Spring扩展接口-BeanFactoryPostProcessor

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简介

Spring启动部分流程：

Spring IOC读取Bean的相关属性，保存到beanDefinition对象中。
加载完所有beanDefinition之后，调用BeanFactoryPostProcessor接口扩展点。
根据beanDefinition创建bean对象。

如果想获取beanDefinition信息或者修改BeanDefinition对象中的属性，可以实现BeanFactoryPostProcessor进行修改。

public interface BeanFactoryPostProcessor {

	void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

用途示例

Spring应用读取配置

抽象类：PropertyResourceConfigurer，主要做的就是读取properties文件，做一些必要的转换，并将这些properties写到bean Definition中。

public abstract class PropertyResourceConfigurer extends PropertiesLoaderSupport

	implements BeanFactoryPostProcessor, PriorityOrdered {

	private int order = Ordered.LOWEST_PRECEDENCE;  // default: same as non-Ordered

	// 略：serOrder / getOrder

	@Override

	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

		try {

			Properties mergedProps = mergeProperties();

			// Convert the merged properties, if necessary.

			convertProperties(mergedProps);

			// Let the subclass process the properties.

			processProperties(beanFactory, mergedProps);

		} catch (IOException ex) {

				throw new BeanInitializationException("Could not load properties", ex);

		}

	}

}

它有两个实现类：

PropertyOverrideConfigurer：比如数据库配置为"dataSource.driverClassName=com.mysql.jdbc.Driver"，这个类负责将这个value从配置文件中（比如叫datasource.properties）读取配置写到相应的beanDefinition中。
PropertyPlaceholderConfigurer ，这个类可以将代码中定义的"${...}"替换为配置文件中的实际的值。

在Spring启动时打印相关信息，比如bean的个数，统计指定特性的bean信息，修改beanDefinition, 甚至注册BeanDefinition。

@Component

public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {

    @Override

    public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {

        DefaultListableBeanFactory defaultListableBeanFactory = (DefaultListableBeanFactory) configurableListableBeanFactory;

		int count = defaultListableBeanFactory.getBeanDefinitionCount();

        System.out.println("[MyBeanFactoryPostProcessor]当前beanFactory共有" + count + "个bean");

		String[] beanDefinitionNames = defaultListableBeanFactory.getBeanDefinitionNames();

        //修改属性

        for (String beanName : beanDefinitionNames) {

            if ("person".equals(beanName)) {

                //获取PersonDefinition对象

                BeanDefinition beanDefinition = defaultListableBeanFactory.getBeanDefinition(beanName);

                MutablePropertyValues propertyValues = beanDefinition.getPropertyValues();

                System.out.println(propertyValues.toString());

                //修改定义中的name属性值

                propertyValues.addPropertyValue("name", "赵四");

                System.out.println("[MyBeanFactoryPostProcessor]postProcessBeanFactory方法中修改了name属性初始值了");

            }

        }

        //手动注册User的BeanDefiniton

        BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(User.class);

        beanDefinitionBuilder.addPropertyValue("id", 123);

        beanDefinitionBuilder.addPropertyValue("name", "修改definition");

        defaultListableBeanFactory.removeBeanDefinition("user");

        defaultListableBeanFactory.registerBeanDefinition("user", beanDefinitionBuilder.getBeanDefinition());

    }

}

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