Context层级的问题，后面文章应该曾经说分明了。
只不过，后面文章是以注解形式举例说明的，通过配置形式怎么体现Context层级呢？有必要也说一下，毕竟当初很多我的项目都是基于xml配置实现的。
web.xml
基于配置的Spring MVC的入口就是web.xml文件，毕竟web.xml是基于web的利用的先人入口……
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
id="WebApp_ID" version="3.1">
<!--1、启动Spring的容器 -->
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:applicationContext.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>肯定要搞清楚的是，context-param配置是用来指定的Spring容器的配置文件所在门路的。必须是和ContextLoaderListener一起配合起作用的。ContextLoaderListener读取context-param的配置来实现Spring IoC容器的初始化的。
Spring IoC容器和Spring MVC的Servlet Web ApplicationContext容器不是必须要离开的，也能够配置为同一个容器。
比方以上配置不指定（不配置Spring IoC容器），也就是去掉contextConfigLocation以及ContextLoaderListener，让Spring MVC的容器担负起Spring IoC容器的职责，也是能够的。
web.xml上面持续配置spring MVC的xml文件所在门路，DispathcerServlet初始化的时候会读取。
<!--2、springmvc的前端控制器，拦挡所有申请 -->
<servlet>
<servlet-name>dispatcherServlet</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:springmvc.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>dispatcherServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>Spring-mvc.xml
名字是能够在web.xml文件中指定的，不指定的话默认就是dispatcherServlet名-dispatcher.xml（须要读一下SpringMVC源码确认）。
如果你想要Spring IoC容器和Spring MvC的web applicationContext容器离开的话，就在Spring-mvc.xml文件中指定包扫描门路仅扫描controller，否则，全扫描即可。反之，Spring Ioc容器存在的话，配置文件的扫描门路也要排除掉Controller的扫描：
<context:component-scan base-package="org.example.service">
<context:exclude-filter type="annotation"
expression="org.springframework.stereotype.Controller" />
</context:component-scan>Servlet和根容器的关系
下图高深莫测的阐明了两者之间的关系：
Servlet容器寄存Controller、VIewResolver、HanderMapping等DispatcherServlet的相干对象，根容器能够寄存其余Service、Repositories等对象。
一个DispatcherServlet能够对应的有一个Servlet WebApplicationContext容器，一个Web利用能够有多个DispatcherServlet（这种利用其实比拟少见），所以一个利用能够有多个Servlet WebApplicationContext容器。然而个别状况下，即便有多个Servlet容器，一个利用也心愿只有一个根容器，以便在不同的Servlet容器之间共享根容器的对象。
根容器初始化过程
xml配置文件的状况下，根容器要依附ContextLoaderListener来初始化。ContextLoaderListener是Spring MVC实现的ServletContextListener接口的实现类，ServletContextListener是Java Servlet的接口。Servlet规范约定，在Servlet容器初始化的过程中，会回调ServletContextListener接口的contextInitialized办法。
所以如果咱们在web.xml文件中配置了ContextLoaderListener，那么，Tomcat在Servlet容器初始化的过程中就会回调ContextLoaderListener的contextInitialized办法：
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}该办法会调用initWebApplicationContext办法，这个办法咱们在后面文章中其实曾经剖析过了，咱们再来看一下：
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!");
}
servletContext.log("Initializing Spring root WebApplicationContext");
Log logger = LogFactory.getLog(ContextLoader.class);
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) {
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);
}
}
...省略n行代码最终会调用到configureAndRefreshWebApplicationContext办法：
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);
}
...configureAndRefreshWebApplicationContext办法会读取web.xml中contextConfigLocation配置，Spring IoC容器的初始化配置文件applicationContext.xml文件名、曾经所在位置就是在这儿被读入的。
读入配置文件信息之后，调用refresh办法：
// 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);
wac.refresh();
}refresh办法是Spring framework的办法，具体就不在这儿深入研究了，咱们只是简略跟中一下配置文件的加载过程：
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();
...在refresh办法的obtainFreshBeanFactory()办法中：
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
return getBeanFactory();
}持续跟踪refreshBeanFactory();办法：
@Override
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
loadBeanDefinitions(beanFactory);
this.beanFactory = beanFactory;
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}持续跟踪，loadBeanDefinitions(beanFactory);办法，会调用到XmlWebApplicationContext类中：
@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(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);
}最初一个办法调用，loadBeanDefinitions办法：
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
String[] configLocations = getConfigLocations();
if (configLocations != null) {
for (String configLocation : configLocations) {
reader.loadBeanDefinitions(configLocation);
}
}
}终于发现了依据configLocation通过loadBeanDefinitions办法读取配置文件、调用beandefinition的代码逻辑。
能够发现代码是能够反对多个配置文件的！
从代码的角度，咱们也实现了xml配置形式下，SpringMVC与spring framework集成，实现spring Ioc容器的初始化过程！
与后面两篇文:Spring MVC 四：Context层级/Spring MVC 五 – Spring MVC的配置和DispatcherServlet初始化过程：Context层级联合,咱们就实现了基于注解的、以及基于xml配置文件两种形式下的Spring MVC框架下，Spring IoC容器的初始化代码的剖析！
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