SpringMVC DispatchServlet 源码解读

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/**
 * Central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers
 * or HTTP-based remote service exporters. Dispatches to registered handlers for processing
 * a web request, providing convenient mapping and exception handling facilities.
 *
 * <p>This servlet is very flexible: It can be used with just about any workflow, with the
 * installation of the appropriate adapter classes. It offers the following functionality
 * that distinguishes it from other request-driven web MVC frameworks:
 *
 * <ul>
 * <li>It is based around a JavaBeans configuration mechanism.
 *
 * <li>It can use any {@link HandlerMapping} implementation - pre-built or provided as part
 * of an application - to control the routing of requests to handler objects. Default is
 * {@link org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping} and
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping}.
 * HandlerMapping objects can be defined as beans in the servlet's application context,
 * implementing the HandlerMapping interface, overriding the default HandlerMapping if
 * present. HandlerMappings can be given any bean name (they are tested by type).
 *
 * <li>It can use any {@link HandlerAdapter}; this allows for using any handler interface.
 * Default adapters are {@link org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter},
 * {@link org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter}, for Spring's
 * {@link org.springframework.web.HttpRequestHandler} and
 * {@link org.springframework.web.servlet.mvc.Controller} interfaces, respectively. A default
 * {@link org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter}
 * will be registered as well. HandlerAdapter objects can be added as beans in the
 * application context, overriding the default HandlerAdapters. Like HandlerMappings,
 * HandlerAdapters can be given any bean name (they are tested by type).
 *
 * <li>The dispatcher's exception resolution strategy can be specified via a
 * {@link HandlerExceptionResolver}, for example mapping certain exceptions to error pages.
 * Default are
 * {@link org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver},
 * {@link org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver}, and
 * {@link org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver}.
 * These HandlerExceptionResolvers can be overridden through the application context.
 * HandlerExceptionResolver can be given any bean name (they are tested by type).
 *
 * <li>Its view resolution strategy can be specified via a {@link ViewResolver}
 * implementation, resolving symbolic view names into View objects. Default is
 * {@link org.springframework.web.servlet.view.InternalResourceViewResolver}.
 * ViewResolver objects can be added as beans in the application context, overriding the
 * default ViewResolver. ViewResolvers can be given any bean name (they are tested by type).
 *
 * <li>If a {@link View} or view name is not supplied by the user, then the configured
 * {@link RequestToViewNameTranslator} will translate the current request into a view name.
 * The corresponding bean name is "viewNameTranslator"; the default is
 * {@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}.
 *
 * <li>The dispatcher's strategy for resolving multipart requests is determined by a
 * {@link org.springframework.web.multipart.MultipartResolver} implementation.
 * Implementations for Apache Commons FileUpload and Servlet 3 are included; the typical
 * choice is {@link org.springframework.web.multipart.commons.CommonsMultipartResolver}.
 * The MultipartResolver bean name is "multipartResolver"; default is none.
 *
 * <li>Its locale resolution strategy is determined by a {@link LocaleResolver}.
 * Out-of-the-box implementations work via HTTP accept header, cookie, or session.
 * The LocaleResolver bean name is "localeResolver"; default is
 * {@link org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver}.
 *
 * <li>Its theme resolution strategy is determined by a {@link ThemeResolver}.
 * Implementations for a fixed theme and for cookie and session storage are included.
 * The ThemeResolver bean name is "themeResolver"; default is
 * {@link org.springframework.web.servlet.theme.FixedThemeResolver}.
 * </ul>
 *
 * <p><b>NOTE: The {@code @RequestMapping} annotation will only be processed if a
 * corresponding {@code HandlerMapping} (for type-level annotations) and/or
 * {@code HandlerAdapter} (for method-level annotations) is present in the dispatcher.</b>
 * This is the case by default. However, if you are defining custom {@code HandlerMappings}
 * or {@code HandlerAdapters}, then you need to make sure that a corresponding custom
 * {@code RequestMappingHandlerMapping} and/or {@code RequestMappingHandlerAdapter}
 * is defined as well - provided that you intend to use {@code @RequestMapping}.
 *
 * <p><b>A web application can define any number of DispatcherServlets.</b>
 * Each servlet will operate in its own namespace, loading its own application context
 * with mappings, handlers, etc. Only the root application context as loaded by
 * {@link org.springframework.web.context.ContextLoaderListener}, if any, will be shared.
 *
 * <p>As of Spring 3.1, {@code DispatcherServlet} may now be injected with a web
 * application context, rather than creating its own internally. This is useful in Servlet
 * 3.0+ environments, which support programmatic registration of servlet instances.
 * See the {@link #DispatcherServlet(WebApplicationContext)} javadoc for details.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @author Chris Beams
 * @author Rossen Stoyanchev
 * @author Sebastien Deleuze
 * @see org.springframework.web.HttpRequestHandler
 * @see org.springframework.web.servlet.mvc.Controller
 * @see org.springframework.web.context.ContextLoaderListener
 */

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@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
    //该方法为 ServletContainerInitializer 的实现方法, 为 servlet3.0 标准
    @Override
    public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
            throws ServletException {

        //....省略的代码

        servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
        AnnotationAwareOrderComparator.sort(initializers);
        for (WebApplicationInitializer initializer : initializers) {
            initializer.onStartup(servletContext);
        }
    }
}

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@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {

    @Configuration(proxyBeanMethods = false)
    @Conditional(DefaultDispatcherServletCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    protected static class DispatcherServletConfiguration {

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServlet dispatcherServlet(WebMvcProperties webMvcProperties) {
            DispatcherServlet dispatcherServlet = new DispatcherServlet();
            dispatcherServlet.setDispatchOptionsRequest(webMvcProperties.isDispatchOptionsRequest());
            dispatcherServlet.setDispatchTraceRequest(webMvcProperties.isDispatchTraceRequest());
            dispatcherServlet.setThrowExceptionIfNoHandlerFound(webMvcProperties.isThrowExceptionIfNoHandlerFound());
            dispatcherServlet.setPublishEvents(webMvcProperties.isPublishRequestHandledEvents());
            dispatcherServlet.setEnableLoggingRequestDetails(webMvcProperties.isLogRequestDetails());
            return dispatcherServlet;
        }

    }
}

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if (this.flashMapManager != null) {
    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);
}

//....省略代码

try {
    doDispatch(request, response);
}
finally {
    if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        if (attributesSnapshot != null) {
            restoreAttributesAfterInclude(request, attributesSnapshot);
        }
    }
    if (this.parseRequestPath) {
        ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
    }
}

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

    // handler 处理链, 此对象包含了匹配的 handlerMethod 和 诸多 interceptor
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);  // 获取一个异步处理器

    try {
        ModelAndView mv = null; // 返回的视图信息
        Exception dispatchException = null; // 异常信息

        try {
            // 检查是否是 multipart 类型的请求, 如果是, 使用 multipartResolver, 把流信息处理一下
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // Determine handler for the current request.
            // 获取 handlerChain, 实际上是调用每个 handlerMapping 的 getHandler 方法
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response); //noHandlerFound的异常处理
                return;
            }

            // Determine handler adapter for the current request.
            // 获取 handlerAdapter, 这个类是最核心的处理类, 由它来实际调用 handlerMethod
            // handlerAdapter 中声明了处理 request 所需要的必要组件, 通过一系列适配后, 最终调用 handlerMethod 来返回最终结果
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            String method = request.getMethod();
            // 处理一下 get 请求的缓存策略
            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;
                }
            }

            // 此处调用拦截器的 preHandler 方法
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // Actually invoke the handler.
            // 处理请求核心方法, 也就是 adapter 的 handle方法, 我们常用的 adapter 就是 requestMappingHandlerAdapter
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) { //如果异步处理器启动了, 就不要往下走了
                return;
            }

            applyDefaultViewName(processedRequest, mv);
            //调用 interceptor 的 postHandle 方法
            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.
            // 这里把一些 error 也转化为异常, 可以被 exceptionHandler 处理
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        //处理结果
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        //不管有没有异常 都要触发 afterCompletion 方法
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    }
    catch (Throwable err) {
        //不管有没有异常 都要触发 afterCompletion 方法
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
    }
    finally {
        if (asyncManager.isConcurrentHandlingStarted()) { //如果异步处理器启动了, 就不要往下走了
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                //清除 request 的 part 数据 -> Part.delete()
                cleanupMultipart(processedRequest);
            }
        }
    }
}

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//该返回值处理器在 requestMappingHandlerAdapter 中声明, 它在 handle 方法中启动了 WebAsyncManager 的 start 方法
public class CallableMethodReturnValueHandler implements HandlerMethodReturnValueHandler {
	@Override
	public boolean supportsReturnType(MethodParameter returnType) {
		return Callable.class.isAssignableFrom(returnType.getParameterType());
	}

	@Override
	public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

		if (returnValue == null) {
			mavContainer.setRequestHandled(true);
			return;
		}

		Callable<?> callable = (Callable<?>) returnValue;
		WebAsyncUtils.getAsyncManager(webRequest).startCallableProcessing(callable, mavContainer);
	}
}

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private void parseRequest(HttpServletRequest request) {
    try {
        Collection<Part> parts = request.getParts();
        this.multipartParameterNames = new LinkedHashSet<>(parts.size());
        MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
        for (Part part : parts) {
            String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
            ContentDisposition disposition = ContentDisposition.parse(headerValue);
            String filename = disposition.getFilename();
            if (filename != null) {
                if (filename.startsWith("=?") && filename.endsWith("?=")) {
                    filename = MimeDelegate.decode(filename);
                }
                files.add(part.getName(), new StandardMultipartFile(part, filename));
            }
            else {
                this.multipartParameterNames.add(part.getName());
            }
        }
        setMultipartFiles(files);
    }
    catch (Throwable ex) {
        handleParseFailure(ex);
    }
}

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public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    Object handler = getHandlerInternal(request);

    // ....省略了一些代码

    // Bean name or resolved handler?
    if (handler instanceof String) {
        String handlerName = (String) handler;
        handler = obtainApplicationContext().getBean(handlerName);
    }

    // Ensure presence of cached lookupPath for interceptors and others
    if (!ServletRequestPathUtils.hasCachedPath(request)) {
        initLookupPath(request);
    }

    HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

    // ....省略了一些代码

    return executionChain;
}

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
    HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ? (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

    for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
        if (interceptor instanceof MappedInterceptor) {
            MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
            if (mappedInterceptor.matches(request)) {
                chain.addInterceptor(mappedInterceptor.getInterceptor());
            }
        } else {
            chain.addInterceptor(interceptor);
        }
    }
    return chain;
}

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// AbstractHandlerMethodMapping, 在 bean 实例化之后, 会生成此 HandlerMapping 的所有 handlerMethod 信息
protected void initHandlerMethods() {
    for (String beanName : getCandidateBeanNames()) {
        if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
            processCandidateBean(beanName);
        }
    }
    handlerMethodsInitialized(getHandlerMethods());
}

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// 可以传入自定义 condition, 用来干预最终匹配的 handlerMethod, 这里需要去看一下RequestMappingInfo这个类, 需要对其内部原理进行解读
// 通过自定义 condition, 我们可以设计不同版本的 handler URI
protected RequestMappingInfo createRequestMappingInfo(
			RequestMapping requestMapping, @Nullable RequestCondition<?> customCondition) {

    RequestMappingInfo.Builder builder = RequestMappingInfo
            .paths(resolveEmbeddedValuesInPatterns(requestMapping.path()))
            .methods(requestMapping.method())
            .params(requestMapping.params())
            .headers(requestMapping.headers())
            .consumes(requestMapping.consumes())
            .produces(requestMapping.produces())
            .mappingName(requestMapping.name());
    if (customCondition != null) {
        builder.customCondition(customCondition);
    }
    return builder.options(this.config).build();
}

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//自定义参数解析器, 可以通过 WebMvcConfigurer 来配置
@Nullable
private List<HandlerMethodArgumentResolver> customArgumentResolvers;

//常规参数解析器集合 可以查看: getDefaultArgumentResolvers方法, 熟悉默认的参数解析器
@Nullable
private HandlerMethodArgumentResolverComposite argumentResolvers;

//自定义绑定器的参数解析器集合 可以查看: getDefaultInitBinderArgumentResolvers方法, 熟悉默认的参数解析器
@Nullable
private HandlerMethodArgumentResolverComposite initBinderArgumentResolvers;

//自定义返回值 handler, 可以通过 WebMvcConfigurer 来配置
@Nullable
private List<HandlerMethodReturnValueHandler> customReturnValueHandlers;

//返回值 handler 集合, 可以查看: getDefaultReturnValueHandlers方法, 来熟悉默认的返回值 handler
@Nullable
private HandlerMethodReturnValueHandlerComposite returnValueHandlers;

//视图解析器 -- 可编程
@Nullable
private List<ModelAndViewResolver> modelAndViewResolvers;

//RESTful 服务中很重要的一个特性: 同一资源可以有多种表述(请求时的 ACCEPT: */*, application/json, text/xml, text/plain)，这就是内容协商(ContentNegotiation)处理器
//该处理器可根据客户端支持的内容格式, 返回适当的消息内容(messageConverter)
private ContentNegotiationManager contentNegotiationManager = new ContentNegotiationManager();

//消息转换器, 用来处理请求的请求数据以及响应数据的序列化工作 -- 可编程
private List<HttpMessageConverter<?>> messageConverters;

//请求 响应消息的增强类 -- 可编程
private final List<Object> requestResponseBodyAdvice = new ArrayList<>();

//数据绑定处理器
@Nullable
private WebBindingInitializer webBindingInitializer;

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@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
        HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

    ServletWebRequest webRequest = new ServletWebRequest(request, response);
    try {
        WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
        ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

        ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
        if (this.argumentResolvers != null) {
            invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
        }
        if (this.returnValueHandlers != null) {
            invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
        }
        invocableMethod.setDataBinderFactory(binderFactory);
        invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

        ModelAndViewContainer mavContainer = new ModelAndViewContainer();
        mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
        modelFactory.initModel(webRequest, mavContainer, invocableMethod);
        mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

        AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
        asyncWebRequest.setTimeout(this.asyncRequestTimeout);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.setTaskExecutor(this.taskExecutor);
        asyncManager.setAsyncWebRequest(asyncWebRequest);
        asyncManager.registerCallableInterceptors(this.callableInterceptors);
        asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

        if (asyncManager.hasConcurrentResult()) {
            Object result = asyncManager.getConcurrentResult();
            mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
            asyncManager.clearConcurrentResult();
            LogFormatUtils.traceDebug(logger, traceOn -> {
                String formatted = LogFormatUtils.formatValue(result, !traceOn);
                return "Resume with async result [" + formatted + "]";
            });
            invocableMethod = invocableMethod.wrapConcurrentResult(result);
        }

        invocableMethod.invokeAndHandle(webRequest, mavContainer);
        if (asyncManager.isConcurrentHandlingStarted()) {
            return null;
        }

        return getModelAndView(mavContainer, modelFactory, webRequest);
    }
    finally {
        webRequest.requestCompleted();
    }
}

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    setResponseStatus(webRequest);

    if (returnValue == null) {
        if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
            disableContentCachingIfNecessary(webRequest);
            mavContainer.setRequestHandled(true);
            return;
        }
    }
    else if (StringUtils.hasText(getResponseStatusReason())) {
        mavContainer.setRequestHandled(true);
        return;
    }

    mavContainer.setRequestHandled(false);
    Assert.state(this.returnValueHandlers != null, "No return value handlers");
    try {
        this.returnValueHandlers.handleReturnValue(
                returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    }
    catch (Exception ex) {
        if (logger.isTraceEnabled()) {
            logger.trace(formatErrorForReturnValue(returnValue), ex);
        }
        throw ex;
    }
}

@Nullable
private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
        ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {

    modelFactory.updateModel(webRequest, mavContainer);
    if (mavContainer.isRequestHandled()) {
        return null;
    }
    ModelMap model = mavContainer.getModel();
    ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, mavContainer.getStatus());
    if (!mavContainer.isViewReference()) {
        mav.setView((View) mavContainer.getView());
    }
    if (model instanceof RedirectAttributes) {
        Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
        HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
        if (request != null) {
            RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
        }
    }
    return mav;
}