1. MVC架构

1.1 FrontController: DispatcherServlet

​ FrontController就是Spring提供的DispatcherServlet , 而不是Spring提供的@Controller，xml配置文件需要指定Tomcat启动时优先加载该Servlet映射路径写/，转发所有请求

 	<servlet>
        <servlet-name>dispatcherServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!--tomcat启动的时候, 优先加载这个Servlet-->
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcherServlet</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

​ DispatcherServlet本质上是一个HttpServlet

追踪HttpServlet.service() 方法的调用链

• FrameworkServlet 继承父类的HttpServlet.service，Spring的拓展实现是processRequest

  FrameworkServlet.processRequest，==Spring会拦截所有类型的请求，包括HttpServlet里没有定义的，==继续追踪processRequest的实现

  	@Override
  	protected void service(HttpServletRequest request, HttpServletResponse response) {
  		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
  		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
  			processRequest(request, response);
  		}
  		else {
  			super.service(request, response);
  		}
  	}
  

• FrameworkServlet.processRequest组合了doService逻辑

  	protected final void processRequest(HttpServletRequest request, HttpServletResponse response) {
  		try {
  			doService(request, response);
  		}
          // 留作后文研究
  		publishRequestHandledEvent(request, response, startTime, failureCause);
  		}
  	}
  

• FrameworkServlet的抽象方法doService 是留给子类必须实现的，也就是DispatcherServlet.doService()

  protected abstract void doService(HttpServletRequest request, HttpServletResponse response)；
  

     /**
   	* DispatcherServlet.doService()
  	*
   	*/
  	@Override
  	protected void doService(HttpServletRequest request, HttpServletResponse response) {		
          // 省略数据准备
  		try {
  			doDispatch(request, response);
  		}
  	}
  

追踪FrameworkServlet.doPost() / doGet() …

• FrameworkServlet.doGet/doPost/..无一例外使用processRequest的实现，也就是DispatcherServlet.doService

探究FrameworkServlet.service() 模板方法实现

• super.service()

  利用了模板设计模式，Spring 兼容了PATCH的实现，其他交给HttpServlet的模板service()

  	@Override
  	protected void service(HttpServletRequest request, HttpServletResponse response)
  			throws ServletException, IOException {
  
  		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
  		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
  			processRequest(request, response);
  		}
  		else {
  			super.service(request, response);
  		}
  	}
  

• HttpServlet.service() 模板，无具体实现。由于FrameworkServlet.doGet/doPost..都已经覆盖了

  HttpServlet的实现，一个Get请求经过DispatcherServlet后，有以下过程

  FrameworkServlet.service() -> super.service() -> FrameworkServlet.doGet()

  -> FrameworkServlet.processRequest() -> DispatcherServlet.doService

  -> DispatcherServlet.doDispatch()

     /**
  	* super.service() 最总会调用到 HttpServlet 的模板方法
  	*
  	*/
  	protected void service(HttpServletRequest req, HttpServletResponse resp) {
          String method = req.getMethod();
          if (method.equals(METHOD_GET)) {
  			doGet(req, resp);
          } else if (method.equals(METHOD_HEAD)) {
              long lastModified = getLastModified(req);
              maybeSetLastModified(resp, lastModified);
              doHead(req, resp);
          } else if (method.equals(METHOD_POST)) {
              doPost(req, resp);     
          } else if (method.equals(METHOD_PUT)) {
              doPut(req, resp);   
          } else if (method.equals(METHOD_DELETE)) {
              doDelete(req, resp); 
          } else if (method.equals(METHOD_OPTIONS)) {
              doOptions(req,resp);
          } else if (method.equals(METHOD_TRACE)) {
              doTrace(req,resp);
          }
      }
  

小结

DispatcherServlet 作为MVC架构中的前端控制器，也是常说的核心控制器, 通过HttpServlet.service()的模板模式把请求 转发给Spring ，DispatcherServlet.doService() 后续解析,

关于DispatcherServlet的其他父类, 网上收集到的作用：

1. HttpServletBean，是FrameworkServlet的父类

   主要做一些初始化的工作，将web.xml中配置的参数设置到Servlet中。比如servlet标签的子标签init-param标签中配置的参数。

2. FrameworkServlet

   将Servlet与Spring容器上下文关联。其实也就是初始化FrameworkServlet的属性webApplicationContext，这个属性代表SpringMVC上下文，它有个父类上下文ServletContext，既web.xml中配置的ContextLoaderListener监听器初始化的容器上下文。

3. DispatcherServlet (后文会提到)

   初始化各个功能的实现类。比如异常处理、视图处理、请求映射处理等。
   

2. MVC交互流程

2.1 Controller: DispatcherServlet.doService()

追踪 DispatcherServlet.doDispatch()

doService() 内部调用的是 doDispatch()

	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) {
    	HttpServletRequest processedRequest = request;
        // Determine handler for the current request.(追踪)
		HandlerExecutionChain mappedHandler = getHandler(processedRequest)
		// Determine handler adapter for the current request.(追踪)
		HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

		// Actually invoke the handler.(追踪)
		mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
		applyDefaultViewName(processedRequest, mv);
		mappedHandler.applyPostHandle(processedRequest, response, mv);	
	}

追踪 DispatcherServlet.getHandler()

	protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		for (HandlerMapping hm : this.handlerMappings) {
			HandlerExecutionChain handler = hm.getHandler(request);
			if (handler != null) {
				return handler;
			}
		}
		return null;
	}

Handler 是根据 HandlerMapping 获取的，对应Controller的@RequestMapping

追踪 DispatcherServlet. getHandlerAdapter()

	protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
		for (HandlerAdapter ha : this.handlerAdapters) {
			if (ha.supports(handler)) {
				return ha;
			}
		}
	}

追踪 HandlerAdapter.handle()

ModelAndView handle(HttpServletRequest request, HttpServletResponse response,
                    Object handler) throws Exception;

在DispatcherServlet中找到ModelAndView 的返回值出处

追踪 HandlerAdapter 实现类 SimpleControllerHandlerAdapter

	public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {

		return ((Controller) handler).handleRequest(request, response);
	}

追踪 Controller 实现

public interface Controller {
	@Nullable
	ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) throws Exception;

}

最终找到了 Controller 返回MV的代码, 也印证了DispatcherServlet.doDispatcher 最终会返回 Controller 拼装而成的 ModelAndView

2.2 Model And View

不同的视图解析器决定如何处理解析 视图名，所以SpringMVC需要配置视图解析器

public interface ViewResolver {
	@Nullable
	View resolveViewName(String viewName, Locale locale) throws Exception;
}

3. 总结

DispatcherServlet 会寻找 HandlerMapping 和 HandlerAdaptor, 解析视图的时候会用到 ViewResolver

如果手动集成SSM，SpringMVC需要手动注入这三个bean。值得一提的是，XML配置如果开启了注解支持，HandlerMapping 和 HandlerAdaptor也随之加载。具体可以见以下图