Spring源码学习之IoC

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Spring学习笔记之初入Ioc

Ioc容器

• BeanFactory:Ioc容器定义最基础的接口，定义了作为Ioc容器所必备最基本的方法。
• ApplicationContext:细化了许多BeanFactory的方法，并且通过继承许多其他接口，实现了更多其他的IoC容器特性

使用IoC容器

编程式使用IoC容器

        ClassPathResource resource = new ClassPathResource("bean.xml");
        DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
        XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory);
        reader.loadBeanDefinitions(resource);

1. 以resource的方式获取封装Bean
2. 创建使用容器的factory
3. 创建一个BeanDefinition读取器，通过这个reader读取XML中的配置信息并回调配置到factory中
4. 读取资源并完成factory的配置

IoC初始化过程

1. Resource定位：对BeanDefinition的资源定位。

2. 载入：将用户定义好的Bean转化成容器内部的数据结构，即BeanDefinition。

3. 注册：将转化好的BeanDefinition注册到IoC容器中，真实实现是通过一个HashMap储存BeanDefinition。

   由于ApplicationContext是对BeanFactory的子类并添加其他接口的实现，功能更加完备，因此以下以ClassPathXmlApplicationContext容器为例，解析Ioc初始化全过程。

   

   ClassPathXmlApplicationContext的类继承结构：

   

   // Ioc容器的构造函数
   public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
           throws BeansException {
   
       super(parent);
           // 设置配置文件地址
       setConfigLocations(configLocations);
       if (refresh) {
           // 初始化的入口
               refresh();
       }
   }

Resource的定位

//AbstractApplicationContext.java

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // 更新前的预处理，影响不大
            prepareRefresh();

            // 获取待使用的BeanFactory
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            ...
            // 其他暂时不涉及资源定位，暂时省去。
        }
    }

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        //获取生成的BeanFactory
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

// AbstractRefreshableApplicationContext.java
// 更新为新的BeanFactory
protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            // 创建BeanFactory
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            // 定制BeanFactory
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

// AbstractXmlApplicationContext.java

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // 创建一个XmlBeanDefinitionReader，用来读取xml并转化为BeanDefinition
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // 配置在当前资源环境下的reader
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        initBeanDefinitionReader(beanDefinitionReader);
        // 用reader读取资源
        loadBeanDefinitions(beanDefinitionReader);
    }

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        // 获取ClassPathXmlApplicationContext初始化时的资源,而这里获取资源在         
        // ClassPathXmlApplicationContext已经定义。
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            reader.loadBeanDefinitions(configLocations);
        }
    }

这里往后就是使用XmlBeanDefinitionReder对资源的读取和解析的过程

BeanDefinition的载入和解析

// AbstractBeanDefinitionReader.java
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
        Assert.notNull(resources, "Resource array must not be null");
        int counter = 0;
        for (Resource resource : resources) {
            counter += loadBeanDefinitions(resource);
        }
        return counter;
    }

// XmlBeanDefinitionReader.java
// 此处由于使用的是XmlBeanDefinitionReader,而如果使用其他的读取器时也有相对应的读取方法
    public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(new EncodedResource(resource));
    }

    public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }
        // 获取当前线程下的资源
        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<EncodedResource>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            // 开始读取资源
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            // 根据资源生成document
            Document doc = doLoadDocument(inputSource, resource);
            // 对doc和资源解析成Spring认识的BeanDefinition
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }

    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        // 创建doc阅读器
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        //在此之前已有的BeanDefinition
        int countBefore = getRegistry().getBeanDefinitionCount();
        // 解析并注册BeanDefinition
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }   

// DefaultBeanDefinitionDocumentReader.java

public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        //获得根节点
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

protected void doRegisterBeanDefinitions(Element root) {
        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createDelegate(getReaderContext(), root, parent);

        if (this.delegate.isDefaultNamespace(root)) {
            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
            if (StringUtils.hasText(profileSpec)) {
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    return;
                }
            }
        }
        // 留给子类实现，前置处理和后面的后置处理
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
                        // 解析默认地元素
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        // 解析定制的元素 根据uri来判定
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }


protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

// 根据节点名称不一致，分类解析处理节点
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

// 导入其他配置文件的解析
protected void importBeanDefinitionResource(Element ele) {
        String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
        if (!StringUtils.hasText(location)) {
            getReaderContext().error("Resource location must not be empty", ele);
            return;
        }

        // Resolve system properties: e.g. "${user.dir}"
        // 将文件中的占位符替换成真实的配置信息
        location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);

        Set<Resource> actualResources = new LinkedHashSet<Resource>(4);

        // Discover whether the location is an absolute or relative URI
        boolean absoluteLocation = false;
        try {
            absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
        }
        catch (URISyntaxException ex) {
            // cannot convert to an URI, considering the location relative
            // unless it is the well-known Spring prefix "classpath*:"
        }

        // Absolute or relative?
        if (absoluteLocation) {
            try {
                //如果是uri模式的资源位置，使用xmlBeanDefinitionReader读取解析资源文件
                int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
                if (logger.isDebugEnabled()) {
                    logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
                }
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error(
                        "Failed to import bean definitions from URL location [" + location + "]", ele, ex);
            }
        }
        else {
            // No URL -> considering resource location as relative to the current file.
            try {
                int importCount;
                // 根据路径获取资源信息，然后使用XmlBeanDefinitionReader解析读取资源信息
                Resource relativeResource = getReaderContext().getResource().createRelative(location);
                if (relativeResource.exists()) {
                    importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
                    actualResources.add(relativeResource);
                }
                else {
                    String baseLocation = getReaderContext().getResource().getURL().toString();
                    importCount = getReaderContext().getReader().loadBeanDefinitions(
                            StringUtils.applyRelativePath(baseLocation, location), actualResources);
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
                }
            }
            catch (IOException ex) {
                getReaderContext().error("Failed to resolve current resource location", ele, ex);
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",
                        ele, ex);
            }
        }
        Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
        getReaderContext().fireImportProcessed(location, actResArray, extractSource(ele));
    }

// 进行别名的注册
protected void processAliasRegistration(Element ele) {
        String name = ele.getAttribute(NAME_ATTRIBUTE);
        String alias = ele.getAttribute(ALIAS_ATTRIBUTE);
        boolean valid = true;
        if (!StringUtils.hasText(name)) {
            getReaderContext().error("Name must not be empty", ele);
            valid = false;
        }
        if (!StringUtils.hasText(alias)) {
            getReaderContext().error("Alias must not be empty", ele);
            valid = false;
        }
        if (valid) {
            try {
                // 将别名和名称绑定起来，一会儿进行bean注册时可以再将别名注册进去
                getReaderContext().getRegistry().registerAlias(name, alias);
            }
            catch (Exception ex) {
                getReaderContext().error("Failed to register alias '" + alias +
                        "' for bean with name '" + name + "'", ele, ex);
            }
            getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
        }
    }

//  解析注册Bean元素
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        //解析Bean元素，并装载到Holder中
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            // 将ele元素的子节点解析装载到Holder中
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to register bean definition with name '" +
                        bdHolder.getBeanName() + "'", ele, ex);
            }
            // Send registration event.
            getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }
    }

// BeanDefinitionParserDelegate.java
// 解析元素
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
        String id = ele.getAttribute(ID_ATTRIBUTE);
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        List<String> aliases = new ArrayList<String>();
        if (StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
            beanName = aliases.remove(0);
            if (logger.isDebugEnabled()) {
                logger.debug("No XML 'id' specified - using '" + beanName +
                        "' as bean name and " + aliases + " as aliases");
            }
        }

        if (containingBean == null) {
            // 之前将所有的节点名称对比看看是否有重复出现
            checkNameUniqueness(beanName, aliases, ele);
        }
        //具体解析节点的操作，无非是一一对应的操作，有兴趣可以自己查看源码
        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
        if (beanDefinition != null) {
            if (!StringUtils.hasText(beanName)) {
                try {
                    if (containingBean != null) {
                        beanName = BeanDefinitionReaderUtils.generateBeanName(
                                beanDefinition, this.readerContext.getRegistry(), true);
                    }
                    else {
                        beanName = this.readerContext.generateBeanName(beanDefinition);
                        // Register an alias for the plain bean class name, if still possible,
                        // if the generator returned the class name plus a suffix.
                        // This is expected for Spring 1.2/2.0 backwards compatibility.
                        String beanClassName = beanDefinition.getBeanClassName();
                        if (beanClassName != null &&
                                beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                                !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                            aliases.add(beanClassName);
                        }
                    }
                    if (logger.isDebugEnabled()) {
                        logger.debug("Neither XML 'id' nor 'name' specified - " +
                                "using generated bean name [" + beanName + "]");
                    }
                }
                catch (Exception ex) {
                    error(ex.getMessage(), ele);
                    return null;
                }
            }
            String[] aliasesArray = StringUtils.toStringArray(aliases);
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }

BeanDefinition的注册

//BeanDefinitionReaderUtils.java
// 注册BeanDefinition
public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {

        // Register bean definition under primary name.
        String beanName = definitionHolder.getBeanName();
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

        // Register aliases for bean name, if any.
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String alias : aliases) {
                registry.registerAlias(beanName, alias);
            }
        }
    }

//DefaultListableBeanFactory.java
// 虽然操作很多，但关键的主要操作就是将name和BeanDefinition放进hashmap中。至此 注册完毕
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {

        Assert.hasText(beanName, "Bean name must not be empty");
        Assert.notNull(beanDefinition, "BeanDefinition must not be null");

        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }

        BeanDefinition oldBeanDefinition;

        oldBeanDefinition = this.beanDefinitionMap.get(beanName);
        if (oldBeanDefinition != null) {
            if (!isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                        "': There is already [" + oldBeanDefinition + "] bound.");
            }
            else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                            "' with a framework-generated bean definition: replacing [" +
                            oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            }
            else if (!beanDefinition.equals(oldBeanDefinition)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName +
                            "' with a different definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            else {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Overriding bean definition for bean '" + beanName +
                            "' with an equivalent definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
        }
        else {
            if (hasBeanCreationStarted()) {
                // Cannot modify startup-time collection elements anymore (for stable iteration)
                synchronized (this.beanDefinitionMap) {
                    this.beanDefinitionMap.put(beanName, beanDefinition);
                    List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
                    updatedDefinitions.addAll(this.beanDefinitionNames);
                    updatedDefinitions.add(beanName);
                    this.beanDefinitionNames = updatedDefinitions;
                    if (this.manualSingletonNames.contains(beanName)) {
                        Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
                        updatedSingletons.remove(beanName);
                        this.manualSingletonNames = updatedSingletons;
                    }
                }
            }
            else {
                // Still in startup registration phase
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }
            this.frozenBeanDefinitionNames = null;
        }

        if (oldBeanDefinition != null || containsSingleton(beanName)) {
            resetBeanDefinition(beanName);
        }
    }

至此，Ioc容器的资源定位，BeanDefinition解析和载入，BeanDefinition的注册都已经完成。意味着作为一个Ioc容器，已经装载了资源中包含的Bean元素。那么，如何从容器中取出元素呢？

依赖注入

上述操作已经将Bean元素载入到Ioc容器中。现在分析用户第一次向容器索要Bean时发生的依赖注入：

//AbstractBeanFactory.java

protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
        // 将输入的名字转化为在BeanDefinition中存储的名称（如果是别名则转化为BeanName）
        final String beanName = transformedBeanName(name);
        Object bean;

        // 获取单例模式的bean，如果之前已经创建，直接从缓存中读取，不需要重复创建
        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 + "'");
                }
            }
            // 根据给定的bean实例，获取到bean元素。要么是直接获取元素本身，要么是通过factoryBean生成             // bean元素 
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // Fail if we're already creating this bean instance:
            // We're assumably within a circular reference.
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 如果当前BeanFactry不包含对应的BeanDefinition，则沿着双亲链一路往上找
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

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

            try {
                // 获得beanName相关的BeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 获取所有依赖的bean，递归获取所有依赖的bean
                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);
                    }
                }

                // 接下来，无论是单例模式还是原型模式，都通过createBean创建bean，然后获取bean。
                // 那么进入createBean了解到底是怎么创建bean的
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            catch (BeansException ex) {
                                // Explicitly remove instance from singleton cache: It might have been put there
                                // eagerly by the creation process, to allow for circular reference resolution.
                                // Also remove any beans that received a temporary reference to the bean.
                                destroySingleton(beanName);
                                throw ex;
                            }
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }

                else if (mbd.isPrototype()) {
                    // It's a prototype -> create a new instance.
                    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;
            }
        }

        // 类型检查，如果通过则正常返回获得的bean
        if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
            try {
                return getTypeConverter().convertIfNecessary(bean, requiredType);
            }
            catch (TypeMismatchException ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to convert bean '" + name + "' to required type [" +
                            ClassUtils.getQualifiedName(requiredType) + "]", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
    }

// AbstractAutowireCapableBeanFactory.java
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
        if (logger.isDebugEnabled()) {
            logger.debug("Creating instance of bean '" + beanName + "'");
        }
        RootBeanDefinition mbdToUse = mbd;

        // 判断是否能实例化，判断能否通过类加载器加载
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

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

        try {
            // 如果Bean配置了PostProcessor，那么返回一个代理
            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;
    }


// 创建bean
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // 创建一个持有bean对象的wrapper
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            // 如果是单例，移除同名缓存
            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;
            }
        }

        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        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);
                }
            });
        }

        // 初始化实例
        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<String>(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.");
                    }
                }
            }
        }

        // Register bean as disposable.
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
    }


// 创建实例
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // 确保bean类型能实例化
        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)  {
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        // Shortcut when re-creating the same bean...
        boolean resolved = false;
        boolean autowireNecessary = false;
        if (args == null) {
            synchronized (mbd.constructorArgumentLock) {
                if (mbd.resolvedConstructorOrFactoryMethod != null) {
                    resolved = true;
                    autowireNecessary = mbd.constructorArgumentsResolved;
                }
            }
        }
        if (resolved) {
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                return instantiateBean(beanName, mbd);
            }
        }

        // 使用构造函数实例化
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        // 使用默认的构造函数实例化
        return instantiateBean(beanName, mbd);
    }


//实例化
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
        try {
            Object beanInstance;
            final BeanFactory parent = this;
            if (System.getSecurityManager() != null) {
                beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    @Override
                    public Object run() {
                        return getInstantiationStrategy().instantiate(mbd, beanName, parent);
                    }
                }, getAccessControlContext());
            }
            else {
                //具体实例化的形式
                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
            }
            BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            initBeanWrapper(bw);
            return bw;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
    }

// SimpleInstantiationStrategy.java
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        // 使用构造器或者工厂方法来实例化对象
        if (bd.getMethodOverrides().isEmpty()) {
            Constructor<?> constructorToUse;
            synchronized (bd.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = bd.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
                                @Override
                                public Constructor<?> run() throws Exception {
                                    return clazz.getDeclaredConstructor((Class[]) null);
                                }
                            });
                        }
                        else {
                            constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);
                        }
                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                    }
                    catch (Exception ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            // 通过jdk反射获取对象实例
            return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
            // 通过cglib来实例化对象
            return instantiateWithMethodInjection(bd, beanName, owner);
        }
    }

// CglibSubclassingInstantiationStrategy.java
// 使用cglib实例化对象实例
public Object instantiate(Constructor<?> ctor, Object... args) {
            // 通过cglib获得生成代理类的Class
            Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
            Object instance;
            if (ctor == null) {
                // 通过Class生成目标实例
                instance = BeanUtils.instantiate(subclass);
            }
            else {
                try {
                    Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
                    instance = enhancedSubclassConstructor.newInstance(args);
                }
                catch (Exception ex) {
                    throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
                            "Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
                }
            }
            // SPR-10785: set callbacks directly on the instance instead of in the
            // enhanced class (via the Enhancer) in order to avoid memory leaks.
            Factory factory = (Factory) instance;
            factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
                    new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
                    new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
            return instance;
        }

getBean的流程图

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创建Bean：