Spring源码九：BeanFactoryPostProcessor

上一篇Spring源码八：容器扩展一，我们看到ApplicationContext容器通过refresh方法中的prepareBeanFactory方法对BeanFactory扩展的一些功能点，包括对SPEL语句的支持、添加属性编辑器的注册器扩展解决Bean属性只能定义基础变量的问题、以及一些对Aware接口的实现。

这一篇，我们回到refresh方法中，接着往下看：

{synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing. 1、初始化上下文信息，替换占位符、必要参数的校验prepareRefresh();// Tell the subclass to refresh the internal bean factory. 2、解析类Xml、初始化BeanFactoryConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // 这一步主要是对初级容器的基础设计// Prepare the bean factory for use in this context. 	3、准备BeanFactory内容：prepareBeanFactory(beanFactory); // 对beanFactory容器的功能的扩展：try {// Allows post-processing of the bean factory in context subclasses. 4、扩展点加一：空实现，主要用于处理特殊Bean的后置处理器//!!!!!!!!!!!!  这里 这里 今天看这里  ！！！！！！！！！！！//postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context. 	5、spring bean容器的后置处理器invokeBeanFactoryPostProcessors(beanFactory);//!!!!!!!!!!!!  这里 这里 今天看这里  ！！！！！！！！！！！//// Register bean processors that intercept bean creation. 	6、注册bean的后置处理器registerBeanPostProcessors(beanFactory);// Initialize message source for this context.	7、初始化消息源initMessageSource();// Initialize event multicaster for this context.	8、初始化事件广播器initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses. 9、扩展点加一：空实现；主要是在实例化之前做些bean初始化扩展onRefresh();// Check for listener beans and register them.	10、初始化监听器registerListeners();// Instantiate all remaining (non-lazy-init) singletons.	11、实例化：非兰加载BeanfinishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.	 12、发布相应的事件通知finishRefresh();}catch (BeansException ex) {if (logger.isWarnEnabled()) {logger.warn("Exception encountered during context initialization - " +"cancelling refresh attempt: " + ex);}// Destroy already created singletons to avoid dangling resources.destroyBeans();// Reset 'active' flag.cancelRefresh(ex);// Propagate exception to caller.throw ex;}finally {// Reset common introspection caches in Spring's core, since we// might not ever need metadata for singleton beans anymore...resetCommonCaches();}}}

工厂后置处理方法：postProcessBeanFactory

上一篇我们看到了方法prepareBeanFactory方法，我们接着这个方法往下可以看到postProcessBeanFactory，进入该方法如下：

没错，又是一个空实现，看到这里我的风格都会说：扩展点加一；postProcessBeanFactory方法使用保护修饰且留给子类自己去具体实现。结合注释我们可以简单猜测一下，该方法是Spring暴露给子类去修改初级容器BeanFactory的。

那到底是什么时候可以允许子类对BeanFactory容器进行修改呢？通过方法在refresh方法的位置在prepareBeanFactory方法之后，说明是在BeanDefiniton都已经注册到BeanFactory以后，但是还未进行实例化的时候进行修改。

我们在前面8篇内容，一点点拨开Spring的方法，到这里其实我们的进程，仅仅进行到将xml文件中的Bean标签内容解析成BeanDefinition对象然后注入到Spring的BeanFactory容器里而已，这个和我们真正意义上的Bean还有一段距离呢。BeanDefintiion从名字上来看是Bean的定义，解释下应该算是Bean属性信息的初始化。

而我们在Java代码里面使用的Bean的实例化对象，需要利用我们BeanDefinition定义的属性信息去创建我们的实例化对象。在Spring中最常见的一个场景就是我们通过Spring容器getBean方法获取一个实例。这个过程我们经常称之为bean的实例化。

所以方法postBeanFactory在Bean初始化以后实例化之前为我们提供了一个可以修改BeanDefinition的机会，我们可以通过实现postBeanFactory方法去修改beanFactory的参数。

重写postProcessBeanFactory示例

package org.springframework;import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;/*** @author Jeremy* @version 1.0* @description: 自定义容器* @date 2024/7/1 20:17*/
public class MyselfClassPathXmlApplicationContext extends ClassPathXmlApplicationContext {/*** Create a new ClassPathXmlApplicationContext, loading the definitions* from the given XML file and automatically refreshing the context.* @param configLocations resource location* @throws BeansException if context creation failed*/public MyselfClassPathXmlApplicationContext(String... configLocations) throws BeansException {super(configLocations);}/**** Spring refresh 方法第扩展点加一：refresh方法中的prepareRefresh();* 重写initPropertySources 设置环境变量和设置requiredProperties**/@Overrideprotected void initPropertySources() {System.out.println("自定义 initPropertySources");getEnvironment().getSystemProperties().put("systemOS", "mac");getEnvironment().setRequiredProperties("systemOS");}/**** Spring refresh 方法第扩展点加一：refresh方法中的postProcessBeanFactory();* 重写该方法，修改基础容器BeanFactory内容* @param beanFactory**/@Overrideprotected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {System.out.println("自定义 postProcessBeanFactory");// 获取容器中的对象BeanDefinition beanDefinition = beanFactory.getBeanDefinition("jsUser");beanDefinition.getScope();// 修改属性beanDefinition.setScope(BeanDefinition.SCOPE_PROTOTYPE);}
}

如上述方法，我们重写了postProcessorBeanFactory方法，通过beanFactory参数获取Bean Definition对象，然后修改BeanDefinition属性值。当让我还可以修改BeanFactory的其他属性，因为这个参数是将整个BeanDefinition委托给我们自己定义的。

package org.springframework;import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.dto.JmUser;/*** @author Jeremy* @version 1.0* @description: TODO* @date 2024/6/30 02:58*/
public class Main {public static void main(String[] args) {ApplicationContext context = new MyselfClassPathXmlApplicationContext("applicationContext.xml");JmUser jmUser = (JmUser)context.getBean("jmUser");System.out.println(jmUser.getName());System.out.println(jmUser.getAge());}
}

运行上述代码，可以看到结果如上图所示，Spring的postProcessorBeanFactory有回调我自定义的方法。正因为Spring在这立留了钩子函数，所以很多框架可以通过这个方法或有类似功能的扩展点来操作整个beanFactory容器，从而可以自定义自己的框架使之拥有更加丰富与个性化的功能。后面会单独留一篇用来说Spring的扩展点，接口SpringCloud相关组件来来看看。

invokeBeanFactoryPostProcessors

看到这里其实今天的核心内容已经结束了，但是我们在文章开始的时候给大家圈里了两个方法，这里我们在提一下和上述方法功能上一致的一个类。

/*** 实例化并调用所有已经注册BeanFactoryPostProcessor* Instantiate and invoke all registered BeanFactoryPostProcessor beans,* respecting explicit order if given.* <p>Must be called before singleton instantiation.*/protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {//PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));}}/*** Return the list of BeanFactoryPostProcessors that will get applied* to the internal BeanFactory.*/public List<BeanFactoryPostProcessor> getBeanFactoryPostProcessors() {return this.beanFactoryPostProcessors;}

在上述代码里的注释里面，已经解释了这个代码的功能是为了实例化并调用所有已经注册的BeanFactoryPostProcessor。所以能知道我这个放的核心其实是BeanFactoryPostProcessor，进入BeanFactoryPostProcessor类看下：

/** Copyright 2002-2019 the original author or authors.** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at**      https://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/package org.springframework.beans.factory.config;import org.springframework.beans.BeansException;/*** Factory hook that allows for custom modification of an application context's* bean definitions, adapting the bean property values of the context's underlying* bean factory.** <p>Useful for custom config files targeted at system administrators that* override bean properties configured in the application context. See* {@link PropertyResourceConfigurer} and its concrete implementations for* out-of-the-box solutions that address such configuration needs.** <p>A {@code BeanFactoryPostProcessor} may interact with and modify bean* definitions, but never bean instances. Doing so may cause premature bean* instantiation, violating the container and causing unintended side-effects.* If bean instance interaction is required, consider implementing* {@link BeanPostProcessor} instead.** <h3>Registration</h3>* <p>An {@code ApplicationContext} auto-detects {@code BeanFactoryPostProcessor}* beans in its bean definitions and applies them before any other beans get created.* A {@code BeanFactoryPostProcessor} may also be registered programmatically* with a {@code ConfigurableApplicationContext}.** <h3>Ordering</h3>* <p>{@code BeanFactoryPostProcessor} beans that are autodetected in an* {@code ApplicationContext} will be ordered according to* {@link org.springframework.core.PriorityOrdered} and* {@link org.springframework.core.Ordered} semantics. In contrast,* {@code BeanFactoryPostProcessor} beans that are registered programmatically* with a {@code ConfigurableApplicationContext} will be applied in the order of* registration; any ordering semantics expressed through implementing the* {@code PriorityOrdered} or {@code Ordered} interface will be ignored for* programmatically registered post-processors. Furthermore, the* {@link org.springframework.core.annotation.Order @Order} annotation is not* taken into account for {@code BeanFactoryPostProcessor} beans.** @author Juergen Hoeller* @author Sam Brannen* @since 06.07.2003* @see BeanPostProcessor* @see PropertyResourceConfigurer*/
@FunctionalInterface
public interface BeanFactoryPostProcessor {/*** Modify the application context's internal bean factory after its standard* initialization. All bean definitions will have been loaded, but no beans* will have been instantiated yet. This allows for overriding or adding* properties even to eager-initializing beans.* @param beanFactory the bean factory used by the application context* @throws org.springframework.beans.BeansException in case of errors*/void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;}

看到这个接口只有一个方法且名称也叫postProcessBeanFactory，在看代码的上面的这是与参数和我们refresh中一样，说明我们也可以通过实现Bean FactoryPostProcessor接口的形式来修改我们的BeanFactory对象。如下所示：

自定义BeanFactoryPostProcessor

package org.springframework;import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;/*** 自定义Bean的后置处理器*/
public class MySelfBeanFactoryPostProcessor implements BeanFactoryPostProcessor {@Overridepublic void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {System.out.println("自定义Bean的后置处理器,重写postProcessBeanFactory方法");// 获取容器中的对象BeanDefinition beanDefinition = beanFactory.getBeanDefinition("jmUser");beanDefinition.getScope();// 修改属性beanDefinition.setScope(BeanDefinition.SCOPE_PROTOTYPE);}
}

Spring已经提供了空实现的postProcessorBeanFactory方法，为啥还要单独搞一个接口来定一个这个方法呢？

Spring提供BeanFactoryPostProcessor接口，而不是在refresh方法中直接进行beanFactory修改，主要是为了提高代码的解耦性、可维护性和扩展性。通过将不同的修改操作分散到不同的实现类中，每个类都只关注特定的任务，这样不仅符合单一职责原则，还使得整个框架更加灵活和易于扩展。这种设计理念是

Spring成功的重要原因之一。

整体小结