spring 在web中启动过程

        在web环境中集成spring环境，也就是在tomcat启动过程中初始化spring容器，tomcat提供ServletContextListener监听类，spring的ServletContextListener继承了他。使用我们一般在web中是这样配置的。

//配置spring配置文件路径

<context-param>

     //名字要取contextConfigLocation
        <param-name>contextConfigLocation</param-name>
        <param-value>
            classpath:/spring/*-config.xml
        </param-value>
    </context-param>

//配置监听

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

接下来我们看下ContextLoaderListener这个类是如何初始化spring容器的，ContextLoaderListener从写了下面的方法

public void contextInitialized(ServletContextEvent event) {
        this.contextLoader = createContextLoader();
        if (this.contextLoader == null) {
            this.contextLoader = this;
        }
        this.contextLoader.initWebApplicationContext(event.getServletContext());
    }

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 {
			// Determine parent for root web application context, if any.
			ApplicationContext parent = loadParentContext(servletContext);

		//这里初始化spring容器，默认是ConfigurableWebApplicationContext
			this.context = createWebApplicationContext(servletContext, parent);
               //把容器放到了servletContext属性里，key是WebApplicationContext.root
                 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;
		}
	}

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

		// Assign the best possible id value.
		String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
		if (idParam != null) {
			wac.setId(idParam);
		}
		else {
			// Generate default id...
			if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
				// Servlet <= 2.4: resort to name specified in web.xml, if any.
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
						ObjectUtils.getDisplayString(sc.getServletContextName()));
			}
			else {
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
						ObjectUtils.getDisplayString(sc.getContextPath()));
			}
		}

		wac.setParent(parent);
		wac.setServletContext(sc);
                  //这里的CONFIG_LOCATION_PARAM就是上面配置的contextConfigLocation名字
		wac.setConfigLocation(sc.getInitParameter(CONFIG_LOCATION_PARAM));
		//spring 提供入口可修改容器
               customizeContext(sc, wac);
                //初始化容器
		wac.refresh();
		return wac;
	}