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Spring Boot - 扩展接口一览

org.springframework.beans.factory.config.BeanFactoryPostProcessor

这个接口是beanFactory的扩展接口，调用时机在spring在读取beanDefinition信息之后，实例化bean之前。

在这个时机，用户可以通过实现这个扩展接口来自行处理一些东西，比如修改已经注册的beanDefinition的元信息

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;

}

spring容器初始化时，从资源中读取到bean的相关定义后，保存在beanFactory的成员变量中（DefaultListableBeanFactory#beanDefinitionMap）

private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);

实例化bean的操作就是依据这些BeanDefinition来做的。

在实例化之前，spring允许我们通过自定义扩展来改变bean的定义，定义一旦变了，后面的实例也就变了，而beanFactory后置处理器BeanFactoryPostProcessor就是用来改变bean定义的。

源码探究

AbstractApplicationContext#refresh 继续走起

继续

// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);

主要功能是： 找出所有beanFactory后置处理器，并且调用这些处理器来改变bean的定义

继续 org.springframework.context.support.AbstractApplicationContext#invokeBeanFactoryPostProcessors

    /**
     * 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());

        .......

我们重点关注PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors

PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

代码比较长，我们逐段看一下

1 是否实现BeanDefinitionRegistryPostProcessor 接口，分别写入集合

    for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryProcessors.add(registryProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

遍历入参 beanFactoryPostProcessors ， 是否实现了BeanDefinitionRegistryPostProcessor，分别放入两个集合：registryProcessors和regularPostProcessors；

2 处理实现了的PriorityOrdered和 BeanDefinitionRegistryPostProcessors 的 bean

// 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);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
            currentRegistryProcessors.clear();

找出所有实现了BeanDefinitionRegistryPostProcessor接口和PriorityOrdered接口的bean，放入registryProcessors集合，根据PriorityOrdered接口来排序，然后这些bean会被invokeBeanDefinitionRegistryPostProcessors方法执行；

3. 处理实现了的Ordered和BeanDefinitionRegistryPostProcessors 的 bean

// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
            currentRegistryProcessors.clear();

找出所有实现了BeanDefinitionRegistryPostProcessor接口和Ordered接口的bean，放入registryProcessors集合，根据Ordered接口来排序，然后这些bean会被invokeBeanDefinitionRegistryPostProcessors方法执行；

4 处理其他实现了BeanDefinitionRegistryPostProcessor的bean

    // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
            boolean reiterate = true;
            while (reiterate) {
                reiterate = false;
                postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
                for (String ppName : postProcessorNames) {
                    if (!processedBeans.contains(ppName)) {
                        currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                        processedBeans.add(ppName);
                        reiterate = true;
                    }
                }
                sortPostProcessors(currentRegistryProcessors, beanFactory);
                registryProcessors.addAll(currentRegistryProcessors);
                invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
                currentRegistryProcessors.clear();
            }

对于那些实现了BeanDefinitionRegistryPostProcessor接口，但是没有实现PriorityOrdered和Ordered的bean也被找出来，然后这些bean会被invokeBeanDefinitionRegistryPostProcessors方法执行

5 invokeBeanFactoryPostProcessors

// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);

6 除了试下BeanDefinitionRegistryPostProcessor之外的其他 实现了BeanFactoryPostProcessor接口的bean 分类

    // 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<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        List<String> orderedPostProcessorNames = new ArrayList<>();
        List<String> 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);
            }
        }

找出实现了BeanFactoryPostProcessor接口的bean，注意这里已将前面实现了BeanDefinitionRegistryPostProcessor接口的bean给剔除了

if (processedBeans.contains(ppName)) {
    // skip - already processed in first phase above
}

将这些bean分为三类：

• 实现了PriorityOrdered接口的放入priorityOrderedPostProcessors，
• 实现了Ordered接口的放入orderedPostProcessorNames，
• 其他的放入nonOrderedPostProcessorNames，这段代码是关键，自定义的实现BeanFactoryPostProcessor接口的bean就会在此处被查找出来。

7 处理 PriorityOrdered ，invokeBeanDefinitionRegistryPostProcessors

// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

8 处理 Ordered ，invokeBeanDefinitionRegistryPostProcessors

// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(orderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

9 处理剩下的 ，invokeBeanDefinitionRegistryPostProcessors

// Finally, invoke all other BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

10 invokeBeanDefinitionRegistryPostProcessors & invokeBeanFactoryPostProcessors 分析

从上面的分析可以发现，所有实现了BeanFactoryPostProcessor接口的bean，都被作为入参，然后调用了invokeBeanDefinitionRegistryPostProcessors或者invokeBeanFactoryPostProcessors方法去处理

    /**
     * Invoke the given BeanDefinitionRegistryPostProcessor beans.
     */
    private static void invokeBeanDefinitionRegistryPostProcessors(
            Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, BeanDefinitionRegistry registry, ApplicationStartup applicationStartup) {

        for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
            StartupStep postProcessBeanDefRegistry = applicationStartup.start("spring.context.beandef-registry.post-process")
                    .tag("postProcessor", postProcessor::toString);
            postProcessor.postProcessBeanDefinitionRegistry(registry);
            postProcessBeanDefRegistry.end();
        }
    }

    /**
     * Invoke the given BeanFactoryPostProcessor beans.
     */
    private static void invokeBeanFactoryPostProcessors(
            Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {

        for (BeanFactoryPostProcessor postProcessor : postProcessors) {
            StartupStep postProcessBeanFactory = beanFactory.getApplicationStartup().start("spring.context.bean-factory.post-process")
                    .tag("postProcessor", postProcessor::toString);
            postProcessor.postProcessBeanFactory(beanFactory);
            postProcessBeanFactory.end();
        }
    }

对每个BeanFactoryPostProcessor接口的实现类，都调用了其接口方法。
不同的是 对于实现了BeanDefinitionRegistryPostProcessor接口的bean，调用其postProcessBeanDefinitionRegistry方法的时候，入参是BeanDefinitionRegistry，而非BeanFactory，因此，实现了BeanDefinitionRegistryPostProcessor接口的bean，其postProcessBeanDefinitionRegistry在被调用时，可以通过入参BeanDefinitionRegistry来做更多和bean的定义有关的操作，例如注册bean等等、

BeanFactoryPostProcessor的处理流程

小结一下：

• ApplicationContext扩展类可以调用AbstractApplicationContext.addBeanFactoryPostProcessor方法，将自定义的BeanFactoryPostProcessor实现类保存到ApplicationContext中；
• spring容器初始化时，上一步中被加入到ApplicationContext的bean会被优先调用其postProcessBeanFactory方法；
• 自定义的BeanFactoryPostProcessor接口实现类，也会被找出来，然后调用其postProcessBeanFactory方法；
• postProcessBeanFactory方法被调用时，beanFactory会被作为参数传入，自定义类中可以使用该参数来处理bean的定义，达到业务需求；
• 此时的spring容器还没有开始实例化bean，因此自定义的BeanFactoryPostProcessor实现类不要做与bean实例有关的操作，而是做一些与bean定义有关的操作，例如修改某些字段的值，这样后面实例化的bean的就会有相应的改变；

扩展方式

package com.artisan.bootspringextend.testextends;

import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.annotation.Configuration;

/**
 * @author 小工匠
 * @version 1.0 
 * @date 2022/11/27 16:58
 * @mark: show me the code , change the world
 */

@Slf4j
@Configuration
public class ExtendBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        log.info("----->postProcessBeanFactory called ");
    }
}