【转自】http://geeekr.com/read-spring-source-1-how-to-load-bean/

最近几天跟同事聊起Spring的一些问题，对一些地方有些疑问，趁这两天有点空，看看Spring的源码，了解下具体的实现细节。本文基于Spring 4.0.5版本。

首先Web项目使用Spring是通过在web.xml里面配置
org.springframework.web.context.ContextLoaderListener初始化IOC容器的。

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

那就以此为切入点顺藤摸瓜。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener

ContextLoaderListener继承了ContextLoader，并且实现ServletContextListener接口。当Server容器（一般指tomcat）启动时，会收到事件初始化。

@Override
public void contextInitialized(ServletContextEvent event) {
    initWebApplicationContext(event.getServletContext());
}

initWebApplicationContext方法是在org.springframework.web.context.ContextLoader类里面。方法太长，分段读一下。

if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
    throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
    logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();

首先是判断servletContext中是否已经注册了WebApplicationContext，如果有则抛出异常，避免重复注册。然后就是启用log，启动计时。本方法的关键就在于try代码块里的内容

try {
    // Store context in local instance variable, to guarantee that
    // it is available on ServletContext shutdown.
    if (this.context == null) {
        this.context = createWebApplicationContext(servletContext);
    }
    if (this.context instanceof ConfigurableWebApplicationContext) {
        ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
        if (!cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, etc
            if (cwac.getParent() == null) {
                // The context instance was injected without an explicit parent ->
                // determine parent for root web application context, if any.
                ApplicationContext parent = loadParentContext(servletContext);
                cwac.setParent(parent);
            }
            configureAndRefreshWebApplicationContext(cwac, servletContext);
        }
    }
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

    ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    if (ccl == ContextLoader.class.getClassLoader()) {
        currentContext = this.context;
    }
    else if (ccl != null) {
        currentContextPerThread.put(ccl, this.context);
    }

    if (logger.isDebugEnabled()) {
        logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
                WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
    }
    if (logger.isInfoEnabled()) {
        long elapsedTime = System.currentTimeMillis() - startTime;
        logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
    }

    return this.context;
}

这里面有几个关键的方法。首先看一下createWebApplicationContext()

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    Class<?> contextClass = determineContextClass(sc);
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    }
    return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

首先determineContextClass()方法查明具体的Context类，他会读取servletContext的初始化参数contextClass，此参数我们一半不配置，所以Spring就会读取跟org.springframework.web.context.WebApplicationContext同一个包下面的ContextLoader.properties文件读取默认设置，反射出org.springframework.web.context.support.XmlWebApplicationContext类来。接下来就是在configureAndRefreshWebApplicationContext()方法里将新创建的XmlWebApplicationContext进行初始化。首先会设置一个默认ID，即org.springframework.web.context.WebApplicationContext:+你项目的ContextPath。

if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
    // The application context id is still set to its original default
    // value
    // -> assign a more useful id based on available information
    String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
    if (idParam != null) {
        wac.setId(idParam);
    } else {
        // Generate default id...
        wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath()));
    }
}

紧接着就是将ServletContext设置成XmlWebApplicationContext的属性，这样Spring就能在上下文里轻松拿到ServletContext了。

wac.setServletContext(sc);

接下来就是读取web.xml文件中的contextConfigLocation参数。如果没有配置就会去读WEB-INF下的applicationContext.xml文件。

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:beans.xml</param-value>
</context-param>

并将值设置(就是我们的Spring配置文件的路径)进XmlWebApplicationContext中。然后就会在指定的路径加载配置文件。

String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
    wac.setConfigLocation(configLocationParam);
}

接下来就是customizeContext(sc, wac)方法，此方法会根据用户配置的globalInitializerClasses参数来初始化一些用户自定义的属性，一般我们不配置，所以这里什么也不做。

最后登场的就是最核心的方法了，

wac.refresh();

在这个方法里，会完成资源文件的加载、配置文件解析、Bean定义的注册、组件的初始化等核心工作，我们一探究竟。

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();
        }

        catch (BeansException ex) {
            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            // Propagate exception to caller.
            throw ex;
        }
    }
}

方法是同步的，避免重复刷新，每个步骤都放在单独的方法内，流程清晰，是值得学习的地方。这里面有个重要的方法是finishBeanFactoryInitialization(beanFactory);，里面的内容是Spring如何实例化bean，并注入依赖的，这个内容下一节讲，本节只说明Spring是如何加载class文件的。

首先就是prepareRefresh()方法。

protected void prepareRefresh() {
    this.startupDate = System.currentTimeMillis();

    synchronized (this.activeMonitor) {
        this.active = true;
    }

    if (logger.isInfoEnabled()) {
        logger.info("Refreshing " + this);
    }

    // Initialize any placeholder property sources in the context environment
    initPropertySources();

    // Validate that all properties marked as required are resolvable
    // see ConfigurablePropertyResolver#setRequiredProperties
    getEnvironment().validateRequiredProperties();
}

此方法做一些准备工作，如记录开始时间，输出日志，initPropertySources();和getEnvironment().validateRequiredProperties();一般没干什么事。

接下来就是初始化BeanFactory，是整个refresh()方法的核心，其中完成了配置文件的加载、解析、注册

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

看看它里面都做了些什么？

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

首先refreshBeanFactory()：

protected final void refreshBeanFactory() throws BeansException {
    if (hasBeanFactory()) {
        destroyBeans();
        closeBeanFactory();
    }
    try {
        DefaultListableBeanFactory beanFactory = createBeanFactory();
        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);
    }
}

我们看到会创建一个DefaultListableBeanFactory实例

DefaultListableBeanFactory beanFactory = createBeanFactory();

再设置一个ID

beanFactory.setSerializationId(getId());

然后设置一些自定义参数：

customizeBeanFactory(beanFactory);

这里面最重要的就是loadBeanDefinitions(beanFactory);方法了。

@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
    // Create a new XmlBeanDefinitionReader for the given BeanFactory.
    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

    // Configure the bean definition reader with this context's
    // resource loading environment.
    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);
    loadBeanDefinitions(beanDefinitionReader);
}

此方法会通过XmlBeanDefinitionReader加载bean定义。具体的实现方法是在org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions方法中定义的。这里设计了层层调用，有好多重载方法，主要就是加载Spring所有的配置文件(可能会有多个)，以备后面解析，注册之用。我一路追踪到org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root)

protected void doRegisterBeanDefinitions(Element 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;
        }
    }
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = createDelegate(this.readerContext, root, parent);
    preProcessXml(root);
    parseBeanDefinitions(root, this.delegate);
    postProcessXml(root);
    this.delegate = parent;
}

这里创建了一个BeanDefinitionParserDelegate示例，解析XML的过程就是委托它完成的，我们不关心它是怎样解析XML的，我们只关心是怎么加载类的，所以就要看parseBeanDefinitions(root, this.delegate)方法了。

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);
    }
}

我们看到最终解析XML元素的是delegate.parseCustomElement(ele)方法，最终会走到一下方法.

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
    String namespaceUri = getNamespaceURI(ele);
    NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
    if (handler == null) {
        error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
        return null;
    }
    return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

这里会根据不同的XML节点，会委托NamespaceHandlerSupport找出合适的BeanDefinitionParser，如果我们配置了

<context:component-scan
    base-package="com.geeekr.service,com.geeekr.dao" />

那么对应BeanDefinitionParser就是org.springframework.context.annotation.ComponentScanBeanDefinitionParser，来看看它的parse方法。

@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
    String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
            ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);

    // Actually scan for bean definitions and register them.
    ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
    Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
    registerComponents(parserContext.getReaderContext(), beanDefinitions, element);

    return null;
}

不难看出这里定义了一个ClassPathBeanDefinitionScanner，通过它去扫描包中的类文件，注意：这里是类文件而不是类，因为现在这些类还没有被加载，只是ClassLoader能找到这些class的路径而已。到目前为止，感觉真想距离我们越来越近了。顺着继续往下摸。进入doSacn方法里，映入眼帘的又是一大坨代码，但是我们只关心观点的部分。

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Assert.notEmpty(basePackages, "At least one base package must be specified");
    Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
    for (String basePackage : basePackages) {
        Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
        for (BeanDefinition candidate : candidates) {
            ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
            candidate.setScope(scopeMetadata.getScopeName());
            String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
            if (candidate instanceof AbstractBeanDefinition) {
                postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
            }
            if (candidate instanceof AnnotatedBeanDefinition) {
                AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
            }
            if (checkCandidate(beanName, candidate)) {
                BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                beanDefinitions.add(definitionHolder);
                registerBeanDefinition(definitionHolder, this.registry);
            }
        }
    }
    return beanDefinitions;
}

一眼就能看出是通过

Set<BeanDefinition> candidates = findCandidateComponents(basePackage);

有时候不得不佩服这些外国人起名字的功力，把扫描出来的类叫做candidates(候选人)；真是不服不行啊，这种名字真的很容易理解有不有？哈哈，貌似扯远了。继续往下看。这里只列出方法的主题部分。

public Set<BeanDefinition> findCandidateComponents(String basePackage) {
    Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
    try {
        String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
                resolveBasePackage(basePackage) + "/" + this.resourcePattern;
        Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
        boolean traceEnabled = logger.isTraceEnabled();
        boolean debugEnabled = logger.isDebugEnabled();
        for (Resource resource : resources) {
            if (traceEnabled) {
                logger.trace("Scanning " + resource);
            }
            if (resource.isReadable()) {
                try {
                    MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
                    if (isCandidateComponent(metadataReader)) {
                        ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
                        sbd.setResource(resource);
                        sbd.setSource(resource);

先看这两句：

String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX + resolveBasePackage(basePackage) + "/" + this.resourcePattern;

假设我们配置的需要扫描的包名为com.geeekr.service，那么packageSearchPath的值就是classpath:com.geeekr.service//.class，意思就是com.geeekr.service包(包括子包)下所有class文件；如果配置的是，那么packageSearchPath的值就是classpath://.class。这里的表达式是Spring自己定义的。Spring会根据这种表达式找出相关的class文件。

Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);

这一句就把相关class文件加载出来了，那我们就要看看，Spring究竟是如何把class文件找到的了。首先看看resourcePatternResolver的定义:

private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();

进入getResources方法

@Override
public Resource[] getResources(String locationPattern) throws IOException {
    Assert.notNull(locationPattern, "Location pattern must not be null");
    if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) {
        // a class path resource (multiple resources for same name possible)
        if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) {
            // a class path resource pattern
            return findPathMatchingResources(locationPattern);
        }
        else {
            // all class path resources with the given name
            return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()));
        }
    }
    else {
        // Only look for a pattern after a prefix here
        // (to not get fooled by a pattern symbol in a strange prefix).
        int prefixEnd = locationPattern.indexOf(":") + 1;
        if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) {
            // a file pattern
            return findPathMatchingResources(locationPattern);
        }
        else {
            // a single resource with the given name
            return new Resource[] {getResourceLoader().getResource(locationPattern)};
        }
    }
}

这里会先判断表达式是否以classpath:开头。前面我们看到Spring已经给我们添加了这个头，这里当然符合条件了。接着会进入findPathMatchingResources方法。在这里又把/.class去掉了，然后在调用getResources方法，然后在进入findAllClassPathResources方法。这里的参数只剩下包名了例如com/geeekr/service/。

protected Resource[] findAllClassPathResources(String location) throws IOException {
    String path = location;
    if (path.startsWith("/")) {
        path = path.substring(1);
    }
    ClassLoader cl = getClassLoader();
    Enumeration<URL> resourceUrls = (cl != null ? cl.getResources(path) : ClassLoader.getSystemResources(path));
    Set<Resource> result = new LinkedHashSet<Resource>(16);
    while (resourceUrls.hasMoreElements()) {
        URL url = resourceUrls.nextElement();
        result.add(convertClassLoaderURL(url));
    }
    return result.toArray(new Resource[result.size()]);
}

真相大白了，Spring也是用的ClassLoader加载的class文件。一路追踪，原始的ClassLoader是Thread.currentThread().getContextClassLoader();。到此为止，就拿到class文件了。
Spring会将class信息封装成BeanDefinition，然后再放进DefaultListableBeanFactory的beanDefinitionMap中。

首先来看一段代码，看过上一节的朋友肯定对这段代码并不陌生。这一段代码诠释了Spring加载bean的完整过程，包括读取配置文件，扫描包，加载类，实例化bean，注入bean属性依赖。

public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();
        }
    }
}

上一节介绍了Spring是如何加载class文件的，本节主要围绕finishBeanFactoryInitialization(beanFactory)方法，聊聊Spring是如何实例化bean的，从上面代码片段中的注解不难看出，此方法主要的任务就是实例化非懒加载的单例bean。闲话少叙，看代码。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // Instantiate all remaining (non-lazy-init) singletons.
    beanFactory.preInstantiateSingletons();
}

上面代码主要看最后一句beanFactory.preInstantiateSingletons()。

public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }
    List<String> beanNames;
    synchronized (this.beanDefinitionMap) {
        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        beanNames = new ArrayList<String>(this.beanDefinitionNames);
    }
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                        @Override
                        public Boolean run() {
                            return ((SmartFactoryBean<?>) factory).isEagerInit();
                        }
                    }, getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
            else {
                getBean(beanName);
            }
        }
    }
}

此方法首先将加载进来的beanDefinitionNames循环分析，如果是我们自己配置的bean就会走else中的getBean(beanName)，接着看。

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}

doGetBean方法内容太多，一段一段看。

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;

    // Eagerly check singleton cache for manually registered singletons.
    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);
    }

这里主要看Object sharedInstance = getSingleton(beanName)。

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

这里能看到，Spring会把实例化好的bean存入singletonObjects，这是一个ConcurrentHashMap，

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);

当然这里我们bean并未实例化过，所以这里应该也不能get出什么东西来，也就是返回null了。if子句也就不会执行了。那么接着看else子句的内容。

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.
    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);
        }
    }

这两条验证也都不会实现，接写来就是重点了。

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("Circular depends-on relationship between '" +
                        beanName + "' and '" + dependsOnBean + "'");
            }
            registerDependentBean(dependsOnBean, beanName);
            getBean(dependsOnBean);
        }
    }

    // Create bean instance.
    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 '" + 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);
        }
    }
}

在这里拿到RootBeanDefinition并check，并获得bean的依赖，并循环迭代实例化bean。例如class A依赖于class B，就会先实例化B。下面的if ... else ...就是真正实例化bean的地方。其实真正实例化bean的方法是createBean(beanName, mbd, args)，只是区分了isSingleton或isPrototype，两者的区别在于，单例的(Singleton)被缓存起来，而Prototype是不用缓存的。首先看一下createBean(beanName, mbd, args)。createBean方法中除了做了一些实例化bean前的检查准备工作外，最核心的方法就是

Object beanInstance = doCreateBean(beanName, mbd, args);

由于这个过程涉及到的代码都是一大坨，就不贴出所有代码了。

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);

首先就是创建一个bean的实例且封装到BeanWrapper中，在这里bean已经实例化了。具体的实现方法是在org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(RootBeanDefinition beanDefinition, String beanName, BeanFactory owner)中。

@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);
    }
}

在这里不难看出实例化分两种情况，如果没有无参构造器是就生成CGLIB子类，否则就直接反射成实例。

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);
    }

既然已经有了实例对象了，那么，Spring是如何将bean的属性注入到bean的呢？返回到上面的doCreateBean方法中。往下看找到populateBean(beanName, mbd, instanceWrapper);，内幕就在这里。只贴部分代码：

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) {
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }
}

这里是调用InstantiationAwareBeanPostProcessor的具体子类的ibp.postProcessPropertyValues方法注入属性。当我们使用@Resource注解的时候，具体的子类是CommonAnnotationBeanPostProcessor；如果使用的是@Autowired注解，则具体的子类是AutowiredAnnotationBeanPostProcessor。此方法内是委托InjectionMetadata对象来完成属性注入。

@Override
public PropertyValues postProcessPropertyValues(
        PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass());
    try {
        metadata.inject(bean, beanName, pvs);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}

findAutowiringMetadata方法能拿到使用了特定注解的属性(Field)、方法(Method)及依赖的关系保存到checkedElements集合<Set>里，然后再执行自己的inject方法。

public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
    Collection<InjectedElement> elementsToIterate =
            (this.checkedElements != null ? this.checkedElements : this.injectedElements);
    if (!elementsToIterate.isEmpty()) {
        boolean debug = logger.isDebugEnabled();
        for (InjectedElement element : elementsToIterate) {
            if (debug) {
                logger.debug("Processing injected method of bean '" + beanName + "': " + element);
            }
            element.inject(target, beanName, pvs);
        }
    }
}

真正干事的还是InjectedElement的inject方法。

@Override
protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
    Field field = (Field) this.member;
    try {
        Object value;
        if (this.cached) {
            value = resolvedCachedArgument(beanName, this.cachedFieldValue);
        }
        else {
            DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
            desc.setContainingClass(bean.getClass());
            Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
            TypeConverter typeConverter = beanFactory.getTypeConverter();
            value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
            synchronized (this) {
                if (!this.cached) {
                    if (value != null || this.required) {
                        this.cachedFieldValue = desc;
                        registerDependentBeans(beanName, autowiredBeanNames);
                        if (autowiredBeanNames.size() == 1) {
                            String autowiredBeanName = autowiredBeanNames.iterator().next();
                            if (beanFactory.containsBean(autowiredBeanName)) {
                                if (beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                    this.cachedFieldValue = new RuntimeBeanReference(autowiredBeanName);
                                }
                            }
                        }
                    }
                    else {
                        this.cachedFieldValue = null;
                    }
                    this.cached = true;
                }
            }
        }
        if (value != null) {
            ReflectionUtils.makeAccessible(field);
            field.set(bean, value);
        }
    }
    catch (Throwable ex) {
        throw new BeanCreationException("Could not autowire field: " + field, ex);
        }
    }
}

其实别看代码这么多，最关键的部分就是：

if (value != null) {
    ReflectionUtils.makeAccessible(field);
    field.set(bean, value);
}

在这里也就真相大白了，就是通过JDK反射特性，直接set值的。