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spring中BeanProcessor接口解析

BeanProcessor的理解

BeanProcessor是spring中的一个重要接口，他有两个接口方法一个是 postProcessBeforeInitialization 前置初始化，另一个是 postProcessAfterInitialization 后置初始化。从名称上就可以大概清楚这个接口的作用：在一个业务流程的前后加入两个接口方法，当执行这个业务流程时，就会触发这两个接口方法的执行。简单的总结一下有两个要点：
1. 在业务流程中，根据BeanProcessor接口方法加在不同的位置(一般是前后)，可以实现对业务逻辑的扩展。
2. 在业务逻辑执行前，BeanProcessor的实现类必须已经被创建完成(BeanProcessor接口类必须要优先实例化)。
而在spring中，就有很多实现了BeanProcessor的bean，通过在重要的业务流程(如bean的生命周期流程)的前后加上BeanProcessor接口方法，就可以对业务逻辑进行修改或补充。
一个BeanProcessor的使用实例

在spring的bean生命周期中，BeanProcessor接口方法会在bean创建后的初始化方法(init-method或@PostConstruct指向的方法)前后执行 before 和 after 方法；那有没有在bean创建前后执行的接口方法呢？答案是肯定有的，这个功能是由BeanProcessor的子接口 InstantiationAwareBeanPostProcessor 来实现的，他也是有 before 和 after 方法，会在bean实例化前后执行。

我们先定义一个 BeanProcessor 接口实现类和一个 InstantiationAwareBeanPostProcessor 接口实现类。

BeanPostProcessor 实现类:

//net.postProcessor.CustomerPostProcessor
@Component
public class CustomerPostProcessor implements BeanPostProcessor {

   @PostConstruct
   public void init(){
      System.out.println("执行CustomerPostProcessor的PostConstruct");
   }

   public CustomerPostProcessor(){
      System.out.println("执行CustomerPostProcessor的构造方法");
   }

   @Override
   public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
      System.out.println(bean+"======BeforeInitialization======"+ beanName);
      return bean;
   }

   @Override
   public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
      System.out.println(bean+"======AfterInitialization======"+ beanName);
      return bean;
   }

}
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InstantiationAwareBeanPostProcessor 实现类:

//net.postProcessor.CustomerInitialPostProcessor
@Component
public class CustomerInitialPostProcessor implements InstantiationAwareBeanPostProcessor {

   @PostConstruct
   public void init(){
      System.out.println("执行CustomerInitialPostProcessor的PostConstruct");
   }

   public CustomerInitialPostProcessor(){
      System.out.println("执行CustomerInitialPostProcessor的构造方法");
   }

   @Override
   public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
      System.out.println("bean初始化前执行：class为"+beanClass.getName()+"|beanName为"+beanName);
      return null;
   }

   @Override
   public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
      System.out.println("bean初始化后执行：Object为"+bean+"|beanName为"+beanName);
      return false;
   }
}
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再创建一个普通的bean对象：

//net.postProcessor.FirstBean
@Component
public class FirstBean implements InitializingBean {

   private String msg = "hello";

   @PostConstruct
   public void init(){
      System.out.println("执行FirstBean的PostConstruct");
   }

   public FirstBean(){
      System.out.println("FirstBean构造方法！"+msg);
   }

   public String getMsg() {
      return msg;
   }

   public void setMsg(String msg) {
      this.msg = msg;
   }

   @Override
   public void afterPropertiesSet() throws Exception {
      System.out.println("执行FirstBean的afterPropertiesSet");
   }
}
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我们创建一个spring工厂对象将上述bean加载进去：

@Test
public void test(){
   AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext("net.postProcessor");
}
//执行得到以下结果：
执行CustomerInitialPostProcessor的构造方法
执行CustomerInitialPostProcessor的PostConstruct
执行CustomerPostProcessor的构造方法
执行CustomerPostProcessor的PostConstruct
    
bean初始化前执行：class为net.postProcessor.FirstBean|beanName为firstBean
FirstBean构造方法！hello
bean初始化后执行：Object为net.postProcessor.FirstBean@79179359|beanName为firstBean
    
net.postProcessor.FirstBean@79179359======BeforeInitialization======firstBean
执行FirstBean的PostConstruct
执行FirstBean的afterPropertiesSet
net.postProcessor.FirstBean@79179359======AfterInitialization======firstBean
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通过上述结果证明了我们之前的说法是正确的：

1.BeanPostProcessor接口类会优先实例化，且在实例化中无法不会调用BeanPostProcessor接口方法的

2.InstantiationAwareBeanPostProcessor接口方法会在FirstBean构造方法构造方法前后执行

3.BeanPostProcessor接口方法会在FirstBean实例化后进行初始化的前后执行

注意：若@PostConstruct注解方法方法未执行，请加入 javax.annotation:javax.annotation-api:1.3.2 jar包依赖，原因是@PostConstruct是J2EE标准的注解，不是spring自己的接口，而在JDK8往上的版本中设计者打算弃用这些注解，所以做了处理，我们是没有办法直接使用J2EE标准注解的(@Resource、@PostConstruct、@PreDestroy等几个注解)，为了兼容这种情况，所以有了 javax.annotation-api jar包的产生(或者降低JDK版本)。

2. BeanProcessor的实现思路和简化实例

BeanProcessor大概的实现思路

通过之前的了解BeanProcessor的使用，我们可以知道BeanProcessor并不复杂，但是却十分的重要，下面来分析下BeanProcessor的实现思路：
1. 创建个接口A，接口包含一些切点方法(Before、After、Around之类的)，实现这个接口A的类要在使用前就创建好
2. 我们需要有个业务流程，这个业务流程由若干步组成；将接口A的接口方法插入到这些业务步骤之间(需要扩展的地方)
3. 要执行这个业务流程时，把接口A的实现类对象赋值到业务流程中，在执行业务流程中，就会触发接口方法的执行完成功能扩展
当我们更换赋值到业务流程中的接口A的实现类时，对应的扩展逻辑也会随之变化，这样就实现了可插拔式的扩展逻辑(策略模式)。

一个BeanProcessor的简化逻辑实例

在spring中我们可以创建任意数量的bean实现BeanProcessor接口，所以实际上我们是要一个全局的 beanProcessorList 对象用来存储这些BeanProcessor对象；在执行业务代码时，要循环这个 beanProcessorList 对象，获取你需要的BeanProcessor对象来执行接口方法。下面是一个模拟spring bean生命周期的简化版，来帮助你理解spring中BeanProcessor的工作原理。

net.postProcessor.SecondBean.java

@Component
public class SecondBean {

   private String msg = "world";

   public SecondBean(){
      System.out.println("SecondBean构造方法！"+msg);
   }

   public String getMsg() {
      return msg;
   }

   public void setMsg(String msg) {
      this.msg = msg;
   }
}
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net.postProcessor.CustomerPostProcessor.java

@Component
public class CustomerPostProcessor implements BeanPostProcessor {

   @PostConstruct
   public void init(){
      System.out.println("执行CustomerPostProcessor的PostConstruct");
   }

   public CustomerPostProcessor(){
      System.out.println("执行CustomerPostProcessor的构造方法");
   }

   @Override
   public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
      System.out.println(bean+"======BeforeInitialization======"+ beanName);
      return bean;
   }

   @Override
   public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
      System.out.println(bean+"======AfterInitialization======"+ beanName);
      return bean;
   }

}
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net.postProcessor.PostProcessor.java

public class PostProcessor {

   //模拟扫描到的bean信息<"SecondBean", "net.postProcessor.SecondBean">
   Map<String, String> scanBeanMap = new HashMap<>();

   //模拟spring的beanPostProcessors列表
   List<BeanPostProcessor> processorBeanList = new ArrayList<>();

   //模拟bean对象缓存
   Map<String, Object> beanCache = new HashMap<>();

   //添加扫描的bean信息
   public PostProcessor addBeanInfo(String beanName, String classPath){
      this.scanBeanMap.put(beanName, classPath);
      return this;
   }

   //模拟bean创建流程
   public Object execute(){
      try {
         //先临时存储实现了postProcessor接口的bean对象
         List<BeanPostProcessor> postProcessorStrList = new ArrayList<>();
         //循环scanBeanMap，获取bean列表中实现了postProcessor接口的类，加入processorBeanList中
         for(String temp: scanBeanMap.keySet()){
            Class<?> clazz = Class.forName(scanBeanMap.get(temp));
            //判断是否实现了BeanPostProcessor接口
            if(BeanPostProcessor.class.isAssignableFrom(clazz)){
               //实例化让如临时容器
               postProcessorStrList.add((BeanPostProcessor)createBean(temp));
            }
         }
         //将实现了postProcessor接口的bean加入processorBeanList中
         for(BeanPostProcessor obj: postProcessorStrList){
            processorBeanList.add(obj);
         }

         //再次循环scanBeanMap初始化所用bean
         for(String temp: scanBeanMap.keySet()){
            createBean(temp);
         }

      } catch (ClassNotFoundException e) {
         e.printStackTrace();
      }
      return null;
   }

   //bean实例化
   public Object createBean(String beanName){
      //从缓存中获取
      if(beanCache.containsKey(beanName)){
         return beanCache.get(beanName);
      }else{
         //缓存中取不到，则进行创建后加入缓存
         try {
            Class<?> clazz = Class.forName(scanBeanMap.get(beanName));
            //processor前置方法执行
            for(BeanPostProcessor processor : processorBeanList){
               processor.postProcessBeforeInitialization(clazz, beanName);
            }

            //bean实例化
            Object result = clazz.getConstructor().newInstance();

            //processor后置方法执行
            for(BeanPostProcessor processor : processorBeanList){
               processor.postProcessAfterInitialization(result, beanName);
            }

            //将bean加入缓存
            beanCache.put(beanName, result);
            return result;
         } catch (ClassNotFoundException e) {
            e.printStackTrace();
         } catch (IllegalAccessException e) {
            e.printStackTrace();
         } catch (InstantiationException e) {
            e.printStackTrace();
         } catch (NoSuchMethodException e) {
            e.printStackTrace();
         } catch (InvocationTargetException e){
            e.printStackTrace();
         }
      }
      return null;
   }

}
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代码调用

public static void main(String[] args) {
   PostProcessor postProcessor = new PostProcessor();
   //添加扫描到的bean
   postProcessor
   .addBeanInfo("SecondBean", "net.postProcessor.SecondBean")
   .addBeanInfo("CustomerPostProcessor", "net.postProcessor.CustomerPostProcessor");
   postProcessor.execute();
}

//执行结果
执行CustomerPostProcessor的构造方法
class net.postProcessor.SecondBean======BeforeInitialization======SecondBean
SecondBean构造方法！world
net.postProcessor.SecondBean@1b40d5f0======AfterInitialization======SecondBean
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代码逻辑如下：

循环bean信息列表，将BeanPostProcessor接口bean分离出来优先实例化(实例化中缓存bean对象)，并将之放入临时容器。
循环完成，将临时容器中的BeanPostProcessor接口bean赋值到全局BeanPostProcessor接口列表中
再次循环bean信息列表，缓存存在则直接返回缓存对象，不存在则进行bean实例化，期间循环调用全局BeanPostProcessor接口对象方法

3. spring中BeanProcessor的源码解析

我们要从spring中的refresh()开始看起：

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

      // Tell the subclass to refresh the internal bean factory.
      //告诉子类刷新内部bean工厂。
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // Prepare the bean factory for use in this context.
      //为容器准备bean工程
      prepareBeanFactory(beanFactory);

      try {
         // Allows post-processing of the bean factory in context subclasses.
         //允许在上下文bean的后处理工厂子类。
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         //优先将BeanDefinitionRegistryPostProcessor/BeanFactoryPostProcessor接口的bean对象实例化
         //属于spring内部组件调用
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         //处理用户自定义PostProcessor接口对象，之后加入spring的beanPostProcessors列表，
         // 供之后预实例化其他bean时触发这些PostProcessor方法
         registerBeanPostProcessors(beanFactory);

		//...省略代码
        //实例化所有(non-lazy-init)单件。
		finishBeanFactoryInitialization(beanFactory);
      }

      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.
         //bean销毁
         destroyBeans();

         // Reset 'active' flag.
         //取消刷新
         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...
         //重置公共缓存
         resetCommonCaches();
      }
   }
}
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其中包含有postProcess字段都有可能和BeanProcessor相关，这里有三个相关方法：

postProcessBeanFactory(beanFactory)，这个是一共空的扩展方法，显然无关
invokeBeanFactoryPostProcessors(beanFactory)，处理spring中实现了BeanProcessor接口的内部组件直接调用接口方法
registerBeanPostProcessors(beanFactory)，实例化用户自定义BeanProcessor接口bean组件，之后循环赋值到全局BeanProcessor列表中

所以registerBeanPostProcessors()就是我们要找的对象，来跟进看下registerBeanPostProcessors()：

//AbstractApplicationContext#registerBeanPostProcessors
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    //委托给PostProcessorRegistrationDelegate.registerBeanPostProcessors进行处理
	PostProcessorRegistrationDelegate.registerBeanPostProcessors进行处理(beanFactory, this);
}
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继续跟进PostProcessorRegistrationDelegate.registerBeanPostProcessors()：

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

    //查询实现了BeanPostProcessor接口的beanName
   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.
   List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
   List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
   List<String> orderedPostProcessorNames = new ArrayList<>();
   List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    //根据beanName循环调用getBean进行实例化
   for (String ppName : postProcessorNames) {
      if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
         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.
    //对BeanPostProcessor接口对象进行排序
   sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
   //将获取到的PostProcessors接口对象加入到spring的beanPostProcessors列表
   registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

   // Next, register the BeanPostProcessors that implement Ordered.
   List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
   for (String ppName : orderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      orderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
         internalPostProcessors.add(pp);
      }
   }
   sortPostProcessors(orderedPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, orderedPostProcessors);

   // Now, register all regular BeanPostProcessors.
   List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
   for (String ppName : nonOrderedPostProcessorNames) {
      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.
   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));
}
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果然这里就是处理BeanPostProcessor接口的地方，逻辑和之前的思路类似：

循环扫描到的bean列表，获取实现了BeanPostProcessor接口的beanName数组
循环beanName数组数组，调用beanFactory.getBean()将bean实例化，并放入priorityOrderedPostProcessors列表中
调用sortPostProcessors对priorityOrderedPostProcessors列表进行排序(处理BeanPostProcessor调用的顺序)
调用registerBeanPostProcessors将priorityOrderedPostProcessors列表中的bean对象赋值到全局列表beanPostProcessors中
回到refresh()中，当调用finishBeanFactoryInitialization()对所用bean进行预实例化时就会调用这些BeanPostProcessor接口方法

原文 https://juejin.im/post/5eee2614e51d4573d47d3463

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