[Java知识库]Spring为@Configuration注解修改的类生成代理对象原因分析

前言

在Spring中只要被@Configuration注解修饰的类，Spring就会为其生成代理对象，至于这样做的主要原因就是为了解决生成对象的单例问题。

说明

实际上作者在ConfigurationClassEnhancer这个类也有注解说明

场景

如果Spring不做处理，下面输出的一定的是false，但是实际上输出的结果是true，那么只有可能是代理类做了特殊处理。

@Configuration
public class MyConfiguration {
    @Bean
    public TestA a(){
        return new TestA();
    }
    @Bean
    public TestB b(){
        TestA a = a();
        TestA b = a();
        System.out.println(a == b);
        return new TestB();
    }
}

处理分析

生成的代理对象，最终会调用ConfigurationClassEnhancer内部类BeanMethodInterceptor的intercept方法，如果不是当前调用的Bean对象（也就是isCurrentlyInvokedFactoryMethod(beanMethod)返回false），则会调用resolveBeanReference方法。

public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,
			MethodProxy cglibMethodProxy) throws Throwable {
	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);
		}
	}
	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()));
		}
		return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
	}
	return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
}

此方法会通过getBean来获取对象，这样就可以控制对象的生成了。

private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,
		ConfigurableBeanFactory beanFactory, String beanName) {
	// The user (i.e. not the factory) is requesting this bean through a call to
	// the bean method, direct or indirect. The bean may have already been marked
	// as 'in creation' in certain autowiring scenarios; if so, temporarily set
	// the in-creation status to false in order to avoid an exception.
	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;
				}
			}
		}
		// 通过getBean就可以控制对象的生成，对象如果生成过，则可以直接从一级缓存中获取
		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);
		}
	}
}