currentRegistryProcessors = new ArrayList<>();

			// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
	

		}

		else {
			// Invoke factory processors registered with the context instance.
			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
		}

		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let the bean factory post-processors apply to them!
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		List priorityOrderedPostProcessors = new ArrayList<>();
		List orderedPostProcessorNames = new ArrayList<>();
		List nonOrderedPostProcessorNames = new ArrayList<>();
		for (String ppName : postProcessorNames) {
			if (processedBeans.contains(ppName)) {
				// skip - already processed in first phase above
			}
			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
			}
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}

	}

spring-config.xml

MyBeanDefinitionRegistryPostProcessor

public class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
		BeanDefinition beanDefinition = BeanDefinitionBuilder.genericBeanDefinition(CustomBean.class).getBeanDefinition();
		beanDefinition.getPropertyValues().add("name", "hello lsq");
		registry.registerBeanDefinition("customBean", beanDefinition);
	}

	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

	}
}

CustomBean

public class CustomBean {
	private String name;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String print(){
		return this.name;
	}
}

TestApplication

public class TestApplication {

	public static void main(String[] args) {
		ApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath*:spring-config.xml");
		CustomBean customBean = (CustomBean) applicationContext.getBean("customBean");
		System.out.println(customBean.print());
	}
}

运行会输出：

hello lsq

用过mybatis的同学应该都熟悉下面这段代码，MapperScannerConfigurer接口就是很好的利用该接口动态添加特性来实现的例子：

其实MapperScannerConfigurer类的定义如下，只保留部分代码：

public class MapperScannerConfigurer
    implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {


  @Override
  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    // left intentionally blank
  }

  
  @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
    scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
    scanner.setResourceLoader(this.applicationContext);
    scanner.setBeanNameGenerator(this.nameGenerator);
    scanner.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
    if (StringUtils.hasText(lazyInitialization)) {
      scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization));
    }
    scanner.registerFilters();
    scanner.scan(
        StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

}

具体执行逻辑请参考scan方法，执行完成之后，会把这些都动态注入到spring容器中