@Controller
public class HelloController {
    @GetMapping("/hello")
    public String handle(Model model) {
        model.addAttribute("message", "Hello World!");
        return "index";
    }
}

//通过java config
@Configuration
@ComponentScan("org.example.web")
public class WebConfig {
    // ...
}

@RestController
@RequestMapping("/persons")
class PersonController {
    @GetMapping("/{id}")
    public Person getPerson(@PathVariable Long id) {
        // ...
    }
    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public void add(@RequestBody Person person) {
        // ...
    }
}

@GetMapping("/owners/{ownerId}/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
    // ...
}

@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
public void handle(@PathVariable String version, @PathVariable String ext) {
    // /spring-web-3.0.5 .jar
}

@PostMapping(path = "/pets", consumes = "application/json")
public void addPet(@RequestBody Pet pet) {
    // ...
}

@GetMapping(path = "/pets/{petId}", produces = "application/json;charset=UTF-8")
@ResponseBody
public Pet getPet(@PathVariable String petId) {
    // ...
}

@GetMapping(path = "/pets/{petId}", params = "myParam=myValue")
public void findPet(@PathVariable String petId) {
    // ...
}

@GetMapping(path = "/pets", headers = "myHeader=myValue")
public void findPet(@PathVariable String petId) {
    // ...
}

// Make framework objects available to handlers and view objects.
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
    request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
try {
    //属性设置完后将request和response交给doDispatch
    doDispatch(request, response);
}
finally {

HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
    ModelAndView mv = null;
    Exception dispatchException = null;
    try {
        processedRequest = checkMultipart(request);
        multipartRequestParsed = (processedRequest != request);
        //通过getHandler决定用哪个handler，并返回一个HandlerExecutionChain对象。
        //这个对象不仅包含handler，还提供了prehandler,posthandler,completionHandler拦截器的方法。
        mappedHandler = getHandler(processedRequest);
        if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
        }

///可以看到getHandler，实际上就是轮询哪个HandlerMapping能够处理这个request
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    for (HandlerMapping hm : this.handlerMappings) {
        if (logger.isTraceEnabled()) {
            logger.trace(
                    "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        HandlerExecutionChain handler = hm.getHandler(request);
        if (handler != null) {
            return handler;
        }
    }
    return null;
}

///那this.handlerMapping是怎么初始化的呢？就是在所有的ApplicationContext，中找到实现了HandlerMapping的beans.如果我们没特别设置，会有一些默认的handlerMapping。
private void initHandlerMappings(ApplicationContext context) {
    this.handlerMappings = null;
    if (this.detectAllHandlerMappings) {
        // 在所有的ApplicationContext，中找到实现了HandlerMapping的beans.
        Map<String, HandlerMapping> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
            // We keep HandlerMappings in sorted order.
            AnnotationAwareOrderComparator.sort(this.handlerMappings);
        }
    }
    else {
        try {
            HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
            this.handlerMappings = Collections.singletonList(hm);
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Ignore, we'll add a default HandlerMapping later.
        }
    }
    // Ensure we have at least one HandlerMapping, by registering
    // a default HandlerMapping if no other mappings are found.
    if (this.handlerMappings == null) {
        this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
        if (logger.isDebugEnabled()) {
            logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
        }
    }
}

/**
Interface that must be implemented for each handler type to handle a request.
 * This interface is used to allow the {@link DispatcherServlet} to be indefinitely
 * extensible. The {@code DispatcherServlet} accesses all installed handlers through
 * this interface, meaning that it does not contain code specific to any handler type.
 */
大概是说为了扩展性，DispatchServlet不应该和任何具体的handler直接打交道，都通过HanlderAdapter来调用。

// Determine handler adapter for the current request.
//挑选一个HandlerAdapter，所有handler的调用都通过adapter。
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

//如果prehandler返回false，直接结束掉            
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    return;
}
// 通过adapter调用handler。
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
    return;
}
applyDefaultViewName(processedRequest, mv);
//执行posthandler
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
//处理渲染，之后执行completionhandler
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
		HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
	boolean errorView = false;
	if (exception != null) {
		if (exception instanceof ModelAndViewDefiningException) {
			logger.debug("ModelAndViewDefiningException encountered", exception);
			mv = ((ModelAndViewDefiningException) exception).getModelAndView();
		}
		else {
			Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
			mv = processHandlerException(request, response, handler, exception);
			errorView = (mv != null);
		}
	}
	// Did the handler return a view to render?
	if (mv != null && !mv.wasCleared()) {
           //在render中会去根据viewname在viewresolver中找到对应的view进行render。
		render(mv, request, response);
		if (errorView) {
			WebUtils.clearErrorRequestAttributes(request);
		}
	}
	else {
		if (logger.isDebugEnabled()) {
			logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
					"': assuming HandlerAdapter completed request handling");
		}
	}
	if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
		// Concurrent handling started during a forward
		return;
	}
	if (mappedHandler != null) {
           //render完成了，执行completionhander。
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
	// Determine locale for request and apply it to the response.
	Locale locale = this.localeResolver.resolveLocale(request);
	response.setLocale(locale);
	View view;
	if (mv.isReference()) {
		// 根据viewname找到对应的view
		view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
		if (view == null) {
			throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
					"' in servlet with name '" + getServletName() + "'");
		}
	}
	else {
		// No need to lookup: the ModelAndView object contains the actual View object.
		view = mv.getView();
		if (view == null) {
			throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
					"View object in servlet with name '" + getServletName() + "'");
		}
	}
	// Delegate to the View object for rendering.
	if (logger.isDebugEnabled()) {
		logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
	}
	try {
           //渲染view
		view.render(mv.getModelInternal(), request, response);
	}
	catch (Exception ex) {
		if (logger.isDebugEnabled()) {
			logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
					getServletName() + "'", ex);
		}
		throw ex;
	}
}

public class HomeServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter writer = resp.getWriter();
        writer.print("<html>  <body>  <h1> hello home </h1> </body>  </html>");
    }
}

initMultipartResolver(context);  
initLocaleResolver(context);   
initThemeResolver(context);   
initHandlerMappings(context);   
initHandlerAdapters(context);   
initHandlerExceptionResolvers(context);   
initRequestToViewNameTranslator(context);   
initViewResolvers(context);   
initFlashMapManager(context);

import org.springframework.web.WebApplicationInitializer;
public class MyWebApplicationInitializer implements WebApplicationInitializer {
	//没有创建root ApplicaitonContext
    @Override
    public void onStartup(ServletContext container) {
        XmlWebApplicationContext appContext = new XmlWebApplicationContext();
        appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
        ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext));
        registration.setLoadOnStartup(1);
        registration.addMapping("/");
    }
}

//用它来提供RootConfig.class和AppConfig.class
public class MyWebAppInitializer extends 
    AbstractAnnotationConfigDispatcherServletInitializer {
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return null;
    }
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[] { MyWebConfig.class };
    }
    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}

public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {
    @Override
    protected WebApplicationContext createRootApplicationContext() {
        return null;
    }
    @Override
    protected WebApplicationContext createServletApplicationContext() {
        XmlWebApplicationContext cxt = new XmlWebApplicationContext();
        cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
        return cxt;
    }
    @Override
    protected String[] getServletMappings() {
        return new String[] { "/" };
    }
}