详述ContextLoaderListener监听器

通过监听器对DispatcherServlet进行spring文件配置：

对web.xml配置一下内容

...省略标签...
	cassini
	
		index.jsp
	
	
	
	
		org.springframework.web.context.ContextLoaderListener
	
	
	
	
		contextConfigLocation
		classpath:application.xml
	
	
	
		DispatcherServlet
		org.springframework.web.servlet.DispatcherServlet
		
			contextAttribute
		org.springframework.web.context.WebApplicationContext.ROOT
		
		1
	
	
		DispatcherServlet
		*.do
	
...省略标签...

    1、配置+中的contextConfigLocation属性的作用

        org.springframework.web.context.ContextLoaderListener监听器在项目发布时会优先执行，从而执行一下方法

public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

        initWebApplicationContext(event.getServletContext())便可以将contextConfigLocation中的对应配置文件生成相应的IOC容器并将其信息存储（setAttribute）在key为org.springframework.web.context.WebApplicationContext.ROOT的变量中，一下是initWebApplicationContext方法

	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;
		}
	}

    找到其中的

 

if (this.context == null) {
				this.context = createWebApplicationContext(servletContext);
			}

此处创建Ioc容器

configureAndRefreshWebApplicationContext(cwac, servletContext);

此处为将contextConfigLocation中的配置文件实现

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

此处将其保存在org.springframework.web.context.WebApplicationContext.ROOT中

之后由于org.springframework.web.servlet.DispatcherServlet配置了load-on-startup，所以Tomcat发布Web应用时依次执行init方法—>initServletBean()方法—>执行initWebApplicationContext()方法，期待码如下：

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;
	}

 

    其中的找到

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();
		}

        即使是将存在于org.springframework.web.context.WebApplicationContext.ROOT中的IOC对象与服务器相对应，之后执行

WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);

方法后成功为DispatcherServlet进行spring文件配置

 

 

注：

	cassini
	
		index.jsp
	
	
	
		org.springframework.web.context.ContextLoaderListener
	
    
	
		contextConfigLocation
		classpath:root-context.xml
	
    
	
		DispatcherServlet
		org.springframework.web.servlet.DispatcherServlet
		
			contextConfigLocation
			classpath:servlet-context.xml
		
		1
	
	
		DispatcherServlet
		*.do
	

采用以上配置可以将spring配置合理化，其中root-context.xml实例化Service层和Dao层，用于数据库相关配置，而

servlet-context.xml：该文件仅用于实例化Controller层，用于视图解析器等SpringMVC相关配置；

其中执行的代码为initWebApplicationContext()中的

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