@Bean
public Object bean() {
    return new Object();
}

@Configuration
public class BeanMethodConfig {
    @Bean
    public Object bean() {
        System.err.println("bean...");
        return new Object();
    }
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(BeanMethodConfig.class);
        BeanMethodConfig config = context.getBean(BeanMethodConfig.class);
        System.err.println("-----------");
        config.bean();
        config.bean();
        config.bean();
    }
}

@Override
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)) {
        processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
    }
    /**
     * FullConfigurationClass 才會生成代理類
     * 避免@Bean方法被反復調用，生成多個實例，破壞了singleton語義
     * @see ConfigurationClassEnhancer#enhance(Class, ClassLoader)
     */
    enhanceConfigurationClasses(beanFactory);
    beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}

/**
 * 過濾出所有的FullConfigurationClass 加了@Configuration注解的類
 */
Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
for (String beanName : beanFactory.getBeanDefinitionNames()) {
    BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
    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'.");
        }
        configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
    }
}
if (configBeanDefs.isEmpty()) {
    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
        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);
    }
}

public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
    /**
     * 生成的CGLIB代理類會實現EnhancedConfiguration接口，
     * 如果已經實現了EnhancedConfiguration接口，則直接返回
     */
    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;
    }
    // 生成代理類
    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;
}

private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(configSuperClass);
    enhancer.setInterfaces(new Class<?>[]{EnhancedConfiguration.class});
    enhancer.setUseFactory(false);
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
    enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));
    enhancer.setCallbackFilter(CALLBACK_FILTER);
    enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());
    return enhancer;
}

private static final Callback[] CALLBACKS = new Callback[]{
        new BeanMethodInterceptor(),
        new BeanFactoryAwareMethodInterceptor(),
        NoOp.INSTANCE
};
private static final ConditionalCallbackFilter CALLBACK_FILTER = new ConditionalCallbackFilter(CALLBACKS);

public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
    /**
     * 生成的CGLIB代理類會有一個名為$$beanFactory的屬性，類型是BeanFactory
     * setBeanFactory()的邏輯就是給$$beanFactory的屬性賦值
     */
    Field field = ReflectionUtils.findField(obj.getClass(), BEAN_FACTORY_FIELD);
    Assert.state(field != null, "Unable to find generated BeanFactory field");
    field.set(obj, args[0]);
    if (BeanFactoryAware.class.isAssignableFrom(ClassUtils.getUserClass(obj.getClass().getSuperclass()))) {
        return proxy.invokeSuper(obj, args);
    }
    return null;
}

public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,
                        MethodProxy cglibMethodProxy) throws Throwable {
    /**
     * 獲取BeanFactory
     * 生成的子類實現了BeanFactoryAware接口，會把BeanFactory賦值給屬性 $$beanFactory
     */
    ConfigurableBeanFactory beanFactory = getBeanFactory(enhancedConfigInstance);
    // @Bean方法名 決定BeanName
    String beanName = BeanAnnotationHelper.determineBeanNameFor(beanMethod);
    if (BeanAnnotationHelper.isScopedProxy(beanMethod)) {
        String scopedBeanName = ScopedProxyCreator.getTargetBeanName(beanName);
        if (beanFactory.isCurrentlyInCreation(scopedBeanName)) {
            beanName = scopedBeanName;
        }
    }
    /**
     * 如果ConfigurationClass是FactoryBean實現類，需要創(chuàng)建代理類來增強getObject()方法返回緩存的bean實例
     */
    if (factoryContainsBean(beanFactory, BeanFactory.FACTORY_BEAN_PREFIX + beanName) &&
            factoryContainsBean(beanFactory, beanName)) {
        Object factoryBean = beanFactory.getBean(BeanFactory.FACTORY_BEAN_PREFIX + beanName);
        if (factoryBean instanceof ScopedProxyFactoryBean) {
        } else {
            return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);
        }
    }
    /**
     * 判斷是否要調用父類方法，生成bean
     * 以singleton為例：首次getBean時，容器不存在，需要創(chuàng)建bean
     * 1.實例化bean時，會把FactoryMethod寫入ThreadLocal
     * @see SimpleInstantiationStrategy#instantiate(org.springframework.beans.factory.support.RootBeanDefinition, java.lang.String, org.springframework.beans.factory.BeanFactory, java.lang.Object, java.lang.reflect.Method, java.lang.Object...)
     * 2.代理對象判斷method已經被調用，則直接調用父類方法生成bean
     * 3.實例化完，會清空ThreadLocal
     * 4.再次調用，將直接進resolveBeanReference()從容器中獲取緩存bean
     *
     * Spring調用了createBean()，就意味著需要調用父類方法生成bean，Spring本身保證單例語義
     * 用戶觸發(fā)的@Bean方法，需要從BeanFactory#getBean()獲取，當容器內不存在bean時，Spring自然會調用createBean()，
     * 會再次進入到這里
     */
    if (isCurrentlyInvokedFactoryMethod(beanMethod)) {
        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()));
        }
        // 調用父類方法生成bean，對于單例bean，只會觸發(fā)一次
        return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
    }
    // 從容器加載bean
    return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
}

/**
 * FactoryMethod當前是否已調用？
 */
private boolean isCurrentlyInvokedFactoryMethod(Method method) {
    /**
     * Spring createBean()會將FactoryMethod寫入到ThreadLocal
     * 再進這個方法就是true了，也就是回去調用父類方法生成bean
     */
    Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();
    return (currentlyInvoked != null && method.getName().equals(currentlyInvoked.getName()) &&
            Arrays.equals(method.getParameterTypes(), currentlyInvoked.getParameterTypes()));
}

Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
try{
//先將factoryMethod寫入ThreadLocal
currentlyInvokedFactoryMethod.set(factoryMethod);
//再反射調用目標方法-代理方法
Object result = factoryMethod.invoke(factoryBean,args);
if (result == null){
result = new NullBean();
}
return result;
}finally{
if (priorInvokedFactoryMethod != null) {
currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
}else{
currentlyInvokedFactoryMethod.remove();
}
}