SpringIOC源码

概述

控制反转IoC(Inversion of Control)是说创建对象的控制权进行转移，以前创建对象的主动权和创建时机是由自己把控的，而现在这种权力转移到第三方，比如转移交给了IoC容器，它就是一个专门用来创建对象的工厂，你要什么对象，它就给你什么对象，有了 IoC容器，依赖关系就变了，原先的依赖关系就没了，它们都依赖IoC容器了，通过IoC容器来建立它们之间的关系。

1.IoC加载

2.注入

从SpringAOP源码的入口一节，我们可以定位加载bean的源码到doLoadBeanDefinitions：

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            Document doc = doLoadDocument(inputSource, resource); // 取得XML文件的Document对象, 这个解析过程由DefaultDocumentLoader完成，包括验证Document等 
return registerBeanDefinitions(doc, resource);  //解析Document
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
.......
}

doLoadDocument

    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

XML比如
<beans
    xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-4.0.xsd">    //XSD方式，用于验证XML文件的正确性

    <bean id="..">...</bean>
</beans>

//常用的XML文件验证方法有两种：DTD和XSD，DTD现在基本不用了，而上图中的spring-beans-4.0.xsd对应在源码中的org.springframework.beans.factory.xml.spring-beans-4.0.xsd。

protected int getValidationModeForResource(Resource resource) { //获取XML的验证方式
        int validationModeToUse = getValidationMode(); //获取检测模式
        if (validationModeToUse != VALIDATION_AUTO) {
            return validationModeToUse;
        }
        int detectedMode = detectValidationMode(resource);//如果没有启动，则自动检测
        if (detectedMode != VALIDATION_AUTO) { 
            return detectedMode;
        }
        // Hmm, we didn't get a clear indication... Let's assume XSD,
        // since apparently no DTD declaration has been found up until
        // detection stopped (before finding the document's root tag).
        return VALIDATION_XSD;
    }

XML的Document对象就到这里，接下来是registerBeanDefinitions了，启动对beanDefinition的解析

registerBeanDefinitions

BeanDefinition载入过程其实就是把定义的BeanDefinition在IoC容器中转化为一个Spring内部表示的数据结构的过程。IoC容器对Bean的管理和依赖注入的实现，
都是通过对其持有的BeanDefinition进行各种相关的操作来完成的。这些BeanDefinition数据在IoC容器中通过一个HashMap来维护。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        // 得到BeanDefinitionDocumentReader来对XML的BeanDefinition进行解析  
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        documentReader.setEnvironment(this.getEnvironment());
        int countBefore = getRegistry().getBeanDefinitionCount();
 // 具体的解析过程在BeanDefinitionDocumentReader的registerBeanDefinitions方法中完成
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

protected void doRegisterBeanDefinitions(Element root) {
        // Any nested <beans> elements will cause recursion in this method. In
        // order to propagate and preserve <beans> default-* attributes correctly,
        // keep track of the current (parent) delegate, which may be null. Create
        // the new (child) delegate with a reference to the parent for fallback purposes,
        // then ultimately reset this.delegate back to its original (parent) reference.
        // this behavior emulates a stack of delegates without actually necessitating one.
        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createDelegate(this.readerContext, root, parent);

        if (this.delegate.isDefaultNamespace(root)) {
            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
            if (StringUtils.hasText(profileSpec)) {
                Assert.state(this.environment != null, "Environment must be set for evaluating profiles");
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                if (!this.environment.acceptsProfiles(specifiedProfiles)) {
                    return;
                }
            }
        }

        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {//获取Document对象根元素的所有子节点并循环解析
            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);//解析spring bean规则默认节点
                    }
                    else {
                        delegate.parseCustomElement(ele); //解析自定义节点（之前的AOP就是在这里解析的）
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) { //import元素
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) { //alias元素
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) { //bean元素
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {内嵌beans元素，作为根节点递归解析
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

直接从代码就可以看出，Spring首先获取Document的根元素（一般为<beans/>），然后取得根元素所有的子节点并循环解析这些子节点；如果子节点在Spring默认的命名空间内，则按照Spring Bean定义规则来解析，否则按照自定义的节点解析。在按照Spring Bean定义规则进行解析的parseDefaultElement方法中，完成了对<import/>、<alias/>、<bean/>、<beans/>等元素的解析。

这里只关注bean的解析

DefaultBeanDefinitionDocumentReader.java

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 具体的解析委托给BeanDefinitionParserDelegate来完成           
// BeanDefinitionHolder是BeanDefinition的封装类, 封装了BeanDefinition、Bean的名字和别名, 用它来完成向IoC容器注册.
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            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

public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
//解析<bean>元素，获取id和name属性 
        String id = ele.getAttribute(ID_ATTRIBUTE);
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        List<String> aliases = new ArrayList<String>();//处理bean中的alias属性
        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()) {//如果bean没有配置id，将别名中的第一个名赋值给beanName
            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) { //检查name是否唯一
            checkNameUniqueness(beanName, aliases, ele);
        }

        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
        if (beanDefinition != null) {//对bean元素进行详细解析
            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;
    }

public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, BeanDefinition containingBean) {

        this.parseState.push(new BeanEntry(beanName));

        String className = null;
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) { //获取class属性值，载入到beanDefinition中，此时并不实例化
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }

        try {
            String parent = null;
            if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
                parent = ele.getAttribute(PARENT_ATTRIBUTE);
            }
            AbstractBeanDefinition bd = createBeanDefinition(className, parent); //根据class名称及parent属性创建BeanDefinition

            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd); // 对当前<Bean>元素中配置的一些属性进行解析, 如singleton、abstract等 
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

// 对<Bean>元素的meta(元数据)、lookup-method、replaced-method等子元素进行解析
            parseMetaElements(ele, bd);
            parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

            parseConstructorArgElements(ele, bd); //解析构造方法参数
            parsePropertyElements(ele, bd); //解析property设置
            parseQualifierElements(ele, bd);

            bd.setResource(this.readerContext.getResource());
            bd.setSource(extractSource(ele));

            return bd;
        }
        catch (ClassNotFoundException ex) {
            error("Bean class [" + className + "] not found", ele, ex);
        }
        catch (NoClassDefFoundError err) {
            error("Class that bean class [" + className + "] depends on not found", ele, err);
        }
        catch (Throwable ex) {
            error("Unexpected failure during bean definition parsing", ele, ex);
        }
        finally {
            this.parseState.pop();
        }

        return null;
    }

<bean>元素解析已经完了，而<bean>元素属性及其子元素的解析顺序为：1，解析<bean>元素的属性。2，解析<description>子元素。3，解析<meta>子元素。4，解析<lookup-method/>子元素。5，解析<replaced-method>子元素。6，解析<constructor-arg>子元素。7，解析<property>子元素。8，解析<qualifier>子元素。解析过程中像<meta>、<qualifier>等子元素都很少使用，而下面就直接解析最常用的子元素<property>子元素。

/**
     * Parse property sub-elements of the given bean element.
     */
    public void parsePropertyElements(Element beanEle, BeanDefinition bd) {
        NodeList nl = beanEle.getChildNodes();//获取所有子元素
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, PROPERTY_ELEMENT)) {//如果是property元素，则进行解析
                parsePropertyElement((Element) node, bd);
            }
        }
    }

/**
     * Parse a property element.
     */
    public void parsePropertyElement(Element ele, BeanDefinition bd) {
        String propertyName = ele.getAttribute(NAME_ATTRIBUTE);//property的name属性
        if (!StringUtils.hasLength(propertyName)) {
            error("Tag 'property' must have a 'name' attribute", ele);
            return;
        }
        this.parseState.push(new PropertyEntry(propertyName));
        try {
            if (bd.getPropertyValues().contains(propertyName)) {//如果一个bean下存在相同的property，则直接返回
                error("Multiple 'property' definitions for property '" + propertyName + "'", ele);
                return;
            }

                // 解析<property>元素, 返回的对象对应<property>元素的解析结果, 最终封装到PropertyValue中, 并设置到BeanDefinitionHolder中 

            Object val = parsePropertyValue(ele, bd, propertyName);
            PropertyValue pv = new PropertyValue(propertyName, val);
            parseMetaElements(ele, pv);
            pv.setSource(extractSource(ele));
            bd.getPropertyValues().addPropertyValue(pv);
        }
        finally {
            this.parseState.pop();
        }
    }

public Object parsePropertyValue(Element ele, BeanDefinition bd, String propertyName) {
        String elementName = (propertyName != null) ?
                        "<property> element for property '" + propertyName + "'" :
                        "<constructor-arg> element";

        // Should only have one child element: ref, value, list, etc.property子元素只能是这几个
        NodeList nl = ele.getChildNodes();
        Element subElement = null;
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT) &&
                    !nodeNameEquals(node, META_ELEMENT)) {
                // Child element is what we're looking for.
                if (subElement != null) {
                    error(elementName + " must not contain more than one sub-element", ele);
                }
                else {
                    subElement = (Element) node;
                }
            }
        }
// 判断property元素是否含有ref和value属性, 不允许既有ref又有value属性.  
        // 同时也不允许ref和value属性其中一个与子元素共存. 
        boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
        boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
        if ((hasRefAttribute && hasValueAttribute) ||
                ((hasRefAttribute || hasValueAttribute) && subElement != null)) {
            error(elementName +
                    " is only allowed to contain either 'ref' attribute OR 'value' attribute OR sub-element", ele);
        }

        if (hasRefAttribute) {
            String refName = ele.getAttribute(REF_ATTRIBUTE);
            if (!StringUtils.hasText(refName)) {
                error(elementName + " contains empty 'ref' attribute", ele);
            }
            RuntimeBeanReference ref = new RuntimeBeanReference(refName);
            ref.setSource(extractSource(ele));
            return ref;
        }
        else if (hasValueAttribute) {
            TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
            valueHolder.setSource(extractSource(ele));
            return valueHolder;
        }
        else if (subElement != null) {
            return parsePropertySubElement(subElement, bd);
        }
        else {
            // Neither child element nor "ref" or "value" attribute found.
            error(elementName + " must specify a ref or value", ele);
            return null;
        }
    }

就这样beanDefinition加载好了

再经过其他的一些初始化，已经可以getBean来获取bean 了

ApplicationContext ac = new ClassPathXmlApplicationContext("spring.xml");
        ac.getBean("iUserDao");

ClassPathXmlApplicationContext中的getBean方法哪来的呢？看一下继承关系就知道了

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory(); //beanFactory来源，最终loadBeanDefinitions为DefaultListableBeanFactory
            beanFactory.setSerializationId(getId());
            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);
        }
    }

再看下继承关系图

现在来看下getBean的实现

AbstractBeanFactory.java

protected <T> T doGetBean(
            final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
//指定名称获取被管理的bean名称
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
//单例模式
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            if (logger.isDebugEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {//如果单例模式已经存在
                    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 + "'");
                }
            }
                //这里完成FactoryBean的相关处理，对于FactoryBean，BeanFactory已经有所提及，在后面会详细分析

            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);
            }

            // Check if bean definition exists in this factory.
 //对容器中的BeanDefinition进行检查，检查能否在当前的BeanFactory中取得Bean.
                //如果在当前的工厂中取不到，则到父类的BeanFactory中去取
                //如果在父类中取不到，则到父类的父类中取
            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 {
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                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);
                    }
                }

                // Create bean instance.单例模式
                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.创建新的实例
                    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 '" + 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;
            }
        }

        // Check if required type matches the type of the actual bean instance.
        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;
    }

从上面，可以看出doGetBean，是依赖注入的实际入口，他定义了Bean的定义模式，单例模式（Singleton）和原型模式（Prototype），而依赖注入触发的前提是BeanDefinition数据已经建立好的前提下。其实对于Ioc容器的使用，Spring提供了许多的参数配置，每一个参数配置实际上代表了一个Ioc实现特性，而这些特性的实现很多都需要在依赖注入的过程中或者对Bean进行生命周期管理的过程中完成。Spring Ioc容器作为一个产品，其真正的价值体现在一系列产品特征上，而这些特征都是以依赖反转模式作为核心，他们为控制反转提供了很多便利，从而实现了完整的Ioc容器。

再看核心createBean方法

getBean是依赖注入的起点，之后就会调用createBean，createBean可以生成需要的Bean以及对Bean进行初始化，但对createBean进行跟踪，发现他在AbstractBeanFactory中仅仅是声明，而具体实现是在AbstractAutowireCapableBeanFactory类里。

直接定位到createBeanInstance

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); //创建bean
        }
        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);
                }
            });
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            populateBean(beanName, mbd, instanceWrapper); //依赖注入
            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;
    }

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // Make sure bean class is actually resolved at this point.// 确认需要创建的Bean实例的类可以实例化
        Class<?> beanClass = resolveBeanClass(mbd, beanName);

//这里使用工厂方法对Bean进行实例化
        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...//使用容器自动装配方法进行实例化
        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) {//配置自动装配属性，使用容器自动实例化
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {//无参构造方法实例化
                return instantiateBean(beanName, mbd);
            }
        }

        // Need to determine the constructor...
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);//使用容器自动装配，调用构造方法实例化
        }

        // No special handling: simply use no-arg constructor.//使用默认构造函数对Bean进行实例化
        return instantiateBean(beanName, mbd);
    }

看个默认的实例化方式instantiateBean

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 {
                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);
        }
    }

    /**
     * Return the instantiation strategy to use for creating bean instances.
     */
    protected InstantiationStrategy getInstantiationStrategy() {
        return this.instantiationStrategy;
    }

    private InstantiationStrategy instantiationStrategy = new CglibSubclassingInstantiationStrategy();  //这里用到了cglib，但是一定用到了cglib吗？

Object instantiate(Constructor<?> ctor, Object[] args) {
            Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
            Object instance;
            if (ctor == null) {
                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(), String.format(
                            "Failed to invoke constructor for CGLIB enhanced subclass [%s]", 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;
        }

SimpleInstantiationStrategy.java

@Override
    public Object instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner) {
        // Don't override the class with CGLIB if no overrides.
        if (beanDefinition.getMethodOverrides().isEmpty()) {
            Constructor<?> constructorToUse;
            synchronized (beanDefinition.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) beanDefinition.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = beanDefinition.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);
                        }
                        beanDefinition.resolvedConstructorOrFactoryMethod = constructorToUse;
                    }
                    catch (Exception ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
            // Must generate CGLIB subclass.
            return instantiateWithMethodInjection(beanDefinition, beanName, owner);
        }
    }

如果有需要覆盖或者动态替换的方法使用Cglib进行动态代理，如果没有需要动态改变的方法那就直接使用反射进行实例化。所谓的动态替换或覆盖的情况，就是使用了replace或者lookup的配置方法。这两个方法的作用就是在调用目标方法的时候，对调用过程进行拦截，调用实现增强功能的拦截器，返回原来实例的代理。关于拦截和代理这部分会在AOP中进行详细的分析。 
接下来就是分别使用Jdk的反射和Cglib进行实例化的具体过程了。 

这所以对象实例化，Spring反射不一定用的是Jdk的反射机制，有可能也会有cglib。

下面分别为jdk和cglib的方式

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
        Assert.notNull(ctor, "Constructor must not be null");
        try {
            ReflectionUtils.makeAccessible(ctor);
            return ctor.newInstance(args);
        }
        catch (InstantiationException ex) {
            throw new BeanInstantiationException(ctor.getDeclaringClass(),
                    "Is it an abstract class?", ex);
        }
        catch (IllegalAccessException ex) {
            throw new BeanInstantiationException(ctor.getDeclaringClass(),
                    "Is the constructor accessible?", ex);
        }
        catch (IllegalArgumentException ex) {
            throw new BeanInstantiationException(ctor.getDeclaringClass(),
                    "Illegal arguments for constructor", ex);
        }
        catch (InvocationTargetException ex) {
            throw new BeanInstantiationException(ctor.getDeclaringClass(),
                    "Constructor threw exception", ex.getTargetException());
        }
    }

Object instantiate(Constructor<?> ctor, Object[] args) {
            Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
            Object instance;
            if (ctor == null) {
                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(), String.format(
                            "Failed to invoke constructor for CGLIB enhanced subclass [%s]", 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;
        }

标准的使用Enhancer实例化对象的方法，包括设置基类，回调方法等。这个方法中出现了LookupOverrideMethodInterceptor和ReplaceOverrideMethodInterceptor两个拦截器，作用就是拦截目标方法，实现方法增强的功能。 
Bean对象已经创建好了，接下来就该进行依赖注入了。回到populateBean方法

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
//获取beanDefinition中property的值
        PropertyValues pvs = mbd.getPropertyValues();

        if (bw == null) {//实例对象为空
            if (!pvs.isEmpty()) {//属性不为空
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
            }
            else {//实例为空，属性也为空直接返回
                // Skip property population phase for null instance.
                return;
            }
        }

        // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
        // state of the bean before properties are set. This can be used, for example,
        // to support styles of field injection.
//在设置属性之前调用Bean的PostProcessor后置处理器
        boolean continueWithPropertyPopulation = true;

        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }

        if (!continueWithPropertyPopulation) {
            return;
        }
//开始进行依赖注入过程，先处理autowire的注入
//根据bean名字或类型自动装配处理
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
            MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

            // Add property values based on autowire by name if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
                autowireByName(beanName, mbd, bw, newPvs);
            }

            // Add property values based on autowire by type if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {//根据类型自动装配注入
                autowireByType(beanName, mbd, bw, newPvs);
            }

            pvs = newPvs;
        }

//检查日期是否持有用于单态模式Bean关闭时的后置处理器
        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

        if (hasInstAwareBpps || needsDepCheck) {
            PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            if (hasInstAwareBpps) {
                for (BeanPostProcessor bp : getBeanPostProcessors()) {
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvs == null) {
                            return;
                        }
                    }
                }
            }
            if (needsDepCheck) {
 //对要配置的属性进行依赖检查
                checkDependencies(beanName, mbd, filteredPds, pvs);
            }
        }
 //对属性进行依赖注入
        applyPropertyValues(beanName, mbd, bw, pvs);
    }

1、依赖注入发生的时间

当Spring IoC容器完成了Bean定义资源的定位、载入和解析注册以后，IoC容器中已经管理类Bean定义的相关数据，但是此时IoC容器还没有对所管理的Bean进行依赖注入，依赖注入在以下两种情况发生：

(1).用户第一次通过getBean方法向IoC容索要Bean时，IoC容器触发依赖注入。

(2).当用户在Bean定义资源中为<Bean>元素配置了lazy-init属性，即让容器在解析注册Bean定义时进行预实例化，触发依赖注入。