基于源码简要分析springmvc的启动过程

1、前言

总是总结一些皮毛的知识点，今天来学习点高大尚的东西，根据源码简单分析springmvc的启动过程。

2、springmvc的架构流程图

3、源码分析

       /**
	 * Process the actual dispatching to the handler.
	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
	 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
	 * to find the first that supports the handler class.
	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
	 * themselves to decide which methods are acceptable.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @throws Exception in case of any kind of processing failure
	 */
	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
                /*
                    1、springmvc容器初始化的完成后，handermapping就已经被加载了，这些handermappings被放在一个List集合中。该List集合中存储了
                    HTTP请求对应的映射数据。
                */
                HttpServletRequest processedRequest = request;
                
                //拦截器链
                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.
                                /*
                                    2、通过HanderMapping接口中getHander（）方法，获取hander执行链。
                                    获取的执行链中包含需要的hander以及interceptors（拦截器链）
                                */
                                mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
                                /*
                                    通过hander执行链获取HanderAdapter    
                                */
                                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
                                /*
                                    3、通过获取到的HanderAdapter，校验（适配）hander的合法性                               
                                */
                                String method = request.getMethod();
				boolean isGet = "GET".equals(method);
				if (isGet || "HEAD".equals(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (logger.isDebugEnabled()) {
						logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

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

				// Actually invoke the handler.
                                /*
                                    校验合法后，会返回ModelAndView：数据模型和视图名称
                                */
                                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
                                /*
                                    4、渲染视图
                                */
				applyDefaultViewName(request, mv);
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
		}
		catch (Error err) {
			triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
				}
			}
		}
	}

4、解释项目启动HanderMapping被加载

通过项目启动的日志，可以看出加载过程,如图：

5、解释HandlerExecutionChain中包含拦截器

直接看源码：

public class HandlerExecutionChain {

	private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);

	private final Object handler;
        //拦截器数组
	private HandlerInterceptor[] interceptors;

	private List<HandlerInterceptor> interceptorList;

	private int interceptorIndex = -1;
        
        //...
}

6、解释通过适配器如何校验Hander的合法性的源码

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

7、解释渲染视图的源码

/**
	 * Do we need view name translation?
	 */
	private void applyDefaultViewName(HttpServletRequest request, ModelAndView mv) throws Exception {
		if (mv != null && !mv.hasView()) {
			mv.setViewName(getDefaultViewName(request));
		}
	}

	/**
	 * Handle the result of handler selection and handler invocation, which is
	 * either a ModelAndView or an Exception to be resolved to a ModelAndView.
	 */
	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()) {
			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) {
			mappedHandler.triggerAfterCompletion(request, response, null);
		}
	}

8、结束

通过上述一系列的操作，用户通过请求就可以看到想要的视图效果了。