spring DispatcherServlet源码解析

DispatcherServlet继承FrameworkServlet，FrameworkServlet继承HttpServletBean，HttpServletBean继承HttpServlet，Web容器（比如Resin、Tomcat）启动时是调用HttpServlet的init方法初始化Servlet，HttpServletBean重写了init方法：

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

		// Set bean properties from init parameters.
		try {
			PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, this.environment));
			initBeanWrapper(bw);
			bw.setPropertyValues(pvs, true);
		}
		catch (BeansException ex) {
			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");
		}
	}

 FrameworkServlet重写了initServletBean方法：

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

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

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

 在initWebApplicationContext方法会从ServletContext中获取由ContextLoadListener加载的WebApplicationContext对象，然后调用createWebApplicationContext方法实例化一个XmlWebApplicationContext，并将Listener加载的context作为该context的parent：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
		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 + "]");
		}
		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");
		}
		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

		wac.setParent(parent);
		wac.setConfigLocation(getContextConfigLocation());

		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...
				ServletContext sc = getServletContext();
				if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
					// Servlet <= 2.4: resort to name specified in web.xml, if any.
					String servletContextName = sc.getServletContextName();
					if (servletContextName != null) {
						wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + servletContextName +
								"." + getServletName());
					}
					else {
						wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + getServletName());
					}
				}
				else {
					// Servlet 2.5's getContextPath available!
					wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
							ObjectUtils.getDisplayString(sc.getContextPath()) + "/" + getServletName());
				}
			}
		}

		wac.setServletContext(getServletContext());
		wac.setServletConfig(getServletConfig());
		wac.setNamespace(getNamespace());
		wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

		postProcessWebApplicationContext(wac);

		applyInitializers(wac);

		wac.refresh();
	}

 在这个方法中又看到了熟悉的refresh方法，下面的过程跟IOC的初始化过程一样。

在该方法中设置了namespace属性，而namespace属性的默认值是’servletName‘-servlet，IOC初始化中会去加载资源文件：

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
		Resource[] configResources = getConfigResources();
		if (configResources != null) {
			reader.loadBeanDefinitions(configResources);
		}
		String[] configLocations = getConfigLocations();
		if (configLocations != null) {
			reader.loadBeanDefinitions(configLocations);
		}
	}

protected String[] getConfigLocations() {
		return (this.configLocations != null ? this.configLocations : getDefaultConfigLocations());
	}

protected String[] getDefaultConfigLocations() {
		if (getNamespace() != null) {
			return new String[] {DEFAULT_CONFIG_LOCATION_PREFIX + getNamespace() + DEFAULT_CONFIG_LOCATION_SUFFIX};
		}
		else {
			return new String[] {DEFAULT_CONFIG_LOCATION};
		}
	}

 从这个流程可以看出如果没有显示指定contextConfigLocation，就会使用namespace的值作为配置文件来加载。

通过DispatcherServlet创建的context最终会放到ServletContext的attribute中，key是FrameworkServlet.class.getName() + ".CONTEXT." + getServletName()