1.web创建IOC容器

web服务器（tomcat）启动会加载web.xml（启动ContextLoaderListener监听器）：

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }


    /**　　
     * serveltContext域对象创建的时候调用这个方法
     * Initialize the root web application context.
     */
    @Override
    public void contextInitialized(ServletContextEvent event) {　
        //初始化web环境中的spring容器
        initWebApplicationContext(event.getServletContext());
    }


    /**
     * Close the root web application context.
     */
    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }

}

当我们点击initWebApplicationContext方法中的时候会发现以下源码 这个方法在contextLoader之中 也就是Contextloader创建spring容器并初始化spring bean实例

try {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {　　　　　　　　　　//这一步就是创建spring容器
                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);
                    }//spring 容器初始化单例bean的实例
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }

configureAndRefreshWebApplicationContext 方法中调用最终初始化Bean的refresh方法

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
        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()));
            }
        }

        wac.setServletContext(sc);
        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);
        }

        customizeContext(sc, wac);
//不管是web还是java环境都是会调用这个方法实例化bean
        wac.refresh();
    }

如果web.xml中没有contextClass的元素,那么spring会找到自己的contextLoad.properties文件来加载

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.

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

2.IOC容器如何加载bean对象

跟着上面的refresh()方法我们来到他的源码看看

源码来自AbstractApplicationContext

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();
　　　　　　　　//1. 创建真正的bean容器 ConfiurabaleBeanFactroy 
   　　　　　　 //2.加载beandefiniton（描述要初始化的Bean的信息）

              //3.将beandefiniton注册到BeanDefitionRegistry
 
            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);
       　　　　　　//执行实现了BeanFactoryPostProcessor接口的Bean
                  //比如PropertyPlaceHolderConfigurer（context:property-placeholer）就是此处被调用的，替换掉BeanDefition中的占位符（${}）中的内容
// Invoke factory processors registered as beans in the context.　　　　　　　　　//针对beandifition实例进行操作
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.　　　　　　　　//主要针对bean实例进行操作的的,             //比如容器自动装载了一个AutowiredAnnotationBeanPostProcessor后置处理器（实现@Autowired注解功能）
                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.　　　　　　　　　　　　　　　　//初始化非懒加载方式的单例bean实例 自定的的java类 compent 之类的　　　　　　　　　　这里的单例可以用过 scope作用域来进行自定义 
                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();
            }
        }
    }