基于spring 4.2.9

org.springframework.web.context.ContextLoaderListener

web.xml中都会配置

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

ContextLoaderListener.java

public class ContextLoaderListener extends ContextLoader implements ServletContextListener 

启动时会调用

@Override
    public void contextInitialized(ServletContextEvent event) {
              //这个是ContextLoader 的方法
        initWebApplicationContext(event.getServletContext());
    }

这是一个典型的 ServletContextListener，Servlet 容器（如 Tomcat 等）在启动时会找到 ContextLoaderListener 并执行其 contextInitialized(ServletContextEvent event) 方法。

org.springframework.web.context.ContextLoader

public class ContextLoader {

public static final String CONTEXT_ID_PARAM = "contextId";

    public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";


    public static final String CONTEXT_CLASS_PARAM = "contextClass";

    
    public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";

    public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";


    public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";


    public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";

    
    private static final String INIT_PARAM_DELIMITERS = ",; \t\n";

    private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
....
}

初始化 时会加载静态代码块

private static final Properties defaultStrategies;

    static {
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
//step1 
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
                      //step2 加载一个属性文件
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
    }

• step1
  private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

image.png

ContextLoader.properties 内容

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

这段代码配置了 XML 默认使用的 Context 为 org.springframework.web.context.WebApplicationContext = org.springframework.web.context.support.XmlWebApplicationContext。根据该定义，如果开发人员没有从 web.xml 指定 contextClass 参数，则默认使用 XmlWebApplicationContext 作为 root WebApplicationContext 工具类。

• step2

public static Properties loadProperties(Resource resource) throws IOException {
        Properties props = new Properties();
        fillProperties(props, resource);
        return props;
    }

public static void fillProperties(Properties props, Resource resource) throws IOException {
        InputStream is = resource.getInputStream();
        try {
            String filename = resource.getFilename();
            if (filename != null && filename.endsWith(XML_FILE_EXTENSION)) {
                props.loadFromXML(is);
            }
            else {
                props.load(is);
            }
        }
        finally {
            is.close();
        }
    }

一个普通的加载资源文件方法

XML_FILE_EXTENSION
private static final String XML_FILE_EXTENSION = ".xml";

用于加载

      <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring/spring.xml,classpath:spring/dubbo-client.xml
        </param-value>
    </context-param>

回到刚才的核心方法 initWebApplicationContext() 中去，

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        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();
                //以上没什么用
        try {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {
//step1,核心方法
                this.context = createWebApplicationContext(servletContext);
            }
....

createWebApplicationContext方法

ContextLoader .java中

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//step1 获取到这个
        Class<?> contextClass = determineContextClass(sc);
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                    "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
        }
//step2
        return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    }

*step1

protected Class<?> determineContextClass(ServletContext servletContext) {
        String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
        if (contextClassName != null) {
            try {
                return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load custom context class [" + contextClassName + "]", ex);
            }
        }
        else {
            contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
            try {
//反射获取到org.springframework.web.context.support.XmlWebApplicationContext
                return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
            }
            catch (ClassNotFoundException ex) {
                throw new ApplicationContextException(
                        "Failed to load default context class [" + contextClassName + "]", ex);
            }
        }
    }

首先determineContextClass()方法查明具体的Context类，他会读取servletContext的初始化参数contextClass，此参数我们一般不配置。

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，
*step2
转换为ConfigurableWebApplicationContext实际上还是XmlWebApplicationContext

ContextLoader -ContextLoader -initWebApplicationContext

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
        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();

        try {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {
//这个上面介绍过
                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);
                    }
                                //step1 配置并刷新Contet
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }

configureAndRefreshWebApplicationContext方法

ContextLoader  .java

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
//step1
        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...
                //step2
                wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                        ObjectUtils.getDisplayString(sc.getContextPath()));
            }
        }

        wac.setServletContext(sc);
//step3
        String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
        if (configLocationParam != null) {
            wac.setConfigLocation(configLocationParam);
        }

        // The wac environment's #initPropertySources will be called in any case when the context
        // is refreshed; do it eagerly here to ensure servlet property sources are in place for
        // use in any post-processing or initialization that occurs below prior to #refresh
        ConfigurableEnvironment env = wac.getEnvironment();
        if (env instanceof ConfigurableWebEnvironment) {
            ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
        }
  //step4
        customizeContext(sc, wac);
  //step5 核心方法
        wac.refresh();
    }

• step1
  首先determineContextClass()方法查明具体的Context类，他会读取servletContext的初始化参数contextClass，此参数我们一般不配置。

*step2
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));

对于得到的这个 (ConfigurableWebApplicationContext) XmlWebApplicationContext
然后对之中的属性进行一系列的设置，首先会设置一个默认ID，即org.springframework.web.context.WebApplicationContext:+你项目的ContextPath。

• step3

wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));
//CONFIG_LOCATION_PARAM = "contextConfigLocation"
//这里设置了你在web.xml中对于applicationContext.xml的地址配置

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

• step4
  customizeContext(sc, wac);
  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。
• step5
  wac.refresh();
  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。

org.springframework.context.support.AbstractApplicationContext -refresh方法

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

            // Tell the subclass to refresh the internal bean factory.
//step1
            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.
//step2
                finishBeanFactoryInitialization(beanFactory);

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

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

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

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

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

• step1 obtainFreshBeanFactory -org.springframework.context.support.AbstractRefreshableApplicationContext

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
//step1
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

• step1 refreshBeanFactory

    @Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
  //step1
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

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。

• step1 loadBeanDefinitions(beanFactory)--org.springframework.context.support.AbstractXmlApplicationContext

@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);
            //step1加载
        loadBeanDefinitions(beanDefinitionReader);
    }

• step1
  DefaultListableBeanFactory beanFactory = createBeanFactory();
  在这个beanFactory中有个beanDefinitionMap，此时size=0

再跟进loadBeanDefinitions(beanFactory)

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
//step1 加载配置文件
            reader.loadBeanDefinitions(configLocations);
        }
    }

• step1
  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
  这里设计了层层调用，有很多重载方法，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。

然后追踪到 org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root) 中

reader.loadBeanDefinitions(configLocations);

org.springframework.beans.factory.support.AbstractBeanDefinitionReader.java
@Override
    public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
        Assert.notNull(locations, "Location array must not be null");
        int counter = 0;
        for (String location : locations) {
//step1
            counter += loadBeanDefinitions(location);
        }
        return counter;
    }

• counter += loadBeanDefinitions(location);

org.springframework.beans.factory.support.AbstractBeanDefinitionReader
@Override
    public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(location, null);
    }

return loadBeanDefinitions(location, null);

public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
        ResourceLoader resourceLoader = getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException(
                    "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        }

        if (resourceLoader instanceof ResourcePatternResolver) {
            // Resource pattern matching available.
            try {
                Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                int loadCount = loadBeanDefinitions(resources);
                if (actualResources != null) {
                    for (Resource resource : resources) {
                        actualResources.add(resource);
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                }
                return loadCount;
            }
            catch (IOException ex) {
                throw new BeanDefinitionStoreException(
                        "Could not resolve bean definition resource pattern [" + location + "]", ex);
            }
        }
        else {
            // Can only load single resources by absolute URL.
            Resource resource = resourceLoader.getResource(location);
            int loadCount = loadBeanDefinitions(resource);
            if (actualResources != null) {
                actualResources.add(resource);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
            }
            return loadCount;
        }
    }

int loadCount = loadBeanDefinitions(resource);

org.springframework.beans.factory.support.AbstractBeanDefinitionReader.java
    @Override
    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;
    }

counter += loadBeanDefinitions(resource);

org.springframework.beans.factory.xml.XmlBeanDefinitionReader.java
@Override
    public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(new EncodedResource(resource));
    }

return loadBeanDefinitions(new EncodedResource(resource));

org.springframework.beans.factory.xml.XmlBeanDefinitionReader.java
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());
        }

        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 {
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
//step1
                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();
            }
        }
    }

• step1 -return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
  读取并加载资源文件

doLoadBeanDefinitions方法

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
Document doc = doLoadDocument(inputSource, resource);
//step1 核心方法
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);
}
}

• step1

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
//step1
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        int countBefore = getRegistry().getBeanDefinitionCount();
//step2
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

• step1
  建立默认的Bean文件读取

protected BeanDefinitionDocumentReader createBeanDefinitionDocumentReader() {
        return BeanDefinitionDocumentReader.class.cast(BeanUtils.instantiateClass(this.documentReaderClass));
    }

• step2 注册-documentReader.registerBeanDefinitions(doc, createReaderContext(resource));

org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.java

@Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        Element root = doc.getDocumentElement();
//注册整个xml
        doRegisterBeanDefinitions(root);
    }

doRegisterBeanDefinitions(root);

protected void doRegisterBeanDefinitions(Element root) {
          //step1
        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;
                }
            }
        }

        preProcessXml(root);
//xml解析和加载类
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }

• step1
  BeanDefinitionParserDelegate parent = this.delegate;
  这里创建了一个BeanDefinitionParserDelegate实例，解析XML的过程就是委托它完成的。实际上你跟进该类，可以发现里面定义了大量的常量，这些常量实际上就是我们在xml中使用到的节点和属性名。

org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.java
public class BeanDefinitionParserDelegate {

    public static final String BEANS_NAMESPACE_URI = "http://www.springframework.org/schema/beans";

    public static final String MULTI_VALUE_ATTRIBUTE_DELIMITERS = ",; ";

    /**
     * Value of a T/F attribute that represents true.
     * Anything else represents false. Case seNsItive.
     */
    public static final String TRUE_VALUE = "true";

    public static final String FALSE_VALUE = "false";

    public static final String DEFAULT_VALUE = "default";

    public static final String DESCRIPTION_ELEMENT = "description";

    public static final String AUTOWIRE_NO_VALUE = "no";

    public static final String AUTOWIRE_BY_NAME_VALUE = "byName";

    public static final String AUTOWIRE_BY_TYPE_VALUE = "byType";

    public static final String AUTOWIRE_CONSTRUCTOR_VALUE = "constructor";

    public static final String AUTOWIRE_AUTODETECT_VALUE = "autodetect";

    public static final String DEPENDENCY_CHECK_ALL_ATTRIBUTE_VALUE = "all";

    public static final String DEPENDENCY_CHECK_SIMPLE_ATTRIBUTE_VALUE = "simple";

    public static final String DEPENDENCY_CHECK_OBJECTS_ATTRIBUTE_VALUE = "objects";

    public static final String NAME_ATTRIBUTE = "name";

    public static final String BEAN_ELEMENT = "bean";

    public static final String META_ELEMENT = "meta";

    public static final String ID_ATTRIBUTE = "id";

    public static final String PARENT_ATTRIBUTE = "parent";

    public static final String CLASS_ATTRIBUTE = "class";

    public static final String ABSTRACT_ATTRIBUTE = "abstract";

    public static final String SCOPE_ATTRIBUTE = "scope";

    private static final String SINGLETON_ATTRIBUTE = "singleton";

    public static final String LAZY_INIT_ATTRIBUTE = "lazy-init";

    public static final String AUTOWIRE_ATTRIBUTE = "autowire";

    public static final String AUTOWIRE_CANDIDATE_ATTRIBUTE = "autowire-candidate";

    public static final String PRIMARY_ATTRIBUTE = "primary";

    public static final String DEPENDENCY_CHECK_ATTRIBUTE = "dependency-check";

    public static final String DEPENDS_ON_ATTRIBUTE = "depends-on";

    public static final String INIT_METHOD_ATTRIBUTE = "init-method";

    public static final String DESTROY_METHOD_ATTRIBUTE = "destroy-method";

    public static final String FACTORY_METHOD_ATTRIBUTE = "factory-method";

    public static final String FACTORY_BEAN_ATTRIBUTE = "factory-bean";

    public static final String CONSTRUCTOR_ARG_ELEMENT = "constructor-arg";

    public static final String INDEX_ATTRIBUTE = "index";

    public static final String TYPE_ATTRIBUTE = "type";

    public static final String VALUE_TYPE_ATTRIBUTE = "value-type";

    public static final String KEY_TYPE_ATTRIBUTE = "key-type";

    public static final String PROPERTY_ELEMENT = "property";

    public static final String REF_ATTRIBUTE = "ref";

    public static final String VALUE_ATTRIBUTE = "value";

    public static final String LOOKUP_METHOD_ELEMENT = "lookup-method";

    public static final String REPLACED_METHOD_ELEMENT = "replaced-method";

    public static final String REPLACER_ATTRIBUTE = "replacer";

    public static final String ARG_TYPE_ELEMENT = "arg-type";

    public static final String ARG_TYPE_MATCH_ATTRIBUTE = "match";

    public static final String REF_ELEMENT = "ref";

    public static final String IDREF_ELEMENT = "idref";

    public static final String BEAN_REF_ATTRIBUTE = "bean";
.....

org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.java

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes(); //将节点获取存入collection
            for (int i = 0; i < nl.getLength(); i++) { //对collection中存储的节点进行依次遍历
                Node node = nl.item(i); //返回当前序号的节点
                if (node instanceof Element) { //判断节点是否属于元素类（我们不需要文本型）
 //判断是否为默认的命名空间，其实就是根据配置文件的命名空间来判定
                    //如果是beans下的则认为是默认的命名空间，如果不是则认为是自定义的，我们使用的Aop、Tx等都是属于自定义标签的范畴
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
//解析
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

最终，我们可以看到解析XML的是 parseDefaultElement(ele, delegate); 方法，它会判断并调用对应的解析，

• parseDefaultElement(ele, delegate);

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);
        }
    }

再进 processBeanDefinition

    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        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));
        }
    }

把beanName放到队列里
把BeanDefinition放到map中

到此，bean的注册就完成了（当然，这里是指所有的bean都注册完）。在后面实例化的时候，就是把beanDefinitionMap中的BeanDefinition取出来，逐一实例化。

obtainFreshBeanFactory() 总算结束了，我们继续看refresh()方法中另一个核心方法，它是将bean进行实例化的重要角色。

回到 finishBeanFactoryInitialization()

经过obtainFreshBeanFactory() 这个方法，我们的beanFactory就准备好了，接下来我们主要围绕finishBeanFactoryInitialization(beanFactory)方法，
Spring是如何实例化bean的。

org.springframework.beans.factory.support.DefaultListableBeanFactory.java

    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();

        // step1
        beanFactory.preInstantiateSingletons();
    }

• step1 这个方法，就是为了实例化非懒加载的单例bean，我们走进 beanFactory.preInstantiateSingletons();

@Override
    public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Pre-instantiating singletons in " + this);
        }

        // 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.
        List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);

        // Trigger initialization of all non-lazy singleton beans...
        for (String beanName : beanNames) { //将加载进来的beanDefinitionNames循环分析
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {  //如果不是抽象类, 且是单例, 且不是延迟加载
                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);
                }
            }
        }

        // Trigger post-initialization callback for all applicable beans...
        for (String beanName : beanNames) {
            Object singletonInstance = getSingleton(beanName);
            if (singletonInstance instanceof SmartInitializingSingleton) {
                final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
                if (System.getSecurityManager() != null) {
                    AccessController.doPrivileged(new PrivilegedAction<Object>() {
                        @Override
                        public Object run() {
                            smartSingleton.afterSingletonsInstantiated();
                            return null;
                        }
                    }, getAccessControlContext());
                }
                else {
                    smartSingleton.afterSingletonsInstantiated();
                }
            }
        }
    }

• 判断这个bean是否是抽象类，是否是单例，是否延迟加载
• 如果不是抽象类, 且是单例, 且不是延迟加载，那么判断是否实现 FactoryBean 接口
• 如果实现了 FactoryBean，则 getBean(FACTORY_BEAN_PREFIX + beanName)，否则 getBean(beanName)

跟进 getBean 这个方法，发现它调用了 doGetBean 这个方法最终完成bean的实例化

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