Spring IoC源码解析——Bean的创建和初始化

Posted 2020-08-16 胆怯的小伙子

Spring介绍

Spring(http://spring.io/)是一个轻量级的Java 开发框架，同时也是轻量级的IoC和AOP的容器框架，主要是针对JavaBean的生命周期进行管理的轻量级容器，可以单独使用，也可以和Struts框架，MyBatis框架等组合使用。

 

IoC介绍

IoC是什么

Ioc—Inversion of Control，即“控制反转”，不是什么技术，而是一种设计思想。在Java开发中，Ioc意味着将你设计好的对象交给容器控制，而不是传统的在你的对象内部直接控制。如何理解好Ioc呢？理解好Ioc的关键是要明确“谁控制谁，控制什么，为何是反转（有反转就应该有正转了），哪些方面反转了”，那我们来深入分析一下：

　　●谁控制谁，控制什么：传统Java SE程序设计，我们直接在对象内部通过new进行创建对象，是程序主动去创建依赖对象；而IoC是有专门一个容器来创建这些对象，即由Ioc容器来控制对 象的创建；谁控制谁？当然是IoC 容器控制了对象；控制什么？那就是主要控制了外部资源获取（不只是对象包括比如文件等）。

　　●为何是反转，哪些方面反转了：有反转就有正转，传统应用程序是由我们自己在对象中主动控制去直接获取依赖对象，也就是正转；而反转则是由容器来帮忙创建及注入依赖对象；为何是反转？因为由容器帮我们查找及注入依赖对象，对象只是被动的接受依赖对象，所以是反转；哪些方面反转了？依赖对象的获取被反转了。

 

IoC能做什么

IoC 不是一种技术，只是一种思想，一个重要的面向对象编程的法则，它能指导我们如何设计出松耦合、更优良的程序。传统应用程序都是由我们在类内部主动创建依赖对象，从而导致类与类之间高耦合，难于测试；有了IoC容器后，把创建和查找依赖对象的控制权交给了容器，由容器进行注入组合对象，所以对象与对象之间是 松散耦合，这样也方便测试，利于功能复用，更重要的是使得程序的整个体系结构变得非常灵活。

其实IoC对编程带来的最大改变不是从代码上，而是从思想上，发生了“主从换位”的变化。应用程序原本是老大，要获取什么资源都是主动出击，但是在IoC/DI思想中，应用程序就变成被动的了，被动的等待IoC容器来创建并注入它所需要的资源了。

IoC很好的体现了面向对象设计法则之一—— 好莱坞法则：“别找我们，我们找你”；即由IoC容器帮对象找相应的依赖对象并注入，而不是由对象主动去找。

那么，IoC容器到底是如何从初始化完成的BeanFactory中对Bean进行创建并初始化的呢？接下来我们就一探究竟。

 

源码解析

准备工作

首先写一个Spring的配置文件spring.xml，为了方便测试，这里面就只有一个名为test的bean。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="test" class="org.study.spring.ioc.Test"></bean>
</beans>

 

编写程序入口代码，可以直接打断点进行调试。

ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
Test bean = context.getBean("test", Test.class);

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开始解析

开始源码解析，紧接着上一节，首先进入AbstractApplicationContext.java的refresh方法，这一节我们重点来看里面的invokeBeanFactoryPostProcessors方法。

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // 在这种情况下刷新
            prepareRefresh();

            // 告诉子类刷新内部bean工厂
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // 在这种情况下,bean工厂准备使用的.
            prepareBeanFactory(beanFactory);

            try {
                // 允许在上下文bean的后处理工厂子类。
                postProcessBeanFactory(beanFactory);

                //在上下文中调用factory工厂的时候注册bean的 实例对象
                invokeBeanFactoryPostProcessors(beanFactory);

                // 注册bean的过程当中拦截所以bean的创建
                registerBeanPostProcessors(beanFactory);

                // 初始化上下文消息资源
                initMessageSource();

                //初始化事物传播属性
                initApplicationEventMulticaster();

                // 在特定上下文初始化其他特殊bean子类。
                onRefresh();

                // 检查侦听器bean并注册。
                registerListeners();

                // 实例化所有剩余(non-lazy-init)单例.
                finishBeanFactoryInitialization(beanFactory);

                // 最后一步:发布对应的事件。
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // 销毁已经创建的单例对象避免浪费资源
                destroyBeans();

                // 重置“活跃”的旗帜。
                cancelRefresh(ex);

                // 异常传播到调用者。
                throw ex;
            }

            finally {
                // 在spring 核心包里重置了内存，因为我们肯不需要元数据单例bean对象了
                resetCommonCaches();
            }
        }
    }

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进入invokeBeanFactoryPostProcessors方法

/**
     * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
     * respecting explicit order if given.
     * <p>Must be called before singleton instantiation.
     */
    protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

        // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
        // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
        if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }
    }

 

打开PostProcessorRegistrationDelegate类中的invokeBeanFactoryPostProcessors方法，可以看到，这个方法里有很多内容，这里我们只分析最关键的部分。从本质上来说，该方法就是去执行BeanFactoryPostProcessor这个接口中的方法去的，上面代码注释也清楚的写到如果想先执行BeanFactoryPostProcessor这个接口的方法，必须先去实例化实现这个接口的Bean,也就是getBean这个方法。

public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

        // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        Set<String> processedBeans = new HashSet<>();

        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<>();
            List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
                    new LinkedList<>();

            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryPostProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryPostProcessors.add(registryPostProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

            // 不初始化factoryBeans：我们需要把所以没有初始化的bean让bean工厂处理他们，单例BeanDefinitionRegistryPostProcessors之间实现PriorityOrdered接口、序列化接口

            String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);

            //首先，调用 BeanDefinitionRegistryPostProcessors 并且实现 PriorityOrdered接口
            List<BeanDefinitionRegistryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
            registryPostProcessors.addAll(priorityOrderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);

            //然后, 调用 BeanDefinitionRegistryPostProcessors 并且实现序列化接口 
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            List<BeanDefinitionRegistryPostProcessor> orderedPostProcessors = new ArrayList<>();
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                    orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, orderedPostProcessors);
            registryPostProcessors.addAll(orderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);

            // 最后，调用其他BeanDefinitionRegistryPostProcessors,直到没有进一步的出现。
            boolean reiterate = true;
            while (reiterate) {
                reiterate = false;
                postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
                for (String ppName : postProcessorNames) {
                    if (!processedBeans.contains(ppName)) {
                        BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
                        registryPostProcessors.add(pp);
                        processedBeans.add(ppName);
                        pp.postProcessBeanDefinitionRegistry(registry);
                        reiterate = true;
                    }
                }
            }

            // 现在，调用的postProcessBeanFactory回调处理器处理
            invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
            invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
        }

        else {
            // 调用该工厂的时候 注册文本的实例对象
            invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
        }

        //不在这里初始化FactoryBeans,我们需要把所有
未初始化的bean让工厂后面处理他们
        String[] postProcessorNames =
                beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

        // 单独beanfactorypostprocessor之间实现PriorityOrdered 接口,下令,休息。
        List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        List<String> orderedPostProcessorNames = new ArrayList<>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        for (String ppName : postProcessorNames) {
            if (processedBeans.contains(ppName)) {
                // 跳过已经处理完的第一阶段
            }
            else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        // 首先, 调用这个 BeanFactoryPostProcessors 并且实现PriorityOrdered 接口
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

        // 然后,调用 BeanFactoryPostProcessors 并且实现 序列化 接口
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

        // 最后, 调用其他所有的 BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

        // 清除缓存合并bean定义自后处理器可能会有修改原来的元数据,例如:替换占位符值. ..
        beanFactory.clearMetadataCache();
    }

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接下来进入AbstractBeanFactory.java类中的doGetBean方法，这个方法的具体实现可以分为三个部分：

第一部分，首先先去singleton缓存中去找实例。由于我们例子中没有把我们的bean手动放入singletonObjects这个Map里面去，所以这里肯定没找到。

第二部分，然后是去获取该BeanFactory父Factory，希望从这些Factory中获取，如果该Beanfactory有父类，则希望用父类去实例化该bean，类似于JVM类加载的双亲委派机制。由于我们例子中的的Beanfactory为null，所以暂不讨论这种情况。

第三部分，这一部分是我们关注的重点，这里我们将这一大部分再分为三个小的部分来进行分析：

1. 先将目前的bean标记为的正在创建
2. 再获取根据beanName得到对应bean在beanfactory中的beanDefinitionMap的BeanDefinition（上一节初始化beanFactory时存入的），然后去获取这个bean依赖的bean。如果依赖的bean还没有创建，则先创建依赖的bean，进行递归调用（这就是依赖注入Dependence Injection）。如果找不到依赖，则忽略。
3. 最后如果是单例（Spring默认是单例），则调用createBean()这个方法进行Bean的创建。

/**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {

        final String beanName = transformedBeanName(name);
        Object bean;

        // 急切地检查手动注册单例单缓存
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isDebugEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.debug("Returning eagerly cached instance of singleton bean \'" + beanName +
                            "\' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.debug("Returning cached instance of singleton bean \'" + beanName + "\'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 如果我们创建bean 实例对象失败了，说明我们在循环引用该实例对象
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 在factory这个工厂里检查bean 对象是否存在
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 当没有发现时，应该检查父类对象
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // 给父类对象提供明确 的参数
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    //没有参数，代表标准的获取.getbean（）方法
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 确保初始化的bean 是当前的这个bean对象
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dependsOnBean : dependsOn) {
                        if (isDependent(beanName, dependsOnBean)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between \'" + beanName + "\' and \'" + dependsOnBean + "\'");
                        }
                        registerDependentBean(dependsOnBean, beanName);
                        getBean(dependsOnBean);
                    }
                }

                // 创建一个 bean 的实例对象
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            catch (BeansException ex) {
                              //从单例明确地删除实例的缓存:这可能是热切的创建过程,允许循环引用的决议。还删除任何bean,收到一个临时bean的引用。
                                destroySingleton(beanName);
                                throw ex;
                            }
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }

                else if (mbd.isPrototype()) {
                    //这是一个原型,创建一个新的实例
                    Object prototypeInstance = null;
                    try {
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName, mbd, args);
                    }
                    finally {
                        afterPrototypeCreation(beanName);
                    }
                    bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                }

                else {
                    String scopeName = mbd.getScope();
                    final Scope scope = this.scopes.get(scopeName);
                    if (scope == null) {
                        throw new IllegalStateException("No Scope registered for scope name \'" + scopeName + "\'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                            @Override
                            public Object getObject() throws BeansException {
                                beforePrototypeCreation(beanName);
                                try {
                                    return createBean(beanName, mbd, args);
                                }
                                finally {
                                    afterPrototypeCreation(beanName);
                                }
                            }
                        });
                        bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }
                    catch (IllegalStateException ex) {
                        throw new BeanCreationException(beanName,
                                "Scope \'" + scopeName + "\' is not active for the current thread; consider " +
                                "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                                ex);
                    }
                }
            }
            catch (BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }

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进入AbstractAutowireCapableBeanFactory.java类的createBean方法，这里面可以分为四个部分：

第一部分：确保该bean的class是真实存在的，也就是该bean是可以classload可以找到加载的

第二部分：准备方法的重写

第三部分：可以看到，这边出现了一个return，也就是说这边可以返回bean了。但看注释：Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. 这样就很清晰了，BeanPostProcessor这个接口是可以临时修改bean的，优先级高于正常实例化bean的，如果beanPostProcessor能返回，则直接返回了。

第四部分：调用doCreateBean方法开始对bean进行创建

/**
     * Central method of this class: creates a bean instance,
     * populates the bean instance, applies post-processors, etc.
     * @see #doCreateBean
     */
    @Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating instance of bean \'" + beanName + "\'");
        }
        RootBeanDefinition mbdToUse = mbd;

        //确保bean类实际上是解决在这一点上,和克隆bean定义的动态解析类不能存储在共享合并bean定义。
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        // 准备方法覆盖
        try {
            mbdToUse.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                    beanName, "Validation of method overrides failed", ex);
        }

        try {
            // .让BeanPostProcessors返回一个代理,而不是目标bean实例
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            if (bean != null) {
                return bean;
            }
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }

        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isDebugEnabled()) {
            logger.debug("Finished creating instance of bean \'" + beanName + "\'");
        }
        return beanInstance;
    }

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打开doCreateBean方法，在这个方法里会做两件事：一是通过createBeanInstance这个方法创建bean，二是通过initializeBean方法初始化bean。先看看createBeanInstance这个方法里有什么玄/**

* Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
     * <p>Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args explicit arguments to use for constructor or factory method invocation
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
        Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

        // Allow post-processors to modify the merged bean definition.
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                mbd.postProcessed = true;
            }
        }

        /// 急切地缓存单件能够解决循环引用
               // 即使像BeanFactoryAware由生命周期接口。.
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isDebugEnabled()) {
                logger.debug("Eagerly caching bean \'" + beanName +
                        "\' to allow for resolving potential circular references");
            }
            addSingletonFactory(beanName, new ObjectFactory<Object>() {
                @Override
                public Object getObject() throws BeansException {
                    return getEarlyBeanReference(beanName, mbd, bean);
                }
            });
        }

        // 初始化 bean 的实例对象
        Object exposedObject = bean;
        try {
            populateBean(beanName, mbd, instanceWrapper);
            if (exposedObject != null) {
                exposedObject = initializeBean(beanName, exposedObject, mbd);
            }
        }
        catch (Throwable ex) {
            if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
                throw (BeanCreationException) ex;
            }
            else {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
            }
        }

        if (earlySingletonExposure) {
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                    if (!actualDependentBeans.isEmpty()) {
                        throw new BeanCurrentlyInCreationException(beanName,
                                "Bean with name \'" + beanName + "\' has been injected into other beans [" +
                                StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                                "] in its raw version as part of a circular reference, but has eventually been " +
                                "wrapped. This means that said other beans do not use the final version of the " +
                                "bean. This is often the result of over-eager type matching - consider using " +
                                "\'getBeanNamesOfType\' with the \'allowEagerInit\' flag turned off, for example.");
                    }
                }
            }
        }

        // 注册一次性使用的 bean
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }

View Code

创建Bean

进入createBeanInstance方法，这块代码主要是再次对bean做安全检查并确定该bean有默认的构造函数。直接看这个方法最后一行，调用instantiateBean方法并返回方法的结果。

/**
     * Create a new instance for the specified bean, using an appropriate instantiation strategy:
     * factory method, constructor autowiring, or simple instantiation.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @param args explicit arguments to use for constructor or factory method invocation
     * @return BeanWrapper for the new instance
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     * @see #instantiateBean
     */
    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // 这一步是确保bean这个类在这个步骤完成解决
        Class<?> beanClass = resolveBeanClass(mbd, beanName);

        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Bean class isn\'t public, and non-public access not allowed: " + beanClass.getName());
        }

        if (mbd.getFactoryMethodName() != null
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