前言

　　開心一刻

　　　　小明：“媽，我被公司開除了”，媽：“啊，為什麼呀？”， 小明：“我罵董事長是笨蛋，公司召開高層會議還要起訴我”，媽：“告你誹謗是吧？”，小明：“不是，他們說要告我洩露公司機密”

BeanPostProcessor定義

　　不管三七二十一，我們先來看看它的定義，看看spring是如何描述BeanPostProcessor的

/*
 * Copyright 2002-2016 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
 *
 *      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,
 * 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;
import org.springframework.lang.Nullable;

/**
 * 允許對新的bean示例進行自定義的修改，例如檢查標誌介面或進行代理封裝
 *
 * spring上下文會在它的beng定義中自動檢測BeanPostProcessor例項，並將它們應用於隨後建立的每一個bean例項
 *
 * implement {@link #postProcessAfterInitialization}.
 * 通常，通過實現BeanPostProcessor的postProcessBeforeInitialization方法（配合標記介面，如@Autowired）來填充bean例項，
 * 通過BeanPostProcessor的postProcessAfterInitialization方法進行bean例項的代理
 *
 */
public interface BeanPostProcessor {

    /**
     * 在bean例項的初始化方法（例如InitializingBean的afterPropertiesSet或自定義的init-method）回撥之前，
     * spring會應用此方法到bean例項上。一般用於bean例項的屬性值的填充
     * 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例項的初始化方法（例如InitializingBean的afterPropertiesSet或自定義的init-method）回撥之後，
     * spring會應用此方法到bean例項上。
     * 在有FactoryBean時，此方法會在FactoryBean例項與FactoryBean的目標物件建立時各呼叫一次
     * 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;
    }

}

　　簡單點來理解，就是spring會自動從它的所有的bean定義中檢測BeanPostProcessor型別的bean定義，然後例項化它們，再將它們應用於隨後建立的每一個bean例項，在bean例項的初始化方法回撥之前呼叫BeanPostProcessor的postProcessBeforeInitialization的方法（進行bean例項屬性的填充），在bean例項的初始化方法回撥之後呼叫BeanPostProcessor的postProcessAfterInitialization的方法（可以進行bean例項的代理封裝）

應用示例

　　我們先來看個簡單的示例，注意：由於spring只是從spring容器中的bean定義中自動檢測BeanPostProcessor型別的bean定義，所以我們自定義的BeanPostProcessor要通過某種方式註冊到spring容器

　　MyBeanPostProcessor

@Component
public class MyBeanPostProcessor implements BeanPostProcessor {

    public MyBeanPostProcessor () {
        System.out.println("MyBeanPostProcessor 例項化......");
    }

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

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("spring中bean例項:" + beanName + " 初始化之後處理......");
        return bean;
    }
}

　　AnimalConfig

@Configuration
public class AnimalConfig {

    public AnimalConfig() {
        System.out.println("AnimalConfig 例項化");
    }

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

}

　　Dog

public class Dog {

    private String name;

    public Dog() {
        System.out.println("Dog 例項化......");
    }

    public String getName() {
        return name;
    }

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

　　完整例項工程：spring-boot-BeanPostProcessor 我們來看看啟動結果

　　有人可能會說了：“你是個逗比把，你舉的這個例子有什麼用？ 實際上，根本就不會出現BeanPostProcessor的這樣用法！”  有這樣的疑問非常正常，示例中的BeanPostProcessor的兩個方法：postProcessBeforeInitialization、postProcessAfterInitialization沒做任何的處理，都只是直接返回bean，這不就是：脫了褲子放屁？

　　我們細看下，會發現postProcessBeforeInitialization、postProcessAfterInitialization中各多了一行列印（），其實示例只是驗證下Spring對BeanPostProcessor的支援、BeanPostProcessor的兩個方法的執行時機，是否如BeanPostProcessor 的註釋所說的那樣，實際應用中肯定不會這麼用的。那問題來了：BeanPostProcessor能用來幹什麼？ 回答這個問題之前，我們先來看看spring對BeanPostProcessor的底層支援

原始碼解析

　　BeanPostProcessor的例項化與註冊　　

　　　　很明顯，我們從spring的啟動過程的refresh方法開始，如下圖

　　　　此時spring容器中所有的BeanPostProcessor都進行了例項化，並註冊到了beanFactory的beanPostProcessors屬性中

　　　　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.
    // 將所有BeanPostProcessor bean定義分三類：實現了PriorityOrdered、實現了Ordered，以及剩下的常規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)) {
            // 例項化實現了PriorityOrdered介面的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.
    // 註冊實現了PriorityOrdered介面的BeanPostProcessor到beanFactory的beanPostProcessors屬性中
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
    for (String ppName : orderedPostProcessorNames) {
        // 例項化實現了Ordered介面的BeanPostProcessor
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    // 註冊實現了Ordered介面的BeanPostProcessor到beanFactory的beanPostProcessors屬性中
    sortPostProcessors(orderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
    for (String ppName : nonOrderedPostProcessorNames) {
        // 例項化剩下的所有的常規的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.
    // 註冊所有常規的的BeanPostProcessor到beanFactory的beanPostProcessors屬性中
    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的生效時機

　　　　前面我們已經知道，spring會應用BeanPostProcessor於隨後建立的每一個bean例項，具體spring是如何做到的了，我們仔細來看看

　　　　finishBeanFactoryInitialization方法例項化所有剩餘的、非延遲初始化的單例（預設情況下spring的bean都是非延遲初始化單例），具體如下

BeanPostProcessor應用場景

　　其實只要我們弄清楚了BeanPostProcessor的執行時機：在bean例項化之後、初始化前後被執行，允許我們對bean例項進行自定義的修改；只要我們明白了這個時機點，我們就能分辨出BeanPostProcessor適用於哪些需求場景，哪些需求場景可以用BeanPostProcessor來實現

　　spring中有很多BeanPostProcessor的實現，我們接觸的比較多的自動裝配：AutowiredAnnotationBeanPostProcessor也是BeanPostProcessor的實現之一，關於自動裝配我會在下篇博文中與大家一起探索

總結

　　spring中bean的生命週期如下圖

　　　　引用自：Spring實戰系列（三）-BeanPostProcessor的妙用