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，同样也用到了相应的扩展点，来完成容器的初始化。这篇文章中将通过源码来分析，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) {
/** * 根据不同的 BeanFactoryPostProcessor 实现 * 去调用不同的 postProcessBeanFactory 方法 * ConfigurationClassPostProcessor 同时也是 BeanFactoryPostProcessor 的子类 */ postProcessor.postProcessBeanFactory(beanFactory); } }
在看具体的postProcessor.postProcessBeanFactory(beanFactory);代码之前，我们先看一下在Spring中这里对应的BeanFactoryPostProcessor的子类实现，如下图： 
从断点调试图一中可以看出，这里的 postProcessor 对应的子类为 ConfigurationClassPostProcessor，下面的 postProcessBeanFactory() 方法对应子类的实现如下：
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)); }
上述方法中，主要的功能就是：①增强配置类；②向容器中添加 BeanPsotProcessor 的子类 ImportAwareBeanPostProcessor。 其中对于配置类的增强，是本文要介绍的，这个就和上一篇文章中提及Spring中对于配置类的 Full和Lite 模式的标记。
增强配置类
public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
for (String beanName : beanFactory.getBeanDefinitionNames()) {
BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
/** * 判断这个类是不是全注解类,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'."); } /** 如果是全注解类，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); } } }
接上篇文章我们提到配置类的 Full和 Lite两种不同的模式。在这篇文章中，我们可以看到，Full模式，也就是全配置类，Spring 通过使用CGLIB动态代理的 方式对其进行了增强。而 Lite 模式的配置类，没有通过代理的方式增强。我们究其原因发现，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()。通过该类，我们应该也要知道，对于Spring 的扩展点 BeanFactoryPostProcessor 的处理。
对于配置类，先通过 resolveBeanClass() 解析得到类的Class对象，然后通过 enhancer.enhance(configClass, this.beanClassLoader ) 得到增强后的类对象。最后再将这个增强后的类对象，设置回对应的 BeanDefinition中去。
CGLIB 代理增强
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; }
newEnhancer() 方法
/**
* Creates a new CGLIB {@link Enhancer} instance.
* 创建一个 CGLIB 实例
*/
private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
Enhancer enhancer = new Enhancer(); /** 增强父类 */ enhancer.setSuperclass(configSuperClass); /** 增强接口， * 便于判断，表示一个类被增强了 * 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 就能获得对象了，不用通过 new 来获取对象了 * 该BeanFactory 的作用是在 this 调用时拦截该调用，并直接在 beanFactory 中获得目标bean * */ enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE); enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader)); enhancer.setCallbackFilter(CALLBACK_FILTER); enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes()); return enhancer; }
createClass()
/**
* 使用增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; }
其中 CALLBACKS 的定义如下：
private static final Callback[] CALLBACKS = new Callback[] {
/**
* 增强方法，主要控制bean的作用域，
* 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 };
对应的 new BeanMethodInterceptor() 拦截方法调用，new BeanFactoryAwareMethodInterceptor() 拦截 BeanFactoryAware 定义的方法 setBeanFactory。对应代码实现如下
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 * 通过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 调用的方法是不是同一个方法 */ 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 创建对象 */ 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) { /** * 判断对象是否正在创建 * 一个对象有三种状态 * a. 没有创建 * b. 正在创建 * c. 创建成功 */ 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); } } } }
通过增强以后，配置类中使用@Bean注解的bean定义方法就不再是普通的方法了，它们具有了如下跟bean作用域有关的能力，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创建bean实例；
2、 再次被调用时，不会再执行创建bean实例，而是根据bean名称返回首次该方法被执行时创建的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，我们定义的Bean，都已经被注册到了 BeanDefinitionMap中。
本文使用 mdnice 排版