一般我们会在web.xml文件中配置DispatcherServlet，比如如下配置方式：

<servlet>
        <servlet-name>dispatcherServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:dispatcher-servlet.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>dispatcherServlet</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

上述代码配置了所有路径的http请求都会交给DispatcherServlet来处理

那么我们来看下DispatcherServlet的具体代码：

@SuppressWarnings("serial")
public class DispatcherServlet extends FrameworkServlet {
	public DispatcherServlet(WebApplicationContext webApplicationContext) {
		super(webApplicationContext);
		setDispatchOptionsRequest(true);
	}

	@Override
	protected void onRefresh(ApplicationContext context) {
		initStrategies(context);
	}

	/**
	 * Initialize the strategy objects that this servlet uses.
	 * <p>May be overridden in subclasses in order to initialize further strategy objects.
	 */
	protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}


	@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		
	}


	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		
	}

可以看到这个类继承了
FrameworkServlet类，再来看下这个类的代码：

@SuppressWarnings("serial")
public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {

	@Override
	protected final void initServletBean() throws ServletException {
		
	}

	protected WebApplicationContext initWebApplicationContext() {

		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);
		}

		
	}
	@Override
	public void destroy() {
		getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");
		// Only call close() on WebApplicationContext if locally managed...
		if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {
			((ConfigurableApplicationContext) this.webApplicationContext).close();
		}
	}


	/**
	 * Override the parent class implementation in order to intercept PATCH requests.
	 */
	@Override
	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (HttpMethod.PATCH == httpMethod || httpMethod == null) {
			processRequest(request, response);
		}
		else {
			super.service(request, response);
		}
	}

	
	@Override
	protected final void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

	
	@Override
	protected final void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

	protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
	}

}

可以看到这个类继承了HttpServletBean类，而HttpServletBean类又继承了HttpServlet类，其中HttpServlet类的init，destroy，service三个方法负责管理Servlet的生命周期。

init方法：负责初始化Servlet对象

destroy方法：负责销毁Servlet对象

service方法：负责处理http请求

由于HttpServletBean实现了HttpServlet的init方法，可以看到该方法的代码如下：

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

		// Set bean properties from init parameters.
		// 获取servlet初始化的参数,对Bean属性进行配置
		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, getEnvironment()));
			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.
		// 调用子类的initServletBean进行具体的初始化
		initServletBean();

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

这里会读取配置在ServletContext中的Bean属性参数，这里可以看到对PropertyValues和BeanWrapper的使用，这里就完成了对Servlet的初始化

接着调用了initServletBean方法来初始化DispatcherServlet持有的IOC容器，当然底层其实是调用initWebApplicationContext方法来初始化，其代码如下：

protected WebApplicationContext initWebApplicationContext() {
		// 得到根上下文,这个根上下文是保存在ServletContext中的,使用这个根上下文作为当前MVC上下文的双亲上下文,当前的上下文就是wac
		// cwac.setParent(rootContext)就是设置当前上下文的根上下文
		WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());
		WebApplicationContext wac = null;


		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);
		}
	}

可以看到createWebApplicationContext方法就是在创建并初始化IOC容器，代码如下：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
		
		//设置IOC容器的各个参数
		wac.setEnvironment(getEnvironment());
		wac.setParent(parent);
		wac.setConfigLocation(getContextConfigLocation());

		configureAndRefreshWebApplicationContext(wac);

		return wac;
	}

这里设置了当前IOC容器上下文的双亲上下文，双亲上下文是如下方法获取到的：

WebApplicationContext rootContext =
				WebApplicationContextUtils.getWebApplicationContext(getServletContext());

这样当从IOC容器中getBean的时候会先从其双亲上下文中获取bean

configureAndRefreshWebApplicationContext方法里面调用了wac.refresh()，也就是refresh方法来真正启动对IOC容器的初始化，这里就跟IOC容器的初始化保持一致了

到这里，对DispatcherServlet所持有的IOC容器的初始化就完成了，初始化完成后，DispatcherServlet就持有一个以自己的Servlet名称命名的IOC容器了，这个IOC容器是一个WebApplicationContext对象。

我们还可以看到initWebApplicationContext方法中还调用了onrefresh方法，该方法是由DispatcherServlet来实现的，
并最终调用了DispatcherServlet的如下方法来进行初始化：

protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

在这个方法里，DispatcherServlet对MVC模块的其他部分进行了初始化，比如handlerMapping，ViewResolver等，它是启动整个Spring MVC框架的初始化

比如，支持国际化的LocalResolver，支持request映射的HandlerMapping，以及视图生成的ViewResolver等的初始化

这里的initHandlerMapping是为HTTP请求找到相应的Controller控制器。

DispatcherServlet的整个初始化过程就到这里结束了，下面说下DispatcherServlet是如何处理http请求的

首先http请求是由doService来处理的，那么我们可以看下HttpServlet类的该方法：

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long errMsg;
        if(method.equals("GET")) {
            errMsg = this.getLastModified(req);
            if(errMsg == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader("If-Modified-Since");
                if(ifModifiedSince < errMsg) {
                    this.maybeSetLastModified(resp, errMsg);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if(method.equals("HEAD")) {
            errMsg = this.getLastModified(req);
            this.maybeSetLastModified(resp, errMsg);
            this.doHead(req, resp);
        } else if(method.equals("POST")) {
            this.doPost(req, resp);
        } else if(method.equals("PUT")) {
            this.doPut(req, resp);
        } else if(method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if(method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if(method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg1 = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg1 = MessageFormat.format(errMsg1, errArgs);
            resp.sendError(501, errMsg1);
        }

    }

这个方法里面会根据不同的请求类型，来调用不同的处理方法，比如会调用doGet或者doPost方法来分别处理get请求和post请求，跟踪这些方法，可以发现，最终是调用了DispatcherServlet的doService方法，代码如下：

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        try {
            this.doDispatch(request, response);
        } finally {
            if(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot1 != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot1);
            }

        }

    }

可以看到该方法调用了doDispatch来进行处理，doDispatch是MVC模式的主要部分