Spring 扩展点之 Bean工厂的后置处理器

BeanFactoryPostProcessor 的子类

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  在Spring 中提供了扩展点来供程序员扩展实现定制化的功能。在Spring中，容器初始化的时候，一样也用到了相应的扩展点，来完成容器的初始化。这篇文章中将经过源码来分析，Spring扩展的实现原理，与使用技巧。以及在Spring中对某一些类的特殊处理。好了，闲言少叙，在下先上图为敬！从下图开始晕车之旅。。。

断点调试图一：

private static void invokeBeanFactoryPostProcessors(  Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {   for (BeanFactoryPostProcessor postProcessor : postProcessors) {  /**  * 根据不一样的 BeanFactoryPostProcessor 实现  * 去调用不一样的 postProcessBeanFactory 方法  * ConfigurationClassPostProcessor 同时也是 BeanFactoryPostProcessor 的子类  */  postProcessor.postProcessBeanFactory(beanFactory);  } } 复制代码

  在看具体的postProcessor.postProcessBeanFactory(beanFactory);代码以前，咱们先看一下在Spring中这里对应的BeanFactoryPostProcessor的子类实现，以下图：

  从断点调试图一中能够看出，这里的 postProcessor 对应的子类为 ConfigurationClassPostProcessor，下面的 postProcessBeanFactory() 方法对应子类的实现以下：

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
 int factoryId = System.identityHashCode(beanFactory);  if (this.factoriesPostProcessed.contains(factoryId)) {  throw new IllegalStateException(  "postProcessBeanFactory already called on this post-processor against " + beanFactory);  }  this.factoriesPostProcessed.add(factoryId);  if (!this.registriesPostProcessed.contains(factoryId)) {  // BeanDefinitionRegistryPostProcessor hook apparently not supported...  // Simply call processConfigurationClasses lazily at this point then.  processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);  }  /**  * 配置类 产生 cglib 代理  */  enhanceConfigurationClasses(beanFactory);  beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory)); } 复制代码

  上述方法中，主要的功能就是：①加强配置类；②向容器中添加 BeanPsotProcessor 的子类 ImportAwareBeanPostProcessor。 其中对于配置类的加强，是本文要介绍的，这个就和上一篇文章中说起Spring中对于配置类的 Full和Lite 模式的标记。

加强配置类

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
 Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();  for (String beanName : beanFactory.getBeanDefinitionNames()) {  BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);  /**  * 判断这个类是否是全注解类,这个地方与前面 {@link Configuration} 注解的类的  * 处理有关  */  if (ConfigurationClassUtils.isFullConfigurationClass(beanDef)) {  if (!(beanDef instanceof AbstractBeanDefinition)) {  throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +  beanName + "' since it is not stored in an AbstractBeanDefinition subclass");  }  else if (logger.isInfoEnabled() && beanFactory.containsSingleton(beanName)) {  logger.info("Cannot enhance @Configuration bean definition '" + beanName +  "' since its singleton instance has been created too early. The typical cause " +  "is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +  "return type: Consider declaring such methods as 'static'.");  }  /** 若是是全注解类，就将其 put 到 configBeanDefs 中*/  configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);  }  }  if (configBeanDefs.isEmpty()) {  /**  * Map 为空 表示没有全注解类，则返回  */  // nothing to enhance -> return immediately  return;  }   ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();  for (Map.Entry<String, AbstractBeanDefinition> entry : configBeanDefs.entrySet()) {  AbstractBeanDefinition beanDef = entry.getValue();  // If a @Configuration class gets proxied, always proxy the target class  beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);  try {  // Set enhanced subclass of the user-specified bean class  /**  * 对全注解类 进行 cglib 代理  * config 类 -> cglib class -> BeanDefinition -> bean  */  Class<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);  if (configClass != null) {  Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);  if (configClass != enhancedClass) {  if (logger.isTraceEnabled()) {  logger.trace(String.format("Replacing bean definition '%s' existing class '%s' with " +  "enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));  }  beanDef.setBeanClass(enhancedClass);  }  }  }  catch (Throwable ex) {  throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);  }  } } 复制代码

  接上篇文章咱们提到配置类的 Full和 Lite两种不一样的模式。在这篇文章中，咱们能够看到，Full模式，也就是全配置类，Spring 经过使用CGLIB动态代理的 方式对其进行了加强。而 Lite 模式的配置类，没有经过代理的方式加强。咱们究其缘由发现，对于@Configuration 类的处理，是Spring 的后置处理器的典型应用。纵观整个 Spring， 在器内部只有一个 ConfigurationClassPostProcessor 该类中处理了 BeanDefinitionRegistryPostProcessor 的方法 postProcessBeanDefinitionRegistry() 也处理了 postProcessBeanFactory的方法 postProcessBeanFactory()。经过该类，咱们应该也要知道，对于Spring 的扩展点 BeanFactoryPostProcessor 的处理。

  对于配置类，先经过 resolveBeanClass() 解析获得类的Class对象，而后经过 enhancer.enhance(configClass, this.beanClassLoader ) 获得加强后的类对象。最后再将这个加强后的类对象，设置回对应的 BeanDefinition中去。

CGLIB 代理加强

public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
 /** 判断是否被代理过*/  if (EnhancedConfiguration.class.isAssignableFrom(configClass)) {  if (logger.isDebugEnabled()) {  logger.debug(String.format("Ignoring request to enhance %s as it has " +  "already been enhanced. This usually indicates that more than one " +  "ConfigurationClassPostProcessor has been registered (e.g. via " +  "<context:annotation-config>). This is harmless, but you may " +  "want check your configuration and remove one CCPP if possible",  configClass.getName()));  }  return configClass;  }  /** 没有被代理 cglib 代理*/  Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));  if (logger.isTraceEnabled()) {  logger.trace(String.format("Successfully enhanced %s; enhanced class name is: %s",  configClass.getName(), enhancedClass.getName()));  }  return enhancedClass; } 复制代码

newEnhancer() 方法

/**  * Creates a new CGLIB {@link Enhancer} instance.  * 建立一个 CGLIB 实例  */ private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {  Enhancer enhancer = new Enhancer();  /** 加强父类 */  enhancer.setSuperclass(configSuperClass);  /** 加强接口，  * 便于判断，表示一个类被加强了  * EnhancedConfiguration 实现了 BeanFactoryAware 接口  */  enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});  enhancer.setUseFactory(false);  /**  * BeanFactoryAwareGeneratorStrategy 是一个生成策略  * 主要为生成的 cglib 类中添加成员变量 $beanFactory  * 同时基于接口 EnhancedConfiguration 的父接口 BeanFactoryAware 中的 setBeanFactory 方法，  * 设置此变量的值为当前 context 中的 beanFactory，这样一来 cglib 代理的对象就有了 beanFactory  * 有了 factory 就能得到对象了，不用经过 new 来获取对象了  * 该BeanFactory 的做用是在 this 调用时拦截该调用，并直接在 beanFactory 中得到目标bean  *  */  enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);  enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));  enhancer.setCallbackFilter(CALLBACK_FILTER);  enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());  return enhancer; } 复制代码

createClass()

/**  * 使用加强器生成超类的子类，  * 确保新的子类注册了回调  * Uses enhancer to generate a subclass of superclass,  * ensuring that callbacks are registered for the new subclass.  */ private Class<?> createClass(Enhancer enhancer) {  Class<?> subclass = enhancer.createClass();  // Registering callbacks statically (as opposed to thread-local)  // is critical for usage in an OSGi environment (SPR-5932)...  Enhancer.registerStaticCallbacks(subclass, CALLBACKS);  return subclass; } 复制代码

  其中 CALLBACKS 的定义以下：

private static final Callback[] CALLBACKS = new Callback[] {
 /**  * 加强方法，主要控制bean的做用域，  * 不用每次都 new  */  // Bean 方法来拦截器  new BeanMethodInterceptor(),  // 拦截BeanFactoryAware 定义的方法 setBeanFactory  new BeanFactoryAwareMethodInterceptor(),  NoOp.INSTANCE }; 复制代码

  对应的 new BeanMethodInterceptor() 拦截方法调用，new BeanFactoryAwareMethodInterceptor() 拦截 BeanFactoryAware 定义的方法 setBeanFactory。对应代码实现以下

拦截方法调用

private static class BeanMethodInterceptor implements MethodInterceptor, ConditionalCallback {
  /**  * Enhance a {@link Bean @Bean} method to check the supplied BeanFactory for the  * existence of this bean object.  * @throws Throwable as a catch-all for any exception that may be thrown when invoking the  * super implementation of the proxied method i.e., the actual {@code @Bean} method  */  @Override  @Nullable  public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,  MethodProxy cglibMethodProxy) throws Throwable {  /**  * enhancedConfigInstance 代理  * 经过enhancedConfigInstance 中 CGLIB 生成的成员变量 &&beanFactory 获取 beanFactory  */  ConfigurableBeanFactory beanFactory = getBeanFactory(enhancedConfigInstance);  String beanName = BeanAnnotationHelper.determineBeanNameFor(beanMethod);   // Determine whether this bean is a scoped-proxy  if (BeanAnnotationHelper.isScopedProxy(beanMethod)) {  String scopedBeanName = ScopedProxyCreator.getTargetBeanName(beanName);  if (beanFactory.isCurrentlyInCreation(scopedBeanName)) {  beanName = scopedBeanName;  }  }   // To handle the case of an inter-bean method reference, we must explicitly check the  // container for already cached instances.   // First, check to see if the requested bean is a FactoryBean. If so, create a subclass  // proxy that intercepts calls to getObject() and returns any cached bean instance.  // This ensures that the semantics of calling a FactoryBean from within @Bean methods  // is the same as that of referring to a FactoryBean within XML. See SPR-6602.  if (factoryContainsBean(beanFactory, BeanFactory.FACTORY_BEAN_PREFIX + beanName) &&  factoryContainsBean(beanFactory, beanName)) {  Object factoryBean = beanFactory.getBean(BeanFactory.FACTORY_BEAN_PREFIX + beanName);  if (factoryBean instanceof ScopedProxyFactoryBean) {  // Scoped proxy factory beans are a special case and should not be further proxied  }  else {  // It is a candidate FactoryBean - go ahead with enhancement  return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);  }  }  /**  * 判断执行的方法 和 调用的方法是否是同一个方法  */  if (isCurrentlyInvokedFactoryMethod(beanMethod)) {  // The factory is calling the bean method in order to instantiate and register the bean  // (i.e. via a getBean() call) -> invoke the super implementation of the method to actually  // create the bean instance.  if (logger.isInfoEnabled() &&  BeanFactoryPostProcessor.class.isAssignableFrom(beanMethod.getReturnType())) {  logger.info(String.format("@Bean method %s.%s is non-static and returns an object " +  "assignable to Spring's BeanFactoryPostProcessor interface. This will " +  "result in a failure to process annotations such as @Autowired, " +  "@Resource and @PostConstruct within the method's declaring " +  "@Configuration class. Add the 'static' modifier to this method to avoid " +  "these container lifecycle issues; see @Bean javadoc for complete details.",  beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName()));  }  /**  * 执行的方法和调用的方法是同一个 执行父类的方法 建立对象  */  return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);  }  /**  * 执行的方法和调用的方法不是同一个，在 &&BeanFactory 中 get 一个 bean 出来  */  return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);  }   private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,  ConfigurableBeanFactory beanFactory, String beanName) {   /**  * 判断对象是否正在建立  * 一个对象有三种状态  * a. 没有建立  * b. 正在建立  * c. 建立成功  */  boolean alreadyInCreation = beanFactory.isCurrentlyInCreation(beanName);  try {  if (alreadyInCreation) {  beanFactory.setCurrentlyInCreation(beanName, false);  }  boolean useArgs = !ObjectUtils.isEmpty(beanMethodArgs);  if (useArgs && beanFactory.isSingleton(beanName)) {  // Stubbed null arguments just for reference purposes,  // expecting them to be autowired for regular singleton references?  // A safe assumption since @Bean singleton arguments cannot be optional...  for (Object arg : beanMethodArgs) {  if (arg == null) {  useArgs = false;  break;  }  }  }  /**  * 调用 beanFactory.getBean() 获取对象  */  Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :  beanFactory.getBean(beanName));  if (!ClassUtils.isAssignableValue(beanMethod.getReturnType(), beanInstance)) {  // Detect package-protected NullBean instance through equals(null) check  if (beanInstance.equals(null)) {  if (logger.isDebugEnabled()) {  logger.debug(String.format("@Bean method %s.%s called as bean reference " +  "for type [%s] returned null bean; resolving to null value.",  beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),  beanMethod.getReturnType().getName()));  }  beanInstance = null;  }  else {  String msg = String.format("@Bean method %s.%s called as bean reference " +  "for type [%s] but overridden by non-compatible bean instance of type [%s].",  beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),  beanMethod.getReturnType().getName(), beanInstance.getClass().getName());  try {  BeanDefinition beanDefinition = beanFactory.getMergedBeanDefinition(beanName);  msg += " Overriding bean of same name declared in: " + beanDefinition.getResourceDescription();  }  catch (NoSuchBeanDefinitionException ex) {  // Ignore - simply no detailed message then.  }  throw new IllegalStateException(msg);  }  }  Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();  if (currentlyInvoked != null) {  String outerBeanName = BeanAnnotationHelper.determineBeanNameFor(currentlyInvoked);  beanFactory.registerDependentBean(beanName, outerBeanName);  }  return beanInstance;  }  finally {  if (alreadyInCreation) {  beanFactory.setCurrentlyInCreation(beanName, true);  }  }  } } 复制代码

经过加强之后，配置类中使用@Bean注解的bean定义方法就再也不是普通的方法了，它们具备了以下跟bean做用域有关的能力，以单例bean为例 ：

1. 它们首次被调用时，相应方法逻辑会被执行用于建立bean实例；
2. 再次被调用时，不会再执行建立bean实例，而是根据bean名称返回首次该方法被执行时建立的bean实例。

总结

  至此，在 refresh()方法中的，invokeBeanFactoryPostProcessors() 部分的代码，已经分析完了。在这一步中，完成了对BeanFactoryPostProcessor的处理，其中有一个极为重要的实现 ConfigurationPostProcessor 对应 Spring中配置类的处理。在这步处理完成的时候，咱们定义的Bean，都已经被注册到了 BeanDefinitionMap中。

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