Spring5源码解析7-ConfigurationClassPostProcessor (下)

ConfigurationClassPostProcessor继承了BeanDefinitionRegistryPostProcessor接口，它实现了postProcessBeanDefinitionRegistry和其父类的BeanFactoryPostProcessor#postProcessBeanFactory方法。

关于postProcessBeanDefinitionRegistry方法的解析能够参看：Spring5 源码学习 (5) ConfigurationClassPostProcessor (上)。

如今咱们来看一下ConfigurationClassPostProcessor#postProcessBeanFactory方法的源码。

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ConfigurationClassPostProcessor#postProcessBeanFactory

调用时机

ConfigurationClassPostProcessor#postProcessBeanFactory方法也在refresh();方法中执行invokeBeanFactoryPostProcessors(beanFactory);方法时被调用的。

源码解析

//ConfigurationClassPostProcessor#postProcessBeanFactory源码
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);
	// 在 this.postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) 方法中
	// 调用this.registriesPostProcessed.add(registryId);
	// if条件不成立
	if (!this.registriesPostProcessed.contains(factoryId)) {
		// BeanDefinitionRegistryPostProcessor hook apparently not supported...
		// Simply call processConfigurationClasses lazily at this point then.
		processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
	}

	// 对配置类进行加强
	enhanceConfigurationClasses(beanFactory);
	// 建立 ImportAwareBeanPostProcessor ,来支持 ImportAware ,调用ImportAware.setImportMetadata方法
	beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}
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主要作了两件事：

1. 对配置类进行加强
2. 建立ImportAwareBeanPostProcessor 来支持ImportAware接口。

主要来看一下对配置类进行加强方法enhanceConfigurationClasses(beanFactory);的源码。

enhanceConfigurationClasses(beanFactory)加强 Full Configuration

Spring会对Full Configuration 进行代理，拦截@Bean方法，以确保正确处理@Bean语义。这个加强的代理类就是在enhanceConfigurationClasses(beanFactory)方法中产生的，源码以下：

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
	Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
	//获取全部的BeanDefinitionName,以前已经完成了bean的扫描,这里会获取到全部的beanName
	for (String beanName : beanFactory.getBeanDefinitionNames()) {
		BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
		// 校验是否为FullConfigurationClass,也就是是否被标记了 @Configuration
		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FullConfigurationClass,则放到变量configBeanDefs中
			configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
		}
	}
	if (configBeanDefs.isEmpty()) {
		// 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
			Class<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);
			if (configClass != null) {
				// 对 FullConfigurationClass 进行加强
				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()));
					}
					//将BeanClass设置为加强后的类
					beanDef.setBeanClass(enhancedClass);
				}
			}
		} catch (Throwable ex) {
			throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
		}
	}
}
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获取全部为FullConfigurationClass的BeanDefinition（即标注@Configuration的配置类），而后依次调用enhancer.enhance(configClass, this.beanClassLoader);方法，对配置类进行加强，将方法返回Class<?> enhancedClass的设置到BeanDefinition中（eanDef.setBeanClass(enhancedClass);），以后Spring建立该BeanDefinition时就会使用这个加强类来建立。

下面是enhancer.enhance(configClass, this.beanClassLoader);方法源码：

public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
	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;
	}
	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;
}
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建立Full Configuration加强类

具体来看一下newEnhancer(configClass, classLoader方法，这个方法负责建立Full Configuration加强类。

private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
	// Spring从新打包了CGLIB（使用Spring专用补丁;仅供内部使用）
	// 这样可避免在应用程序级别或第三方库和框架上与CGLIB的依赖性发生任何潜在冲突
	// https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/cglib/package-summary.html
	Enhancer enhancer = new Enhancer();
	enhancer.setSuperclass(configSuperClass);
	// 设置须要实现的接口,也就是说,咱们的配置类的cglib代理还实现的 EnhancedConfiguration 接口
	enhancer.setInterfaces(new Class<?>[]{EnhancedConfiguration.class});
	enhancer.setUseFactory(false);
	// 设置命名策略
	enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
	// 设置生成器建立字节码策略
	// BeanFactoryAwareGeneratorStrategy 是 CGLIB的DefaultGeneratorStrategy的自定义扩展，主要为了引入BeanFactory字段
	enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));
	// 设置加强
	enhancer.setCallbackFilter(CALLBACK_FILTER);
	enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());
	return enhancer;
}
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这里的Enhancer对象是org.springframework.cglib.proxy.Enhancer，那它和cglib是什么关系呢？

Spring's repackaging of CGLIB 3.2 (with Spring-specific patches; for internal use only).This repackaging technique avoids any potential conflicts with dependencies on CGLIB at the application level or from third-party libraries and frameworks.

引用自：docs.spring.io/spring/docs…

大体就是说，Spring从新打包了CGLIB（使用Spring专用补丁，仅供内部使用） ，这样可避免在应用程序级别或第三方库和框架上与CGLIB的依赖性发生任何潜在冲突。

那具体作了哪些加强呢？

1. 实现EnhancedConfiguration接口。这是一个空的标志接口，仅由Spring框架内部使用，而且由全部@ConfigurationCGLIB子类实现，该接口继承了BeanFactoryAware接口。
2. 设置了命名策略
3. 设置生成器建立字节码的策略。BeanFactoryAwareGeneratorStrategy继承了cglib的DefaultGeneratorStrategy，其主要做用是为了让子类引入BeanFactory字段和设置ClassLoader。
4. 设置加强Callback：

private static final Callback[] CALLBACKS = new Callback[]{
		// 拦截 @Bean 方法的调用,以确保正确处理@Bean语义
		new BeanMethodInterceptor(),
		// 拦截 BeanFactoryAware#setBeanFactory 的调用
		new BeanFactoryAwareMethodInterceptor(),
		NoOp.INSTANCE
};
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• BeanMethodInterceptor：负责拦截@Bean方法的调用，以确保正确处理@Bean语义。
• BeanFactoryAwareMethodInterceptor：负责拦截 BeanFactoryAware#setBeanFactory方法的调用，由于加强的配置类实现了EnhancedConfiguration接口（也就是实现了BeanFactoryAwar接口）。

设置加强Callback

下面，咱们就以AppConfig为例，来学习加强Callback相关源码。

@Configuration
@ComponentScan
public class AppConfig {

	@Bean
	public String name() throws Exception {
		getUserBean().getObject();
		return "程序员小黑";
	}


	@Bean
	public FactoryBean getUserBean() {
		return new FactoryBean<UserBean>() {
			@Override
			public UserBean getObject() throws Exception {
				System.out.println("1111");
				return new UserBean("shen", 17);
			}

			@Override
			public Class<?> getObjectType() {
				return UserBean.class;
			}
		};
	}
}
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BeanMethodInterceptor

主要做用是：拦截@Bean方法的调用，以确保正确处理@Bean语义。当调用@Bean方法时，就会被如下代码所拦截：

//BeanMethodInterceptor#intercept源码
public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs, MethodProxy cglibMethodProxy) throws Throwable {

	// enhancedConfigInstance 已是配置类的加强对象了,在加强对象中,有beanFactory字段的
	// 获取加强对象中的beanFactory
	ConfigurableBeanFactory beanFactory = getBeanFactory(enhancedConfigInstance);
	// 获取beanName
	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.

	// 检查容器中是否存在对应的 FactoryBean 若是存在,则建立一个加强类
	// 经过建立加强类来代理拦截 getObject（）的调用 , 以确保了FactoryBean的语义
	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
			// 建立加强类,来代理 getObject（）的调用
			// 有两种可选代理方式,cglib 和 jdk
			// Proxy.newProxyInstance(
			// factoryBean.getClass().getClassLoader(), new Class<?>[]{interfaceType},
			// (proxy, method, args) -> {
			// if (method.getName().equals("getObject") && args == null) {
			// return beanFactory.getBean(beanName);
			// }
			// return ReflectionUtils.invokeMethod(method, factoryBean, args);
			// });
			return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);
		}
	}

	// 判断当时执行的方法是否为@Bean方法自己
	// 举个例子 : 若是是直接调用@Bean方法,也就是Spring来调用咱们的@Bean方法,则返回true
	// 若是是在别的方法内部,咱们本身的程序调用 @Bean方法,则返回false
	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()));
		}
		// 若是返回true,也就是Spring在调用这个方法,那么就去真正执行该方法
		return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
	}

	//不然,则尝试从容器中获取该 Bean 对象
	// 怎么获取呢? 经过调用 beanFactory.getBean 方法
	// 而这个getBean 方法,若是对象已经建立则直接返回,若是尚未建立,则建立,而后放入容器中,而后返回
	return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
}
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1. enhancedConfigInstance是配置类的加强对象。从加强对象中获取beanFactory和beanName。举个例子：当Spring调用name()方法时，beanName就是name。
2. 检查容器中是否存在对应的FactoryBean，若是存在，则建立一个加强类，来代理getObject()的调用。在本示例中，若是读者将name()方法注释删掉以后程序并不会执行到这一步。由于Spring调用getUserBean()方法时，容器中并无存在对应的FactoryBean。由于只有第二次调用getUserBean()方法容器中才会存在对应的FactoryBean。
3. 判断当时执行的方法是否为@Bean方法自己，若是是，则直接调用该方法，不作加强拦截；不然，则尝试从容器中获取该Bean对象。

BeanFactoryAwareMethodInterceptor

BeanFactoryAwareMethodInterceptor方法就比较简单，其做用为拦截 BeanFactoryAware#setBeanFactory的调用，用于获取BeanFactory对象。

// BeanFactoryAwareMethodInterceptor#intercept 源码
public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
	Field field = ReflectionUtils.findField(obj.getClass(), BEAN_FACTORY_FIELD);
	Assert.state(field != null, "Unable to find generated BeanFactory field");
	field.set(obj, args[0]);

	// Does the actual (non-CGLIB) superclass implement BeanFactoryAware?
	// If so, call its setBeanFactory() method. If not, just exit.
	if (BeanFactoryAware.class.isAssignableFrom(ClassUtils.getUserClass(obj.getClass().getSuperclass()))) {
		return proxy.invokeSuper(obj, args);
	}
	return null;
}
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输出加强类class文件

最后，再补充说明一点，咱们能够经过以下配置来获取Spring为咱们生成的CGLIB代理加强类的class文件：

System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "spring-study/docs/classes");
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源码注释GITHUB地址：github.com/shenjianeng…

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