Spring的后处理器

Spring的后处理器是Spring对外开放的重要扩展点，允许我们介入到Bean的整个实例化流程中来，以达到动态注册BeanDefinition，动态修改BeanDefinition，以及动态修改Bean的作用。Spring主要有两种后处理器：

• BeanFactoryPostProcessor：Bean工厂后处理器，在BeanDefinitionMap填充完毕，Bean实例化之前执行；
• BeanPostProcessor：Bean后处理器，一般在Bean实例化之后，填充到单例池singletonObjects之前执行。

Bean工厂后处理器-BeanFactoryPostProcessor

BeanFactoryPostProcessor是一个接口规范，实现了该接口的类只要交由Spring容器管理的话，那么Spring就会回调该接口的方法，用于对BeanDefinition注册和修改的功能。

BeanFactoryPostProcessor定义如下：

public interface BeanFactoryPostProcessor {
	void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory);
}

使用方法：

package com.luxifa.processor;
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
	System.out.println("beanDefinitionMap填充完毕后回调该方法")
	}
}

xml中：

<bean class="com.luxifa.processor.MyBeanFactoryPostProcessor"></bean>

案例：修改某个BeanDifinition

package com.luxifa.processor;
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
	System.out.println("beanDefinitionMap填充完毕后回调该方法")
	//修改某个BeanDifinition
	BeanDefinition beanDefintion = beanFactory.getBeanDefinition("userService");
	beanDefintion.setBeanClassName("com.luxifa.dao.impl.UserDaoImpl");
	}
}

<bean id="userDao" name="aaa,bbb" class="com.luxifa.dao.impl.UserDaoImpl"></bean>

<bean class="userService" class="com.luxifa.service.impl.UserServeiceImpl"></bean>

public class ApplicationContextTest {
	public static void main(String[] args) throws Exception{
	
	ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
	Object userService = applicationContext.getBean("userServeice");
	System.out.println(userService);
	}
}

打印信息：

beanDefinitionMap填充完毕后回调该方法
com.luxifa.dao.impl.UserDaoImpl@544fe44a

Spring的Bean工厂后处理器注册BeanDefinition

public interface PersonDao {
}

public class PersonDaoImpl implements PersonDao {
}

package com.luxifa.processor;
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
	System.out.println("beanDefinitionMap填充完毕后回调该方法")
	//注册BeanDifinition
	BeanDefinition beanDefintion = new RootBeanDefinition();
	beanDefintion.setBeanClassName("com.luxifa.dao.impl.PersonDaoImpl");
	DefaultListableBeanFactory defaultListableBeanFactory = (DefaultListableBeanFactory) beanFactory;
	defaultListableBeanFactory.registerBeanDefinition(beanName:"personDao",beanDefinition);
	}
}

xml中：

<bean class="com.luxifa.processor.MyBeanFactoryPostProcessor"></bean>

public class ApplicationContextTest {
	public static void main(String[] args) throws Exception{
	
	ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
	PersonDao userService = applicationContext.getBean(PersonDao.class);
	System.out.println(userService);
	}
}

控制台打印：

beanDefinitionMap填充完毕后回调该方法
com.luxifa.dao.impl.PersonDaoImpl@4e1d422d

Bean工厂后处理器 - BeanFactoryPostProcessor

Spring提供了一个BeanFactoryPostProcessor的子接口BeanDefinitionRegisterPostProcessor专门用来注册BeanDefinition操作

public class MyBeanFactoryPostProcessor2 implements BeanDefinitionRegistryPostProcessor {
	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeanException {}

	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeanException {
	BeanDefinitionRegistry beanDefinitionRegistry = new RootBeanDefinition();
	beanDefinitionRegistry.setBeanClassName(com.luxifa.dao.userDaoImpl2);
	beanDefinitionRegistry.registerBeanDefinition("userDao2",beanDefinition);
	}
}

xml中：

<bean class="com.luxifa.processor.MyBeanDefinitionRegistryPostProcessor"></bean>

postProcessBeanDefinitionRegistry是BeanDefinitionRegistryPostProcessor父接口中的方法，也需要实现。

执行顺序 先执行postProcessBeanDefinitionRegistry 方法，后执行 postProcessBeanFactory方法。

Spring的bean工厂后处理器-完善实例化基本流程图

Spring的Bean工厂后处理器-自定义@Component

案例：使用Spring的BeanFactoryPostProcessor扩展点完成自定义注解扫描

• 自定义@MyConponent注解，使用在类上
• 完成指定包的类扫描
• 自定义BeanFactoryPostProcessor完成注解@MyComponent的解析，解析后最终被Spring管理。

package com.luxifa.beans;

@MyComponent("otherBean")
public class OtherBean {
}

package com.luxifa.anno;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyComponent {

	String value;
	
}

package com.luxifa.processor;

public class ComponentBeanFactoryPostProcessor implements BeanDefinitionRegistryPostProcessor {
	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeanException {}

	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeanException {
	//通过扫描工具去扫描指定包及其子包下的所有类,收集使用@Mycomponent的注解的类
	Map<String, Class> myComponentAnnotationMap = BaseClassScanUtils.scanMyComponentAnnotation(basePackage:"com.luxifa");
	//遍历Map,组装BeanDefinition进行注册
	myComponentAnnotationMap.forEach((beanName,clazz)->{
		//获得beanClassName
		String beanClassName = clazz.getName(); //com.luxifa.beans.OtherBean
		//创建BeanDefinition
		BeanDefinition beanDefinition = new RootBeanDefinition();
		beanDefinition.setBeanClassName(beanClassName);
		//注册
		beanDefinitionRegistry.registerBeanDefinition(beanName,beanDefinition);
		});
	}
}

xml中：

<bean class="com.luxifa.processor.MyComponentBeanFactoryPostProcessor"></bean>

public class ApplicationContextTest {
	
	public static void main(String[] args) throws Exception {
		
		ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
		OtherBean otherBean = applicationContext.getBean(OtherBean.class);
		System.out.println(otherBean);
		
	}
}