[Spring] web mvc 코드로 이해하기

web mvc overview

DispatcherServlet 가 모든 역할을 담당해주는 것을 볼 수 있다. 코드를 자세히 살펴보도록 하자!

 

DispatcherServlet.class - doDispatch 메소드 Overview

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

				// Determine handler for the current request.
                //* 1. getHandler() 메소드 통해 핸들러 매핑
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
                //* 2. 핸들러 어댑터 조회 - 핸들러를 처리할 수 있는 어댑터
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				String method = request.getMethod();
				boolean isGet = HttpMethod.GET.matches(method);
				if (isGet || HttpMethod.HEAD.matches(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
                //* 3. 핸들러 어댑터 실행
                //* 4. 핸들러 어댑터 통해 핸들러 실행
                //* 5. ModelAndView 반환
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}

				applyDefaultViewName(processedRequest, mv);
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			catch (Throwable err) {
				// As of 4.3, we're processing Errors thrown from handler methods as well,
				// making them available for @ExceptionHandler methods and other scenarios.
				dispatchException = new NestedServletException("Handler dispatch failed", err);
			}
            //* 6. viewResolver
            //* 7. view
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
		}
		catch (Throwable err) {
			triggerAfterCompletion(processedRequest, response, mappedHandler,
					new NestedServletException("Handler processing failed", err));
		}
		finally {
			if (asyncManager.isConcurrentHandlingStarted()) {
				// Instead of postHandle and afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable Exception exception) throws Exception {

		boolean errorView = false;

		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				//* 7. ModelAndView 반환
                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()) {
        	//* 8. render
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("No view rendering, null ModelAndView returned.");
			}
		}

		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Concurrent handling started during a forward
			return;
		}

		if (mappedHandler != null) {
			// Exception (if any) is already handled..
			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 != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
		response.setLocale(locale);

		View view;
		String viewName = mv.getViewName();
		if (viewName != null) {
			// We need to resolve the view name.
			view = resolveViewName(viewName, 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.isTraceEnabled()) {
			logger.trace("Rendering view [" + view + "] ");
		}
		try {
			if (mv.getStatus() != null) {
				request.setAttribute(View.RESPONSE_STATUS_ATTRIBUTE, mv.getStatus());
				response.setStatus(mv.getStatus().value());
			}
			view.render(mv.getModelInternal(), request, response);
		}
		catch (Exception ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Error rendering view [" + view + "]", ex);
			}
			throw ex;
		}
	}

이제 한 단계씩 살펴보도록 하자.

1. 핸들러 매핑

핸들러 매핑

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

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
                //* 1. getHandler() 메소드 통해 핸들러 매핑
				mappedHandler = getHandler(processedRequest);
                
                ...
				
			}
		}
	}

우선 HandlerMapping 을 모두 찾는다.

 

HandlerMapping 이란?

Interface to be implemented by objects that define a mapping between requests and handler objects. This class can be implemented by application developers, although this is not necessary, as org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping and org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping are included in the framework. The former is the default if no HandlerMapping bean is registered in the application context.

request (key), handler object (value) 를 갖는 Map 인터페이스

 

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

mapping.getHandler(request) 

HandlerMapping 모두를 찾은 후에 그 중에서도 해당 request 를 처리해줄 수 있는 handler object를 반환한다.

디버깅 결과 이렇게 5가지의 handlerMappings 가 나타났다

return 된 handler 는 HandlerMethod

@RequestMapping 으로 찾아진 handler

2. 핸들러 어댑터 조회

핸들러 어댑터 찾기

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

				// Determine handler for the current request.
                //* 1. getHandler() 메소드 통해 핸들러 매핑
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
                //* 2. 핸들러 어댑터 조회 - 핸들러를 처리할 수 있는 어댑터
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
		}
	}

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
		if (this.handlerAdapters != null) {
			for (HandlerAdapter adapter : this.handlerAdapters) {
				if (adapter.supports(handler)) {
					return adapter;
				}
			}
		}
		throw new ServletException("No adapter for handler [" + handler +
				"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
	}

this.handlerAdapters

우리는 이전 단계에서 HandlerMapping 를 모두 탐색하여 request 에 맞는 handler object를 찾았다. 이젠 이 handler를 어떻게 실행할 것인지 알아야하는데 그건 HandlerAdatper에 있다. 그래서 이 handler로 우리는 다시 HandlerAdapter 를 탐색한다.

 

HandlerAdapter 란?

This interface is used to allow the DispatcherServlet to be indefinitely extensible. The DispatcherServlet accesses all installed handlers through this interface, meaning that it does not contain code specific to any handler type. Note that a handler can be of type Object. This is to enable handlers from other frameworks to be integrated with this framework without custom coding, as well as to allow for annotation-driven handler objects that do not obey any specific Java interface. This interface is not intended for application developers. It is available to handlers who want to develop their own web workflow. Note: HandlerAdapter implementors may implement the org.springframework.core.Ordered interface to be able to specify a sorting order (and thus a priority) for getting applied by the DispatcherServlet. Non-Ordered instances get treated as the lowest priority.

자바의 추상과 구현의 예시라고 보면 된다. dispatcher servlet 에서는 수 많은 handler를 처리할 수 있어야한다. 그래서 handlerAdapter로 추상화를 시켜 공통 실행 함수를 만들고 그것을 실행시키는 역할을 한다.

public interface HandlerAdapter {

	boolean supports(Object handler);

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

}

 

supports 메소드를 통해 해당 handler를 실행시킬 수 있는지 확인하고 그렇다면 handle 함수를 실행시킨다.

3. HandlerAdapter - handle() 메소드 실행, 4. handler 호출, 5. ModelAndView 반환

핸들러 어댑터를 찾았으니 이제 handle() 메소드를 실행해보자!

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

				// Determine handler for the current request.
                //* 1. getHandler() 메소드 통해 핸들러 매핑
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
                //* 2. 핸들러 어댑터 조회 - 핸들러를 처리할 수 있는 어댑터
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Actually invoke the handler.
                //* 3. 핸들러 어댑터 실행
                //* 4. 핸들러 어댑터 통해 핸들러 실행
                //* 5. ModelAndView 반환
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
		}
	}

AbstractHandlerMethodAdapter의 handle() 메소드가 호출됐다.

@Override
	@Nullable
	public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {

		return handleInternal(request, response, (HandlerMethod) handler);
	}

내 예시에서는 RequestMappingHandlerAdatper 의 handleInternal() 메소드가 호출된다.

@Override
	protected ModelAndView handleInternal(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

		ModelAndView mav;
		checkRequest(request);

		// Execute invokeHandlerMethod in synchronized block if required.
		if (this.synchronizeOnSession) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				Object mutex = WebUtils.getSessionMutex(session);
				synchronized (mutex) {
					mav = invokeHandlerMethod(request, response, handlerMethod);
				}
			}
			else {
				// No HttpSession available -> no mutex necessary
				mav = invokeHandlerMethod(request, response, handlerMethod);
			}
		}
		else {
			// No synchronization on session demanded at all...
            //* 이 글의 예시에서는 여기로 분기!!
			mav = invokeHandlerMethod(request, response, handlerMethod);
		}

		if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
			if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
				applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
			}
			else {
				prepareResponse(response);
			}
		}

		return mav;
	}

invokeHandlerMethod() 에서 ModelAndView 를 생성해준다.

 

6. viewResolver 호출, 7. view 반환, 8. render

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

				// Determine handler for the current request.
                //* 1. getHandler() 메소드 통해 핸들러 매핑
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
                //* 2. 핸들러 어댑터 조회 - 핸들러를 처리할 수 있는 어댑터
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				String method = request.getMethod();
				boolean isGet = HttpMethod.GET.matches(method);
				if (isGet || HttpMethod.HEAD.matches(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}
				
                // applyPrehandle, applyPostHandle -> Interceptor
          
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
                //* 3. 핸들러 어댑터 실행
                //* 4. 핸들러 어댑터 통해 핸들러 실행
                //* 5. ModelAndView 반환
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            //* 6. viewResolver
            //* 7. view
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
	}

이제 view render를 할 차례다. render() 메소드를 타고 분기하면 view resolver를 만나게 된다.

참고
Interceptor의 posthandle, prehandle은 handle() 이 실행되기 전후로 실행된다.

	private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable 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()) {
        	//* 여기로 분기
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("No view rendering, null ModelAndView returned.");
			}
		}

		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Concurrent handling started during a forward
			return;
		}

		if (mappedHandler != null) {
			// Exception (if any) is already handled..
			mappedHandler.triggerAfterCompletion(request, response, null);
		}
	}

render() 함수는 View를 만들어주는 역할을 한다.

특히 ModelAndView에서 viewName를 받아 해당 View 를 찾아주는 역할을 하도록 한다. 

	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 != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
		response.setLocale(locale);

		View view;
		String viewName = mv.getViewName();
		if (viewName != null) {
			// We need to resolve the view name.
			view = resolveViewName(viewName, 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.isTraceEnabled()) {
			logger.trace("Rendering view [" + view + "] ");
		}
		try {
			if (mv.getStatus() != null) {
				request.setAttribute(View.RESPONSE_STATUS_ATTRIBUTE, mv.getStatus());
				response.setStatus(mv.getStatus().value());
			}
			view.render(mv.getModelInternal(), request, response);
		}
		catch (Exception ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Error rendering view [" + view + "]", ex);
			}
			throw ex;
		}
	}

@Nullable
	protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
			Locale locale, HttpServletRequest request) throws Exception {

		if (this.viewResolvers != null) {
			for (ViewResolver viewResolver : this.viewResolvers) {
				View view = viewResolver.resolveViewName(viewName, locale);
				if (view != null) {
					return view;
				}
			}
		}
		return null;
	}

참고

김영한 이사 WebMvc 강의

 

더 공부할 자료

https://gowoonsori.com/spring/architecture/