Spring中@Configuration注解修改的类生成代理原因解析

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);
        }
    }
}
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