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SpringMVC 流程分析
一. 常用web.xml配置

<web-app version="2.4"    
    xmlns="http://java.sun.com/xml/ns/j2ee"         
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
    http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

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

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

    
    <servlet>
        <servlet-name>dispatcherservlet-name>
        <servlet-class>
            org.springframework.web.servlet.**DispatcherServlet**
        servlet-class>
        <load-on-startup>1load-on-startup>
    servlet>

    <servlet-mapping>
        <servlet-name>dispatcherservlet-name>
        <url-pattern>/url-pattern>
    servlet-mapping>

    <servlet-mapping>
        <servlet-name>defaultservlet-name>
        <url-pattern>*.htmlurl-pattern>
    servlet-mapping>
web-app>

web.xml配置了ContextLoaderListener，contextConfigLocation(classpath:spring/applicationContext.xml)和DispatcherServlet。

二. 服务器启动阶段（DispatcherServlet初始化阶段）
1. 启动后ContextLoaderListener首先会在初始化方法contextInitialized(ServletContextEvent event)中调用
initWebApplicationContext(event.getServletContext())来在创建一个上下文WebApplicationContext，
并将其作为ServletContext的一个属性设置到Servlet环境中。
这个上下文对象为根上下文对象(root WebApplicationContext)。

ContextLoaderListener类代码如下：

//ContextLoaderListener继承自ContextLoader，实现了ServletContextListener
//Bootstrap listener to start up and shut down Spring's root WebApplicationContext.
public class ContextLoaderListener extends ContextLoader  implements ServletContextListener{
   ……
   //Initialize the root web application context.
   //ServletContextEvent：This is the event class(事件类) for notifications about changes to the servlet context of a web application.
   pulic void contextInitialized(ServletContextEvent event) {
     this.contextLoader = createContextLoader();
     if (this.contextLoader == null) {
           this.contextLoader = this;
     }
   this.contextLoader.initWebApplicationContext(event.getServletContext()); 
 }
 ……
}

ContextLoader类代码如下：

/**
* Initialize Spring's web application context for the given servlet context,
* using the application context provided at construction time, or creating a new one
* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and
* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.
* @param servletContext current servlet context
* @return the new WebApplicationContext
* @see #ContextLoader(WebApplicationContext)
* @see #CONTEXT_CLASS_PARAM
* @see #CONFIG_LOCATION_PARAM
*/
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) {
            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;
    }catch (RuntimeException ex) {
        logger.error("Context initialization failed", ex);
        servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
            throw ex;
    }catch (Error err) {
        logger.error("Context initialization failed", err);
        servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
            throw err;
    }
}

1. DispatcherServlet初始化过程中initServletBean()也会创建一个WebApplicationContext对象，并将ContextLoaderListener创建的上下文对象作为父上下文，同样也会将其作为ServletContext的一个属性。

public abstract class FrameworkServlet extends HttpServletBean {
    protected final void initServletBean() throws ServletException {
        this.webApplicationContext = **initWebApplicationContext**();
        **initFrameworkServlet**();
    }

    /**
    * Initialize and publish the WebApplicationContext for this servlet.
    * 
Delegates to {@link #createWebApplicationContext} for actual creation
    * of the context. Can be overridden in subclasses.
    * @return the WebApplicationContext instance
    * @see #FrameworkServlet(WebApplicationContext)
    * @see #setContextClass
    * @see #setContextConfigLocation
    */
    protected WebApplicationContext **initWebApplicationContext**() {
        WebApplicationContext rootContext =         WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                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 -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }

        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }

        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }
        return wac;
    }

}