Spring拓展接口之BeanPostProcessor，我们来看看它的底层实现

前言

　　开心一刻

　　　　小明：“妈，我被公司开除了”，妈：“啊，为什么呀？”， 小明：“我骂董事长是笨蛋，公司召开高层会议还要起诉我”，妈：“告你诽谤是吧？”，小明：“不是，他们说要告我泄露公司机密”

BeanPostProcessor定义

　　不管三七二十一，我们先来看看它的定义，看看spring是如何描述BeanPostProcessor的

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 * 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
 *
 *      http://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,
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package org.springframework.beans.factory.config;

import org.springframework.beans.BeansException;
import org.springframework.lang.Nullable;

/**
 * 允许对新的bean示例进行自定义的修改，例如检查标志接口或进行代理封装
 *
 * spring上下文会在它的beng定义中自动检测BeanPostProcessor实例，并将它们应用于随后创建的每一个bean实例
 *
 * implement {@link #postProcessAfterInitialization}.
 * 通常，通过实现BeanPostProcessor的postProcessBeforeInitialization方法（配合标记接口，如@Autowired）来填充bean实例，
 * 通过BeanPostProcessor的postProcessAfterInitialization方法进行bean实例的代理
 *
 */
public interface BeanPostProcessor {

    /**
     * 在bean实例的初始化方法（例如InitializingBean的afterPropertiesSet或自定义的init-method）回调之前，
     * spring会应用此方法到bean实例上。一般用于bean实例的属性值的填充
     * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean
     * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     * The returned bean instance may be a wrapper around the original.
     * <p>The default implementation returns the given {@code bean} as-is.
     * @param bean the new bean instance
     * @param beanName the name of the bean
     * @return the bean instance to use, either the original or a wrapped one;
     * if {@code null}, no subsequent BeanPostProcessors will be invoked
     * @throws org.springframework.beans.BeansException in case of errors
     * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
     */
    @Nullable
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    /**
     * 在bean实例的初始化方法（例如InitializingBean的afterPropertiesSet或自定义的init-method）回调之后，
     * spring会应用此方法到bean实例上。
     * 在有FactoryBean时，此方法会在FactoryBean实例与FactoryBean的目标对象创建时各调用一次
     * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean
     * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
     * or a custom init-method). The bean will already be populated with property values.
     * The returned bean instance may be a wrapper around the original.
     * <p>In case of a FactoryBean, this callback will be invoked for both the FactoryBean
     * instance and the objects created by the FactoryBean (as of Spring 2.0). The
     * post-processor can decide whether to apply to either the FactoryBean or created
     * objects or both through corresponding {@code bean instanceof FactoryBean} checks.
     * <p>This callback will also be invoked after a short-circuiting triggered by a
     * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
     * in contrast to all other BeanPostProcessor callbacks.
     * <p>The default implementation returns the given {@code bean} as-is.
     * @param bean the new bean instance
     * @param beanName the name of the bean
     * @return the bean instance to use, either the original or a wrapped one;
     * if {@code null}, no subsequent BeanPostProcessors will be invoked
     * @throws org.springframework.beans.BeansException in case of errors
     * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
     * @see org.springframework.beans.factory.FactoryBean
     */
    @Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

}

　　简单点来理解，就是spring会自动从它的所有的bean定义中检测BeanPostProcessor类型的bean定义，然后实例化它们，再将它们应用于随后创建的每一个bean实例，在bean实例的初始化方法回调之前调用BeanPostProcessor的postProcessBeforeInitialization的方法（进行bean实例属性的填充），在bean实例的初始化方法回调之后调用BeanPostProcessor的postProcessAfterInitialization的方法（可以进行bean实例的代理封装）

应用示例

　　我们先来看个简单的示例，注意：由于spring只是从spring容器中的bean定义中自动检测BeanPostProcessor类型的bean定义，所以我们自定义的BeanPostProcessor要通过某种方式注册到spring容器

　　MyBeanPostProcessor

@Component
public class MyBeanPostProcessor implements BeanPostProcessor {

    public MyBeanPostProcessor () {
        System.out.println("MyBeanPostProcessor 实例化......");
    }

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("spring中bean实例:" + beanName + " 初始化之前处理......");
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("spring中bean实例:" + beanName + " 初始化之后处理......");
        return bean;
    }
}

　　AnimalConfig

@Configuration
public class AnimalConfig {

    public AnimalConfig() {
        System.out.println("AnimalConfig 实例化");
    }

    @Bean
    public Dog dog() {
        return new Dog();
    }

}

　　Dog

public class Dog {

    private String name;

    public Dog() {
        System.out.println("Dog 实例化......");
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

　　完整实例工程：spring-boot-BeanPostProcessor 我们来看看启动结果

　　有人可能会说了：“你是个逗比把，你举的这个例子有什么用？ 实际上，根本就不会出现BeanPostProcessor的这样用法！”  有这样的疑问非常正常，示例中的BeanPostProcessor的两个方法：postProcessBeforeInitialization、postProcessAfterInitialization没做任何的处理，都只是直接返回bean，这不就是：脱了裤子放屁？

　　我们细看下，会发现postProcessBeforeInitialization、postProcessAfterInitialization中各多了一行打印（），其实示例只是验证下Spring对BeanPostProcessor的支持、BeanPostProcessor的两个方法的执行时机，是否如BeanPostProcessor 的注释所说的那样，实际应用中肯定不会这么用的。那问题来了：BeanPostProcessor能用来干什么？ 回答这个问题之前，我们先来看看spring对BeanPostProcessor的底层支持

源码解析

　　BeanPostProcessor的实例化与注册　　

　　　　很明显，我们从spring的启动过程的refresh方法开始，如下图

　　　　此时spring容器中所有的BeanPostProcessor都进行了实例化，并注册到了beanFactory的beanPostProcessors属性中

　　　　registerBeanPostProcessors

public static void registerBeanPostProcessors(
        ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

    String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

    // Register BeanPostProcessorChecker that logs an info message when
    // a bean is created during BeanPostProcessor instantiation, i.e. when
    // a bean is not eligible for getting processed by all BeanPostProcessors.
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    // Separate between BeanPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    // 将所有BeanPostProcessor bean定义分三类：实现了PriorityOrdered、实现了Ordered，以及剩下的常规BeanPostProcessor
    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
    List<String> orderedPostProcessorNames = new ArrayList<>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            // 实例化实现了PriorityOrdered接口的BeanPostProcessor
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            priorityOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, register the BeanPostProcessors that implement PriorityOrdered.
    // 注册实现了PriorityOrdered接口的BeanPostProcessor到beanFactory的beanPostProcessors属性中
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
    for (String ppName : orderedPostProcessorNames) {
        // 实例化实现了Ordered接口的BeanPostProcessor
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    // 注册实现了Ordered接口的BeanPostProcessor到beanFactory的beanPostProcessors属性中
    sortPostProcessors(orderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
    for (String ppName : nonOrderedPostProcessorNames) {
        // 实例化剩下的所有的常规的BeanPostProcessors
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        nonOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

    // Finally, re-register all internal BeanPostProcessors.
    // 注册所有常规的的BeanPostProcessor到beanFactory的beanPostProcessors属性中
    sortPostProcessors(internalPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, internalPostProcessors);

    // Re-register post-processor for detecting inner beans as ApplicationListeners,
    // moving it to the end of the processor chain (for picking up proxies etc).
    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

　　BeanPostProcessor的生效时机

　　　　前面我们已经知道，spring会应用BeanPostProcessor于随后创建的每一个bean实例，具体spring是如何做到的了，我们仔细来看看

　　　　finishBeanFactoryInitialization方法实例化所有剩余的、非延迟初始化的单例（默认情况下spring的bean都是非延迟初始化单例），具体如下

BeanPostProcessor应用场景

　　其实只要我们弄清楚了BeanPostProcessor的执行时机：在bean实例化之后、初始化前后被执行，允许我们对bean实例进行自定义的修改；只要我们明白了这个时机点，我们就能分辨出BeanPostProcessor适用于哪些需求场景，哪些需求场景可以用BeanPostProcessor来实现

　　spring中有很多BeanPostProcessor的实现，我们接触的比较多的自动装配：AutowiredAnnotationBeanPostProcessor也是BeanPostProcessor的实现之一，关于自动装配我会在下篇博文中与大家一起探索

总结

　　spring中bean的生命周期如下图

　　　　引用自：Spring实战系列（三）-BeanPostProcessor的妙用