4. Spring MVC 源码深度剖析

4.1 前端控制器 DispatcherServlet 继承结构

4.2 重要时机点分析

1. Handler方法的执行时机

观看调用栈：

doDispathch方法中的1064行代码完成handler方法的调用

2. ⻚面渲染时机

打断点并观察调用栈

SpringMVC处理请求的流程即为 org.springframework.web.servlet.DispatcherServlet#doDispatch方法的执行过程，其中步骤 2、3、4、5是核心步骤

调用getHandler()获取到能够处理当前请求的执行链 HandlerExecutionChain(Handler+拦截器)
调用getHandlerAdapter();获取能够执行1)中Handler的适配器但是如何去getHandlerAdapter的?后面进行分析
适配器调用Handler执行ha.handle(总会返回一个ModelAndView对象)
调用processDispatchResult()方法完成视图渲染跳转

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 {
            // 1 检查是否是文件上传的请求
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // Determine handler for the current request.
            // 2 取得处理当前请求的Controller，这里也称为Handler，即处理器
            // 这里并不是直接返回 Controller，而是返回 HandlerExecutionChain 请求处理链对象
            // 该对象封装了Handler和Interceptor
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                // 如果 handler 为空，则返回404
                noHandlerFound(processedRequest, response);
                return;
            }

            // Determine handler adapter for the current request.
            // 3 获取处理请求的处理器适配器 HandlerAdapter
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            // 处理 last-modified 请求头
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(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.
                // 4 实际处理器处理请求，返回结果视图对象
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                // 结果视图对象的处理
                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);
        }
        // 5 跳转⻚面，渲染视图
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    } catch (Exception ex) {
        //最终会调用HandlerInterceptor的afterCompletion 方法
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    } catch (Throwable err) {
        //最终会调用HandlerInterceptor的afterCompletion 方法
        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);
            }
        }
    }
}

4.3 核心步骤getHandler方法剖析

遍历两个HandlerMapping，试图获取能够处理当前请求的执行链

4.4 核心步骤getHandlerAdapter方法剖析

遍历各个HandlerAdapter，看哪个Adapter支持处理当前Handler

默认配置 DispatcherServlet.properties

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\
org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

4.5 核心步骤ha.handle方法剖析

入口

4.6 核心步骤processDispatchResult方法剖析

4.7 SpringMVC九大组件初始化

在DispatcherServlet中定义了九个属性，每一个属性都对应一种组件

/** MultipartResolver used by this servlet. */ 
// 多部件解析器
@Nullable
private MultipartResolver multipartResolver;

/** LocaleResolver used by this servlet. */ 
// 区域化 国际化解析器
@Nullable
private LocaleResolver localeResolver;

/** ThemeResolver used by this servlet. */ 
// 主题解析器
@Nullable
private ThemeResolver themeResolver;

/** List of HandlerMappings used by this servlet. */ 
// 处理器映射器组件
@Nullable
private List<HandlerMapping> handlerMappings;

/** List of HandlerAdapters used by this servlet. */ 
// 处理器适配器组件
@Nullable
private List<HandlerAdapter> handlerAdapters;

/** List of HandlerExceptionResolvers used by this servlet. */ 
// 异常解析器组件
@Nullable
private List<HandlerExceptionResolver> handlerExceptionResolvers;

/** RequestToViewNameTranslator used by this servlet. */ 
// 默认视图名转换器组件
@Nullable
private RequestToViewNameTranslator viewNameTranslator;

/** FlashMapManager used by this servlet. */ 
// flash属性管理组件
@Nullable
private FlashMapManager flashMapManager;

/** List of ViewResolvers used by this servlet. */ 
// 视图解析器
@Nullable
private List<ViewResolver> viewResolvers;