spring的组件工厂后置处理器——BeanFactoryPostProcessor

作用和调用时机

spring有两种后置处理器：

1. 组件后置处理器——org.springframework.beans.factory.config.BeanPostProcessor；

2. 工厂后置处理器——org.springframework.beans.factory.config.BeanFactoryPostProcessor。

根据名字可以看出，前者是处理组件的，后者是处理组件工厂的。

BeanPostProcessor的调用时机是在bean被实例化之后、初始化之前或之后的；

那么，BeanFactoryPostProcessor的调用时机是什么呢？先来截取一段sring源码。

@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;

}

从源码可以看出，BeanFactoryPostProcessor是一个接口，只有1个方法，用于修改springIOC容器内部的bean工厂，调用时机是容器标准初始化之后。什么是标准初始化？注释后面有解释：所有的bean定义都即将被加载，但是还没有bean被实例化。所以，BeanFactoryPostProcessor的方法肯定是在BeanPostProcessor的两个方法之前被调用的。

示例Demo

自定义BeanFactoryPostProcessor实现类

/**
 * 自定义的BeanFactoryPostProcessor，即工厂后置处理器
 * created on 2019-04-21
 */
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    /**
     * 这个方法来自于BeanFactoryPostProcessor接口
     * 用于修改关于容器的配置
     * 调用时机：容器的标准初始化之后，这时所有的bean定义将要加载、但是并没有bean被实例化
     * @param beanFactory
     * @throws BeansException
     */
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        int count = beanFactory.getBeanDefinitionCount();
        String[] names = beanFactory.getBeanDefinitionNames();
        System.out.println("MyBeanFactoryPostProcessor.postProcessBeanFactory正在执行");
        System.out.println("共有" + count + "个组件，它们是：" + Arrays.asList(names));
    }
}

普通bean类

/**
 * bean类，通过实现接口的方式获取初始化和销毁时调用的方法
 */
@Component
public class Dog implements InitializingBean, DisposableBean {

    //属性
    private String name;
    private int age;

    //构造方法
    public Dog() {
        System.out.println("Dog:正在调用无参构造方法");
    }
    public Dog(String name, int age) {
        System.out.println("正在调用带参构造方法");
        this.name = name;
        this.age = age;
    }

    @Override
    public void destroy() throws Exception {
        System.out.println("Dog:正在调用销毁方法");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("Dog:正在调用初始化方法");
    }

    //getters & setters
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

容器配置类，自定义的工厂后置处理器必须在容器中注册才能生效

/**
 * 容器配置类
 * 注册自定义BeanFactoryPostProcessor
 */
@Configuration
@Import(value = {MyBeanFactoryPostProcessor.class})
public class BeanFactoryPostProcessorConfig {

    //注册普通bean类
    @Bean
    @Scope("singleton")
    public Dog dog() {
        return new Dog();
    }
}

测试类

/**
 * 用于测试自定义BeanFactoryPostProcessor
 */
public class BeanFactoryPostProcessorTest {

    @Test
    public void test() {
        //创建容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(BeanFactoryPostProcessorConfig.class);
        //关闭容器
        applicationContext.registerShutdownHook();
    }
}

测试结果

MyBeanFactoryPostProcessor.postProcessBeanFactory正在执行
共有9个组件，它们是：[org.springframework.context.annotation.internalConfigurationAnnotationProcessor, 
org.springframework.context.annotation.internalAutowiredAnnotationProcessor, 
org.springframework.context.annotation.internalCommonAnnotationProcessor, 
org.springframework.context.event.internalEventListenerProcessor, 
org.springframework.context.event.internalEventListenerFactory, 
beanFactoryPostProcessorConfig, cn.monolog.processors.MyBeanFactoryPostProcessor, 
cn.monolog.processors.MyBeanDefinitionRegistryPostProcessor, 
dog]

Dog:正在调用无参构造方法
Dog:正在调用初始化方法
Dog:正在调用销毁方法

从测试结果可以看出：

BeanFactoryPostProcessor.postProcessBeanFactory方法被调用时，容器中确实已经有关于bean的定义，但是bean的实例化却是后来才执行的。

源码分析

我们从创建springIOC容器开始

org.springframework.context.annotation.AnnotationConfigApplicationContext

/**
 * Create a new AnnotationConfigApplicationContext, deriving bean definitions
 * from the given annotated classes and automatically refreshing the context.
 * @param annotatedClasses one or more annotated classes,
 * e.g. {@link Configuration @Configuration} classes
 */
 public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
　　 this();
    register(annotatedClasses);
    refresh();
 }

跟进refresh →

org.springframework.context.support.AbstractApplicationContext

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);  //1

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);  //2

                // Last step: publish corresponding event.
                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();
            }
        }
    }

在1处调用的是组件工厂后置处理器的方法，在2处才是bean实例化，继续跟进invokeBeanFactoryPostProcessors →

org.springframework.context.support.AbstractApplicationContext

    /**
     * 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()));
        }
    }

首先调用了一个静态方法，后面是一堆关于类加载器的操作，跟进这个静态方法 →

org.springframework.context.support.PostProcessorRegistrationDelegate
    public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

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

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

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

            // Do not initialize FactoryBeans here: We need to leave all regular beans
            // uninitialized to let the bean factory post-processors apply to them!
            // Separate between BeanDefinitionRegistryPostProcessors that implement
            // PriorityOrdered, Ordered, and the rest.
            List<BeanDefinitionRegistryPostProcessor> 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);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
            currentRegistryProcessors.clear();

            // 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);
            currentRegistryProcessors.clear();

            // 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);
                currentRegistryProcessors.clear();
            }

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

        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);  //1

        // 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);
            }
        }

        // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);  //2

        // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(orderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);  //3

        // Finally, invoke all other BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);  //4

        // Clear cached merged bean definitions since the post-processors might have
        // modified the original metadata, e.g. replacing placeholders in values...
        beanFactory.clearMetadataCache();
    }

这个静态方法很长，前面有一段关于beanFactory instanceof BeanDefinitionRegistry的分支，会优先执行。这个分支是用来处理BeanDefinitionRegistryPostProcessor的，它是BeanFactoryPostProcessor的一个子接口，用于向容器中添加新的bean定义，调用时机在别的BeanFactoryPostProcessor之前。我们先不看它，直接看下面的普通BeanFactoryPostProcessor——

1处：从容器中根据类型获取所有的BeanFactoryPostProcessor的bean名称；

2处：首先执行实现了PriorityOrdered接口的BeanFactoryPostProcessor；

3处：然后执行实现了Ordered接口的BeanFactoryPostProcessor；

4处：最后执行其他的BeanFactoryPostProcessor，即没有实现任何排序接口的。