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DispatcherServlet初始化

说起springmvc我们自然会第一个想到，配置一个springmvc的web项目肯定是要先在web.xml中配置一个叫dispatcherServlet的servlet，servlet此处不做过多讨论，只是我们知道，servlet在web项目启动时会先调用他的init方法，我们顺着这个思路往下看，spring的这个dispatcherservlet他就是一个servlet。以下是他在spring中的uml图。他继承自FrameworkServlet，FrameworkServlet继承自HttpServletBean，再向上便是继承自HttpServlet，那他作为一个servlet自然会在项目启动时调用init方法，我们找到他的init() 方法。看一下他是如何一步一步初始化spring的。

init方法在HttpServletBean中我们，首先他在初始化是先加载web.xml中 dispatcherservlet所在servlet对应的所有的标签的参数，整个spring初始化的init中initServletBean（）最为关键。

public final void init() throws ServletException {
		if (logger.isDebugEnabled()) {
			logger.debug("Initializing servlet '" + getServletName() + "'");
		}

		// Set bean properties from init parameters.
		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
                //创建一个bean包装类BeanWrapperImpl
				BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                //用当前上下文创建ResourceLoader资源加载器
				ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                //bw中注册一个用StandardServletEnvironment 创建的ResourceEditor
				bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                //初始化bw用用户自己实现
				initBeanWrapper(bw);

				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				if (logger.isErrorEnabled()) {
					logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
				}
				throw ex;
			}
		}

		// Let subclasses do whatever initialization they like.
		initServletBean();

		if (logger.isDebugEnabled()) {
			logger.debug("Servlet '" + getServletName() + "' configured successfully");
		}
	}

private static class ServletConfigPropertyValues extends MutablePropertyValues {

		public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties)
				throws ServletException {

			Set<String> missingProps = (!CollectionUtils.isEmpty(requiredProperties) ?
					new HashSet<>(requiredProperties) : null);
            //从servlet所对应的servletconfig中获取所有的<init-param>标签的内容 并将其包装成PropertyValue添加到List<PropertyValue> propertyValueList中
			Enumeration<String> paramNames = config.getInitParameterNames();
			while (paramNames.hasMoreElements()) {
				String property = paramNames.nextElement();
				Object value = config.getInitParameter(property);
				addPropertyValue(new PropertyValue(property, value));
				if (missingProps != null) {
					missingProps.remove(property);
				}
			}

			// Fail if we are still missing properties.
			if (!CollectionUtils.isEmpty(missingProps)) {
				throw new ServletException(
						"Initialization from ServletConfig for servlet '" + config.getServletName() +
						"' failed; the following required properties were missing: " +
						StringUtils.collectionToDelimitedString(missingProps, ", "));
			}
		}
	}

此处关键在于初始化webApplicationContext 以下我们看下初始化的流程。

@Override
	protected final void initServletBean() throws ServletException {
		getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
		if (logger.isInfoEnabled()) {
			logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
		}
		long startTime = System.currentTimeMillis();

		try {
			this.webApplicationContext = initWebApplicationContext();
			initFrameworkServlet();
		}
		catch (ServletException | RuntimeException ex) {
			logger.error("Context initialization failed", ex);
			throw ex;
		}

		if (logger.isInfoEnabled()) {
			long elapsedTime = System.currentTimeMillis() - startTime;
			logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
					elapsedTime + " ms");
		}
	}
//////////////////////////////////////////

protected WebApplicationContext initWebApplicationContext() {
        //从servlet上下文中查找是否有叫做org.springframework.web.context.WebApplicationContext.ROOT的根上下文
		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.
			synchronized (this.onRefreshMonitor) {
				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;
	}


	protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
		//DEFAULT_CONTEXT_CLASS XmlWebApplicationContext.class
		Class<?> contextClass = getContextClass();

		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Servlet with name '" + getServletName() +
					"' will try to create custom WebApplicationContext context of class '" +
					contextClass.getName() + "'" + ", using parent context [" + parent + "]");
		}
		//判断是不是他的继承类或实现类
		if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
			throw new ApplicationContextException(
					"Fatal initialization error in servlet with name '" + getServletName() +
					"': custom WebApplicationContext class [" + contextClass.getName() +
					"] is not of type ConfigurableWebApplicationContext");
		}
		//反射实例化XmlWebApplicationContext
		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
		//StandardServletEnvironment
		wac.setEnvironment(getEnvironment());
		//父上下文
		wac.setParent(parent);
		String configLocation = getContextConfigLocation();
		if (configLocation != null) {
			wac.setConfigLocation(configLocation);
		}
		// 初始化关键方法
		configureAndRefreshWebApplicationContext(wac);

		return wac;
	}

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
		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
			if (this.contextId != null) {
				wac.setId(this.contextId);
			}
			else {
				// Generate default id...
				wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
						ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
			}
		}

		wac.setServletContext(getServletContext());
		wac.setServletConfig(getServletConfig());
		wac.setNamespace(getNamespace());
		//添加监听器 参考事件监听设计模式
		wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

		// 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();
		//XmlWebApplicationContext 的环境 空的话默认创建一个 StandardEnvironment 环境
		//StandardEnvironment 实现是ConfigurableEnvironment接口
		//eg：StandardServletEnvironment 替换 PropertySource为ServletPropertySource
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
		}

		postProcessWebApplicationContext(wac);
		//获取当前servlet上下文 的init-param 的globalInitializerClasses
		//初始化 添加 ApplicationContextInitializer
		//遍历调用initialize()初始化方法
		applyInitializers(wac);
		//* 刷新上下文 调用abstractapplicationcontext中的refresh
		wac.refresh();
	}