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Spring中@Configuration注解修改的类生成代理原因解析

2024-04-02 19:55

关注

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

在这里插入图片描述

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);
		}
	}
}

总结

到此这篇关于Spring中@Configuration注解修改的类生成代理原因解析的文章就介绍到这了,更多相关Spring @Configuration注解内容请搜索编程网以前的文章或继续浏览下面的相关文章希望大家以后多多支持编程网!

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