你真的了解Spring MVC处理请求流程吗?

前言

阅读本文章大概需要8分钟左右。相信会让你对Spring MVC的理解更加深刻,更上一层楼。

SpringMVC图解

粒度很粗的图解

自己画的.png

粒度比较粗的图解

自己画的.png

粒度一般的图解

本图来自互联网.png

FrameworkServlet

本图来自互联网.png

FrameworkServlet是Spring MVC框架中的基本Servlet,集成提供了Spring应用的上下文。通过读取我们在web.xml中配置的ContextConfigLocation、ContextLoaderListener、ContextClass属性注入上下文。子类必须重写doService()方法去处理请求。

假如我们要请求http://localhost:8081/order/detail?orderId=1,由于我们的请求方式是GET,会进入到doGet()方法。实际上这个方法会把请求委托给processRequest()和doService()处理。

    /**
     * Delegate GET requests to processRequest/doService.
     * <p>Will also be invoked by HttpServlet's default implementation of {@code doHead},
     * with a {@code NoBodyResponse} that just captures the content length.
     * @see #doService
     * @see #doHead
     */
    @Override
    protected final void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

在processRequest()方法中,会处理这个请求,并且不管结果如何,都会发布一个请求事件。实际上处理请求是子类DispatcherServlet的doService()方法完成的。

    /**
     * Process this request, publishing an event regardless of the outcome.
     * <p>The actual event handling is performed by the abstract
     * {@link #doService} template method.
     */
    protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;

        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

        initContextHolders(request, localeContext, requestAttributes);

        try {
            doService(request, response);
        }
        catch (ServletException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }

        finally {
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }

            if (logger.isDebugEnabled()) {
                if (failureCause != null) {
                    this.logger.debug("Could not complete request", failureCause);
                }
                else {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        logger.debug("Leaving response open for concurrent processing");
                    }
                    else {
                        this.logger.debug("Successfully completed request");
                    }
                }
            }

            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

DispatcherServlet

DispatcherServlet就是一个前端控制器,集中提供请求处理机制。将url映射到指定的Controller处理,Controller处理完毕后将ModelAndView返回给DispatcherServlet,DispatcherServlet通过viewResovler进行视图解析,然后将model填充到view,响应给用户。

doService()方法会将判断该请求是不是包含请求。如果是包含请求,会将request对象的参数进行快照,以便在包含后恢复这些属性。这些属性分别是

javax.servlet.include.request_uri
javax.servlet.include.context_path
javax.servlet.include.servlet_path
javax.servlet.include.path_info
javax.servlet.include.query_string

接着将Spring MVC框架的全局对象注入到request对象中,让handler和view对象可用。接着调用doDispatch()方法

    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (logger.isDebugEnabled()) {
            String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
            logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
                    " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        // Keep a snapshot of the request attributes in case of an include,
        // to be able to restore the original attributes after the include.
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap<String, Object>();
            Enumeration<?> attrNames = request.getAttributeNames();
            while (attrNames.hasMoreElements()) {
                String attrName = (String) attrNames.nextElement();
                if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }

        // 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 {
            doDispatch(request, response);
        }
        finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
                // Restore the original attribute snapshot, in case of an include.
                if (attributesSnapshot != null) {
                    restoreAttributesAfterInclude(request, attributesSnapshot);
                }
            }
        }
    }

doDispatch()这个方法很核心,把请求调度给真正的handler去处理。

    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.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                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.
                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);
            }
            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);
                }
            }
        }
    }

checkMultipart(request)判断这个请求是否是Multipart,比如文件上传就是Multipart请求。如果是Multipart请求就交给multipartResolver处理,如果不是Multipart返回当前的请求。

    protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
                        "this typically results from an additional MultipartFilter in web.xml");
            }
            else if (hasMultipartException(request) ) {
                logger.debug("Multipart resolution failed for current request before - " +
                        "skipping re-resolution for undisturbed error rendering");
            }
            else {
                try {
                    return this.multipartResolver.resolveMultipart(request);
                }
                catch (MultipartException ex) {
                    if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                        logger.debug("Multipart resolution failed for error dispatch", ex);
                        // Keep processing error dispatch with regular request handle below
                    }
                    else {
                        throw ex;
                    }
                }
            }
        }
        // If not returned before: return original request.
        return request;
    }

multipartRequestParsed = (processedRequest != request)判断处理后的请求是否和处理前的请求一致。如果不一致,multipartRequestParsed标志为true,代表这个请求已经被multipartResolver处理过了。

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);

在getHandler(processedRequest)里面通过遍历所有的handlerMapping,调用handlerMapping对象中的getHandler(request)方法获得HandlerExecutionChain对象。实际上这里的handlerMapping对象是RequestMappingHandlerMapping对象。

    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;
    }

进入到AbstractHandlerMapping中的getHandler(request),一看究竟。handler是通过getHandlerInternal(request)获得的。

    /**
     * Look up a handler for the given request, falling back to the default
     * handler if no specific one is found.
     * @param request current HTTP request
     * @return the corresponding handler instance, or the default handler
     * @see #getHandlerInternal
     */
    @Override
    public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Object handler = getHandlerInternal(request);
        if (handler == null) {
            handler = getDefaultHandler();
        }
        if (handler == null) {
            return null;
        }
        // Bean name or resolved handler?
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }

        HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
        if (CorsUtils.isCorsRequest(request)) {
            CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
            CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
            CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
            executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
        }
        return executionChain;
    }

进入到AbstractHandlerMethodMapping中的getHandlerInternal(request)方法,先从request对象获取当前要查询的lookupPath。

    @Override
    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        if (logger.isDebugEnabled()) {
            logger.debug("Looking up handler method for path " + lookupPath);
        }
        this.mappingRegistry.acquireReadLock();
        try {
            HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
            if (logger.isDebugEnabled()) {
                if (handlerMethod != null) {
                    logger.debug("Returning handler method [" + handlerMethod + "]");
                }
                else {
                    logger.debug("Did not find handler method for [" + lookupPath + "]");
                }
            }
            return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
        }
        finally {
            this.mappingRegistry.releaseReadLock();
        }
    }

这里的mappingRegistry说白了就是一个映射关系注册中心,里面维护了所有mapping到处理程序handlerMethod的映射关系,以便查找和提供并发访问。所以每次通过访问显式获得锁,访问结束后要显式释放锁。

    /**
     * A registry that maintains all mappings to handler methods, exposing methods
     * to perform lookups and providing concurrent access.
     *
     * <p>Package-private for testing purposes.
     */
    class MappingRegistry {

        private final Map<T, MappingRegistration<T>> registry = new HashMap<T, MappingRegistration<T>>();

        private final Map<T, HandlerMethod> mappingLookup = new LinkedHashMap<T, HandlerMethod>();

        private final MultiValueMap<String, T> urlLookup = new LinkedMultiValueMap<String, T>();

        private final Map<String, List<HandlerMethod>> nameLookup =
                new ConcurrentHashMap<String, List<HandlerMethod>>();

        private final Map<HandlerMethod, CorsConfiguration> corsLookup =
                new ConcurrentHashMap<HandlerMethod, CorsConfiguration>();

        private final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();

        /**
         * Return all mappings and handler methods. Not thread-safe.
         * @see #acquireReadLock()
         */
        public Map<T, HandlerMethod> getMappings() {
            return this.mappingLookup;
        }

        /**
         * Return matches for the given URL path. Not thread-safe.
         * @see #acquireReadLock()
         */
        public List<T> getMappingsByUrl(String urlPath) {
            return this.urlLookup.get(urlPath);
        }

        /**
         * Return handler methods by mapping name. Thread-safe for concurrent use.
         */
        public List<HandlerMethod> getHandlerMethodsByMappingName(String mappingName) {
            return this.nameLookup.get(mappingName);
        }

        /**
         * Return CORS configuration. Thread-safe for concurrent use.
         */
        public CorsConfiguration getCorsConfiguration(HandlerMethod handlerMethod) {
            HandlerMethod original = handlerMethod.getResolvedFromHandlerMethod();
            return this.corsLookup.get(original != null ? original : handlerMethod);
        }

        /**
         * Acquire the read lock when using getMappings and getMappingsByUrl.
         */
        public void acquireReadLock() {
            this.readWriteLock.readLock().lock();
        }

        /**
         * Release the read lock after using getMappings and getMappingsByUrl.
         */
        public void releaseReadLock() {
            this.readWriteLock.readLock().unlock();
        }

        public void register(T mapping, Object handler, Method method) {
            this.readWriteLock.writeLock().lock();
            try {
                HandlerMethod handlerMethod = createHandlerMethod(handler, method);
                assertUniqueMethodMapping(handlerMethod, mapping);

                if (logger.isInfoEnabled()) {
                    logger.info("Mapped \"" + mapping + "\" onto " + handlerMethod);
                }
                this.mappingLookup.put(mapping, handlerMethod);

                List<String> directUrls = getDirectUrls(mapping);
                for (String url : directUrls) {
                    this.urlLookup.add(url, mapping);
                }

                String name = null;
                if (getNamingStrategy() != null) {
                    name = getNamingStrategy().getName(handlerMethod, mapping);
                    addMappingName(name, handlerMethod);
                }

                CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);
                if (corsConfig != null) {
                    this.corsLookup.put(handlerMethod, corsConfig);
                }

                this.registry.put(mapping, new MappingRegistration<T>(mapping, handlerMethod, directUrls, name));
            }
            finally {
                this.readWriteLock.writeLock().unlock();
            }
        }

        private void assertUniqueMethodMapping(HandlerMethod newHandlerMethod, T mapping) {
            HandlerMethod handlerMethod = this.mappingLookup.get(mapping);
            if (handlerMethod != null && !handlerMethod.equals(newHandlerMethod)) {
                throw new IllegalStateException(
                        "Ambiguous mapping. Cannot map '" + newHandlerMethod.getBean() + "' method \n" +
                        newHandlerMethod + "\nto " + mapping + ": There is already '" +
                        handlerMethod.getBean() + "' bean method\n" + handlerMethod + " mapped.");
            }
        }

        private List<String> getDirectUrls(T mapping) {
            List<String> urls = new ArrayList<String>(1);
            for (String path : getMappingPathPatterns(mapping)) {
                if (!getPathMatcher().isPattern(path)) {
                    urls.add(path);
                }
            }
            return urls;
        }

        private void addMappingName(String name, HandlerMethod handlerMethod) {
            List<HandlerMethod> oldList = this.nameLookup.get(name);
            if (oldList == null) {
                oldList = Collections.<HandlerMethod>emptyList();
            }

            for (HandlerMethod current : oldList) {
                if (handlerMethod.equals(current)) {
                    return;
                }
            }

            if (logger.isTraceEnabled()) {
                logger.trace("Mapping name '" + name + "'");
            }

            List<HandlerMethod> newList = new ArrayList<HandlerMethod>(oldList.size() + 1);
            newList.addAll(oldList);
            newList.add(handlerMethod);
            this.nameLookup.put(name, newList);

            if (newList.size() > 1) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Mapping name clash for handlerMethods " + newList +
                            ". Consider assigning explicit names.");
                }
            }
        }

        public void unregister(T mapping) {
            this.readWriteLock.writeLock().lock();
            try {
                MappingRegistration<T> definition = this.registry.remove(mapping);
                if (definition == null) {
                    return;
                }

                this.mappingLookup.remove(definition.getMapping());

                for (String url : definition.getDirectUrls()) {
                    List<T> list = this.urlLookup.get(url);
                    if (list != null) {
                        list.remove(definition.getMapping());
                        if (list.isEmpty()) {
                            this.urlLookup.remove(url);
                        }
                    }
                }

                removeMappingName(definition);

                this.corsLookup.remove(definition.getHandlerMethod());
            }
            finally {
                this.readWriteLock.writeLock().unlock();
            }
        }

        private void removeMappingName(MappingRegistration<T> definition) {
            String name = definition.getMappingName();
            if (name == null) {
                return;
            }
            HandlerMethod handlerMethod = definition.getHandlerMethod();
            List<HandlerMethod> oldList = this.nameLookup.get(name);
            if (oldList == null) {
                return;
            }
            if (oldList.size() <= 1) {
                this.nameLookup.remove(name);
                return;
            }
            List<HandlerMethod> newList = new ArrayList<HandlerMethod>(oldList.size() - 1);
            for (HandlerMethod current : oldList) {
                if (!current.equals(handlerMethod)) {
                    newList.add(current);
                }
            }
            this.nameLookup.put(name, newList);
        }
    }

我们继续回到AbstractHandlerMethodMapping中的getHandlerInternal(request)方法中,通过调用this.mappingRegistry.acquireReadLock()方法申请获得mapping注册中心中的读锁。

this.mappingRegistry.acquireReadLock();
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);

接着调用lookupHandlerMethod(lookupPath, request),通过url匹配的方式获得合适的hanlderMethod。

    /**
     * Look up the best-matching handler method for the current request.
     * If multiple matches are found, the best match is selected.
     * @param lookupPath mapping lookup path within the current servlet mapping
     * @param request the current request
     * @return the best-matching handler method, or {@code null} if no match
     * @see #handleMatch(Object, String, HttpServletRequest)
     * @see #handleNoMatch(Set, String, HttpServletRequest)
     */
    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        List<Match> matches = new ArrayList<Match>();
                //通过lookupPath,在this.urlLookup.get(urlPath)获取List<RequestMappingInfo>集合
        List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
        if (directPathMatches != null) {
            addMatchingMappings(directPathMatches, matches, request);
        }
        if (matches.isEmpty()) {
            // No choice but to go through all mappings...
            addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
        }

        if (!matches.isEmpty()) {
            Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
            Collections.sort(matches, comparator);
            if (logger.isTraceEnabled()) {
                logger.trace("Found " + matches.size() + " matching mapping(s) for [" +
                        lookupPath + "] : " + matches);
            }
            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                if (CorsUtils.isPreFlightRequest(request)) {
                    return PREFLIGHT_AMBIGUOUS_MATCH;
                }
                Match secondBestMatch = matches.get(1);
                if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                    Method m1 = bestMatch.handlerMethod.getMethod();
                    Method m2 = secondBestMatch.handlerMethod.getMethod();
                    throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" +
                            request.getRequestURL() + "': {" + m1 + ", " + m2 + "}");
                }
            }
            handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        }
        else {
            return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
        }
    }

我们可以关注List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);这个方法实际上是从mappingRegistry中的urlLookup获得List<RequestMappingInfo>集合. urlLookup的结构是Map<K, List<V>>, K是url, List<V>对应着RequestMappingInfo,实际上每一个RequestMapping最后都会被封装成RequestMappingInfo.

我们可以看到directPathMatches不为空,会调用addMatchingMappings(directPathMatches, matches, request),我们仔细关注T match = getMatchingMapping(mapping, request)这一行代码。

    private void addMatchingMappings(Collection<T> mappings, List<Match> matches, HttpServletRequest request) {
        for (T mapping : mappings) {
            T match = getMatchingMapping(mapping, request);
            if (match != null) {
                matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));
            }
        }
    }

当前请求的url和RequestMappingInfo中的PatternsRequestCondition对象中的url集合中是否匹配,如果匹配成功,返回一个新的RequestMappingInfo。

    /**
     * Checks if all conditions in this request mapping info match the provided request and returns
     * a potentially new request mapping info with conditions tailored to the current request.
     * <p>For example the returned instance may contain the subset of URL patterns that match to
     * the current request, sorted with best matching patterns on top.
     * @return a new instance in case all conditions match; or {@code null} otherwise
     */
    @Override
    public RequestMappingInfo getMatchingCondition(HttpServletRequest request) {
        RequestMethodsRequestCondition methods = this.methodsCondition.getMatchingCondition(request);
        ParamsRequestCondition params = this.paramsCondition.getMatchingCondition(request);
        HeadersRequestCondition headers = this.headersCondition.getMatchingCondition(request);
        ConsumesRequestCondition consumes = this.consumesCondition.getMatchingCondition(request);
        ProducesRequestCondition produces = this.producesCondition.getMatchingCondition(request);

        if (methods == null || params == null || headers == null || consumes == null || produces == null) {
            return null;
        }

        PatternsRequestCondition patterns = this.patternsCondition.getMatchingCondition(request);
        if (patterns == null) {
            return null;
        }

        RequestConditionHolder custom = this.customConditionHolder.getMatchingCondition(request);
        if (custom == null) {
            return null;
        }

        return new RequestMappingInfo(this.name, patterns,
                methods, params, headers, consumes, produces, custom.getCondition());
    }

接着关注matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));这一行代码,Match是AbstractHandlerMethodMapping的内部类,this.mappingRegistry.getMappings()是获取映射注册中心的mappingLookup对象,其结构为Map<RequestMappingInfo, HandlerMethod>。

    /**
     * A thin wrapper around a matched HandlerMethod and its mapping, for the purpose of
     * comparing the best match with a comparator in the context of the current request.
     */
    private class Match {

        private final T mapping;

        private final HandlerMethod handlerMethod;

        public Match(T mapping, HandlerMethod handlerMethod) {
            this.mapping = mapping;
            this.handlerMethod = handlerMethod;
        }

        @Override
        public String toString() {
            return this.mapping.toString();
        }
    }

回到AbstractHandlerMethodMapping中的lookupHandlerMethod(String lookupPath, HttpServletRequest request)方法,如果matches为空,则遍历mappingRegistry中的mappingLookup集合,并且填充到matches。最后通过排序比较,获得matches集合中的第一个Match对象,此对象也是最匹配的,返回Match对象中的handlerMethod。

        if (matches.isEmpty()) {
            // No choice but to go through all mappings...
            addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
        }

回到AbstractHandlerMethodMapping中的getHandlerInternal(HttpServletRequest request)方法,如果获得的handlerMethod不为空,调用createWithResolvedBean()方法。其中的逻辑是如果当前handlerMethod中的bean只是bean的名称而不是真正的bean实例时,那么通过名称获得bean的实例。并且返回一个新的HandlerMethod。这里的bean是handlerMethd所属于的类。比如UserController中有一个login()方法,bean就是UserController,login就是HandlerMethod。最后释放mappingRegistry的读锁。

return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
    /**
     * If the provided instance contains a bean name rather than an object instance,
     * the bean name is resolved before a {@link HandlerMethod} is created and returned.
     */
    public HandlerMethod createWithResolvedBean() {
        Object handler = this.bean;
        if (this.bean instanceof String) {
            String beanName = (String) this.bean;
            handler = this.beanFactory.getBean(beanName);
        }
        return new HandlerMethod(this, handler);
    }

回到AbstractHandlerMapping中的getHandler(HttpServletRequest request)方法,接着调用 getHandlerExecutionChain(handler, request),遍历所有的handlerInterceptor,把handler和handlerInterceptor(拦截器)封装成handlerExecutionChain(处理程序链)。还有一点就是MappedInterceptor里面有includePatterns和excludePatterns属性。通过这2个属性,设置需要被拦截的url和不需要被拦截的url。

    /**
     * Build a {@link HandlerExecutionChain} for the given handler, including
     * applicable interceptors.
     * <p>The default implementation builds a standard {@link HandlerExecutionChain}
     * with the given handler, the handler mapping's common interceptors, and any
     * {@link MappedInterceptor}s matching to the current request URL. Interceptors
     * are added in the order they were registered. Subclasses may override this
     * in order to extend/rearrange the list of interceptors.
     * <p><b>NOTE:</b> The passed-in handler object may be a raw handler or a
     * pre-built {@link HandlerExecutionChain}. This method should handle those
     * two cases explicitly, either building a new {@link HandlerExecutionChain}
     * or extending the existing chain.
     * <p>For simply adding an interceptor in a custom subclass, consider calling
     * {@code super.getHandlerExecutionChain(handler, request)} and invoking
     * {@link HandlerExecutionChain#addInterceptor} on the returned chain object.
     * @param handler the resolved handler instance (never {@code null})
     * @param request current HTTP request
     * @return the HandlerExecutionChain (never {@code null})
     * @see #getAdaptedInterceptors()
     */
    protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
        HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
                (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

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

返回到AbstractHandlerMapping中的getHandler(request)中,我们已经获取到executionChain对象,可以返回该对象。

HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

返回到DispatcherServlet中的getHandler(HttpServletRequest request)返回当前request请求中的executionChain对象

HandlerExecutionChain handler = hm.getHandler(request);

我们继续回到DispatchServlet中的doDispatcher()方法,如果当前handlerExecutionChain(处理程序执行链)等于空或者handlerExecutionChain中的handlerMethod为空的话,就会抛出著名的NoHandlerFoundException异常

                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

如果你不信,可以点开noHandlerFound(processedRequest, response);

    /**
     * No handler found -> set appropriate HTTP response status.
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception if preparing the response failed
     */
    protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (pageNotFoundLogger.isWarnEnabled()) {
            pageNotFoundLogger.warn("No mapping found for HTTP request with URI [" + getRequestUri(request) +
                    "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        if (this.throwExceptionIfNoHandlerFound) {
            throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request),
                    new ServletServerHttpRequest(request).getHeaders());
        }
        else {
            response.sendError(HttpServletResponse.SC_NOT_FOUND);
        }
    }

接着看,我要从当前请求获取能够支持当前handlerMethod的适配器。

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

点开代码看看,看看细节。主要是循环当前所有的handlerAdapters,通过supports()判断是否支持当前handlerMethod,这种循环比对思想在Spring MVC源码随处可见。

    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");
    }

进入到AbstractHandlerMethodAdapter中的supports()方法,通过判断当前handler对象是否是HandlerMethod类的实例和是否支持当前handlerMethod。

    /**
     * This implementation expects the handler to be an {@link HandlerMethod}.
     * @param handler the handler instance to check
     * @return whether or not this adapter can adapt the given handler
     */
    @Override
    public final boolean supports(Object handler) {
        return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
    }

点开,进入到RequestMappingHandlerAdapter中的supportsInternal(),恍然大悟。这个方法总是返回true,因为任何方法的参数和返回值都以某种方式处理

    /**
     * Always return {@code true} since any method argument and return value
     * type will be processed in some way. A method argument not recognized
     * by any HandlerMethodArgumentResolver is interpreted as a request parameter
     * if it is a simple type, or as a model attribute otherwise. A return value
     * not recognized by any HandlerMethodReturnValueHandler will be interpreted
     * as a model attribute.
     */
    @Override
    protected boolean supportsInternal(HandlerMethod handlerMethod) {
        return true;
    }

判断request的请求方式,如果是GET或者是HEAD,用户当前请求上一次请求的时间戳,通过checkNotModified()判断是否修改过。如果没有修改过,返回状态码304。

                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;
                    }
                }

如果handlerExecutionChain中的拦截器preHandle返回false,就不会调用postHandle(),直接清理资源,然后返回。

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

进入HandlerExecutionChain中的applyPreHandle(processedRequest, response)方法。遍历HandlerExecutionChain中的所有拦截器,如果拦截器中的preHandle(request, response, this.handler)返回false,那么直接调用triggerAfterCompletion(request, response, null)进行资源清理,返回false。通过记录interceptorIndex来标志当前执行的拦截器。

        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = 0; i < interceptors.length; i++) {
                HandlerInterceptor interceptor = interceptors[i];
                if (!interceptor.preHandle(request, response, this.handler)) {
                    triggerAfterCompletion(request, response, null);
                    return false;
                }
                this.interceptorIndex = i;
            }
        }
        return true;

triggerAfterCompletion()方法,也是大同小异。遍历所有拦截器,调用拦截器中清理资源的方法afterCompletion()。

    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
            throws Exception {

        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = this.interceptorIndex; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                try {
                    interceptor.afterCompletion(request, response, this.handler, ex);
                }
                catch (Throwable ex2) {
                    logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
                }
            }
        }
    }

小高潮来了,handlerAdapter(处理程序适配器)开始调用handlerMethod(处理程序)的功能方法。

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

进入到AbstractHandlerMethodAdapter中的handle()方法

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

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

进入RequestMappingHandlerAdapter中的handleInternal()方法,我们可以仔细看看这个方法做了什么。

    @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) {
                        //获取session,如果为空直接返回null
            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...
            mav = invokeHandlerMethod(request, response, handlerMethod);
        }

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

        return mav;
    }

通过调用WebContentGenerator中的checkRequest()方法,判断支持的请求方式是否包含当前请求的方式,如果supportedMethods不为空且不支持当前请求方式,会抛出著名的HttpRequestMetohdNotSupportedException。如果需要session且从当前请求获得不到session,同样抛出HttpSessionRequiredException异常。

    /**
     * Check the given request for supported methods and a required session, if any.
     * @param request current HTTP request
     * @throws ServletException if the request cannot be handled because a check failed
     * @since 4.2
     */
    protected final void checkRequest(HttpServletRequest request) throws ServletException {
        // Check whether we should support the request method.
        String method = request.getMethod();
        if (this.supportedMethods != null && !this.supportedMethods.contains(method)) {
            throw new HttpRequestMethodNotSupportedException(method, this.supportedMethods);
        }

        // Check whether a session is required.
        if (this.requireSession && request.getSession(false) == null) {
            throw new HttpSessionRequiredException("Pre-existing session required but none found");
        }
    }

通过synchronizeOnSession标识符,判断调用invokeHandlerMethod是否需要同步机制。 然后调用invokeHandlerMethod()

mav = invokeHandlerMethod(request, response, handlerMethod);

进入到RequestMappingHandlerAdapter中的invokeHandlerMethod()中,首先RequestMappingHandlerAdapter支持{@link #setCustomArgumentResolvers}和{@link #setCustomReturnValueHandlers}配置自定义参数和自定义返回值,也支持来配置{@link #setArgumentResolvers}和{@link #setReturnValueHandlers}所有参数和返回值。

            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);
            invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
            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();
                if (logger.isDebugEnabled()) {
                    logger.debug("Found concurrent result value [" + result + "]");
                }
                invocableMethod = invocableMethod.wrapConcurrentResult(result);
            }

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

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

创建WebDataBinderFactory实例,用于创建WebDataBinder对象,用于web参数绑定。

WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);

比如我们现在有一个需求,从前台传来的日期字符串,我们要全部解析成Date类型的。一般有3种方式解决:PropertyEditor、Formatter、Converter去解决。最常见的做法实现WebBindingInitializer接口,通过WebDataBinder注册属性编辑器。

/**
 * WebBindingInitializer
 */
public class WebBindingInitializer implements org.springframework.web.bind.support.WebBindingInitializer {

    /* (non-Javadoc)
     * @see org.springframework.web.bind.support.WebBindingInitializer#initBinder(org.springframework.web.bind.WebDataBinder, org.springframework.web.context.request.WebRequest)
     */
    @Override
    public void initBinder(WebDataBinder binder, WebRequest request) {
        binder.registerCustomEditor(String.class, new DatePropertyEditor());
    }

}

回到正轨,看一下是怎么创建WebDataBinderFactory实例。

    private WebDataBinderFactory getDataBinderFactory(HandlerMethod handlerMethod) throws Exception {
        Class<?> handlerType = handlerMethod.getBeanType();
        Set<Method> methods = this.initBinderCache.get(handlerType);
        if (methods == null) {
            methods = MethodIntrospector.selectMethods(handlerType, INIT_BINDER_METHODS);
            this.initBinderCache.put(handlerType, methods);
        }
        List<InvocableHandlerMethod> initBinderMethods = new ArrayList<InvocableHandlerMethod>();
        // Global methods first
        for (Entry<ControllerAdviceBean, Set<Method>> entry : this.initBinderAdviceCache.entrySet()) {
            if (entry.getKey().isApplicableToBeanType(handlerType)) {
                Object bean = entry.getKey().resolveBean();
                for (Method method : entry.getValue()) {
                    initBinderMethods.add(createInitBinderMethod(bean, method));
                }
            }
        }
        for (Method method : methods) {
            Object bean = handlerMethod.getBean();
            initBinderMethods.add(createInitBinderMethod(bean, method));
        }
        return createDataBinderFactory(initBinderMethods);
    }

获得handlerMethod所在类的类型。通过所在类的类型获得从initBinderCache缓存中获得当前类所有的方法。这些方法应该是被@InitBinder注解的方法。

private final Map<Class<?>, Set<Method>> initBinderCache = new ConcurrentHashMap<Class<?>, Set<Method>>(64);

如果methods等于空,那么我们去获得当前类下被@InitBinder注解的方法,并放入到initBinderCache缓存中。

        if (methods == null) {
            methods = MethodIntrospector.selectMethods(handlerType, INIT_BINDER_METHODS);
            this.initBinderCache.put(handlerType, methods);
        }

优先遍历被@ControllerAdvice注解全局类中的方法,再遍历被@Controller注解的类的方法。通过createInitBinderMethod(bean, method)方法创建InvocableHandlerMethod对象(用于参数准备,准备当中会用到WebDataBinderFactory创建WebDataBinder实例进行参数转换解析绑定,方法调用),并且放入到initBinderMethods集合中。

        // Global methods first
        for (Entry<ControllerAdviceBean, Set<Method>> entry : this.initBinderAdviceCache.entrySet()) {
            if (entry.getKey().isApplicableToBeanType(handlerType)) {
                Object bean = entry.getKey().resolveBean();
                for (Method method : entry.getValue()) {
                    initBinderMethods.add(createInitBinderMethod(bean, method));
                }
            }
        }
        for (Method method : methods) {
            Object bean = handlerMethod.getBean();
            initBinderMethods.add(createInitBinderMethod(bean, method));
        }

创建InvocableHandlerMethod对象,注入initBinderArgumentResolvers属性、parameterNameDiscoverer(属性名字发现器)、DefaultDataBinderFactory实例。我们发现要创建一个DefaultDataBinderFactory必须要传入webBindingInitializer。

    private InvocableHandlerMethod createInitBinderMethod(Object bean, Method method) {
        InvocableHandlerMethod binderMethod = new InvocableHandlerMethod(bean, method);
        binderMethod.setHandlerMethodArgumentResolvers(this.initBinderArgumentResolvers);
        binderMethod.setDataBinderFactory(new DefaultDataBinderFactory(this.webBindingInitializer));
        binderMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
        return binderMethod;
    }

最后调用createDataBinderFactory(initBinderMethods)方法创建ServletRequestDataBinderFactory实例,同样也要传入webBindingInitializer。ServletRequestDataBinderFactory是InitBinderDataBinderFactory的子类。

    createDataBinderFactory(initBinderMethods);

    protected InitBinderDataBinderFactory createDataBinderFactory(List<InvocableHandlerMethod> binderMethods)
            throws Exception {

        return new ServletRequestDataBinderFactory(binderMethods, getWebBindingInitializer());
    }

接着创建出ModelFactory实例,我们首先要搞清楚ModelFactory是干啥的。ModelFactory作用是在控制器方法调用前初始化Model模型,调用后对Model模型进行更新。在初始化时,通过调用被@ModelAttribute注解的方法,Model模型会在会话中被临时存储的属性填充。

ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

我们再来看是如何创建ModelFactory实例的,其实和创建WebDataBinderFactory的逻辑差不多。首先获取sessionAttributesHandler对象,同样这个对象是从sessionAttributesHandlerCache缓存获得到的。key是handlerMethod所在类的类型。如果sessionAttributesHandler没有从缓存中获取到,那么锁住缓存,再从缓存中取一遍。如果sessionAttributesHandler还为空的话,那么自己通过new SessionAttributesHandler(handlerType, sessionAttributeStore)创建一个默认的sessionAttributesHandler对象,并放入到缓存中。这种思想是享元设计模式。

    /**
     * Return the {@link SessionAttributesHandler} instance for the given handler type
     * (never {@code null}).
     */
    private SessionAttributesHandler getSessionAttributesHandler(HandlerMethod handlerMethod) {
        Class<?> handlerType = handlerMethod.getBeanType();
        SessionAttributesHandler sessionAttrHandler = this.sessionAttributesHandlerCache.get(handlerType);
        if (sessionAttrHandler == null) {
            synchronized (this.sessionAttributesHandlerCache) {
                sessionAttrHandler = this.sessionAttributesHandlerCache.get(handlerType);
                if (sessionAttrHandler == null) {
                    sessionAttrHandler = new SessionAttributesHandler(handlerType, sessionAttributeStore);
                    this.sessionAttributesHandlerCache.put(handlerType, sessionAttrHandler);
                }
            }
        }
        return sessionAttrHandler;
    }

创建SessionAttributesHandler过程

    /**
     * Create a new instance for a controller type. Session attribute names and
     * types are extracted from the {@code @SessionAttributes} annotation, if
     * present, on the given type.
     * @param handlerType the controller type
     * @param sessionAttributeStore used for session access
     */
    public SessionAttributesHandler(Class<?> handlerType, SessionAttributeStore sessionAttributeStore) {
        Assert.notNull(sessionAttributeStore, "SessionAttributeStore may not be null");
        this.sessionAttributeStore = sessionAttributeStore;

        SessionAttributes annotation =
                AnnotatedElementUtils.findMergedAnnotation(handlerType, SessionAttributes.class);
        if (annotation != null) {
            this.attributeNames.addAll(Arrays.asList(annotation.names()));
            this.attributeTypes.addAll(Arrays.asList(annotation.types()));
        }
        this.knownAttributeNames.addAll(this.attributeNames);
    }

接着通过modelAttributeCache中获取handlerMethod所在类中所有被@ModelAttribute注解且没有被@RequestMapping注解的方法。如果没有从缓存中查找到,那么通过 MethodIntrospector.selectMethods(handlerType, MODEL_ATTRIBUTE_METHODS)查找,并加入到modelAttributeCache缓存中。

    private ModelFactory getModelFactory(HandlerMethod handlerMethod, WebDataBinderFactory binderFactory) {
        SessionAttributesHandler sessionAttrHandler = getSessionAttributesHandler(handlerMethod);
        Class<?> handlerType = handlerMethod.getBeanType();
        Set<Method> methods = this.modelAttributeCache.get(handlerType);
        if (methods == null) {
            methods = MethodIntrospector.selectMethods(handlerType, MODEL_ATTRIBUTE_METHODS);
            this.modelAttributeCache.put(handlerType, methods);
        }
        List<InvocableHandlerMethod> attrMethods = new ArrayList<InvocableHandlerMethod>();
        // Global methods first
        for (Entry<ControllerAdviceBean, Set<Method>> entry : this.modelAttributeAdviceCache.entrySet()) {
            if (entry.getKey().isApplicableToBeanType(handlerType)) {
                Object bean = entry.getKey().resolveBean();
                for (Method method : entry.getValue()) {
                    attrMethods.add(createModelAttributeMethod(binderFactory, bean, method));
                }
            }
        }
        for (Method method : methods) {
            Object bean = handlerMethod.getBean();
            attrMethods.add(createModelAttributeMethod(binderFactory, bean, method));
        }
        return new ModelFactory(attrMethods, binderFactory, sessionAttrHandler);
    }

接着老操作,通过createModelAttributeMethod(binderFactory, bean, method)方法创建InvocableHandlerMethod对象,并放入到attrMethods集合中。

    private InvocableHandlerMethod createModelAttributeMethod(WebDataBinderFactory factory, Object bean, Method method) {
        InvocableHandlerMethod attrMethod = new InvocableHandlerMethod(bean, method);
        attrMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
        attrMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
        attrMethod.setDataBinderFactory(factory);
        return attrMethod;
    }

终于到了new ModelFactory()这一步。通过handlerMethods集合、WebDataBinderFactory实例,SessionAttributesHandler实例创建出ModelFactory实例。

    public ModelFactory(List<InvocableHandlerMethod> handlerMethods,
            WebDataBinderFactory binderFactory, SessionAttributesHandler attributeHandler) {

        if (handlerMethods != null) {
            for (InvocableHandlerMethod handlerMethod : handlerMethods) {
                this.modelMethods.add(new ModelMethod(handlerMethod));
            }
        }
        this.dataBinderFactory = binderFactory;
        this.sessionAttributesHandler = attributeHandler;
    }

既然binderFactory、modelFactory都被我们造出来了,那肯定要干正紧事情了。对handlerMethod进行下一步包装,填充argumentResolvers(HandlerMethodArgumentResolverComposite)、returnValueHandlers(HandlerMethodReturnValueHandlerComposite)、binderFactory、parameterNameDiscoverer属性包装成ServletInvocableHandlerMethod。ServletInvocableHandlerMethod的作用对处理程序的返回值进行处理和ResponseStatus处理。

            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
            invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
            invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
            invocableMethod.setDataBinderFactory(binderFactory);
            invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

实例化ModelAndViewContainer容器,把request里面的属性名为"org.springframework.web.servlet.DispatcherServlet.INPUT_FLASH_MAP"的重定向参数注入到容器中的model模型中。FlashMap的作用是在redirect中传递参数。重定向是会生成新的request,那么传递参数就不能直接用request进行传递。

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

我们关注到initModel(webRequest, mavCOntainer, invocableMethod)这一行,它到底干了什么事情。首先从request中获取检索@SessionAttribute中名称的属性,以Map<String, Object>的结构存储起来,并且放入到ModelAndViewContainer容器的model中。

    public void initModel(NativeWebRequest request, ModelAndViewContainer container,
            HandlerMethod handlerMethod) throws Exception {

        Map<String, ?> sessionAttributes = this.sessionAttributesHandler.retrieveAttributes(request);
        container.mergeAttributes(sessionAttributes);
        invokeModelAttributeMethods(request, container);

        for (String name : findSessionAttributeArguments(handlerMethod)) {
            if (!container.containsAttribute(name)) {
                Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
                if (value == null) {
                    throw new HttpSessionRequiredException("Expected session attribute '" + name + "'", name);
                }
                container.addAttribute(name, value);
            }
        }
    }

接着调用invokeModelAttributeMethods(request, container)方法,将被@ModelAttribute注解的handlerMethod中的模型填充到ModelAndViewContainer容器中的model。只有当容器中不包含当前@ModelAtrribute中的属性时才添加该属性至容器。同时还要判断当前@ModelAttribute中的属性能不能添加到容器中,如果不能,那么放到容器中的bindingDisabledAttributes进行标记。然后提前调用被@ModelAttribute注解的handlerMethod,只有handlerMethod的返回值类型是void,才能将进行数据绑定(也就是绑定到容器中的model里)。如果handlerMethod的返回类不是void,那太好了可以进行数据绑定。数据绑定的规则是如果@ModelAttribute注解设置value和name属性了,优先选择value和name属性作为model中的key,如果没有设置value和name属性,选择被@ModelAttribute注解的handlerMethod的返回类型名称(首字母小写)作为model中的key。

    private void invokeModelAttributeMethods(NativeWebRequest request, ModelAndViewContainer container)
            throws Exception {

        while (!this.modelMethods.isEmpty()) {
            InvocableHandlerMethod modelMethod = getNextModelMethod(container).getHandlerMethod();
            ModelAttribute ann = modelMethod.getMethodAnnotation(ModelAttribute.class);
            if (container.containsAttribute(ann.name())) {
                if (!ann.binding()) {
                    container.setBindingDisabled(ann.name());
                }
                continue;
            }

            Object returnValue = modelMethod.invokeForRequest(request, container);
            if (!modelMethod.isVoid()){
                String returnValueName = getNameForReturnValue(returnValue, modelMethod.getReturnType());
                if (!ann.binding()) {
                    container.setBindingDisabled(returnValueName);
                }
                if (!container.containsAttribute(returnValueName)) {
                    container.addAttribute(returnValueName, returnValue);
                }
            }
        }
    }

说的通俗点,如果被@ModelAttribute注解的handlerMethod返回类型是Collection或者是数组类型,那么填充到model中的key就是方法返回类型名称(首字母小写)再拼接上List。看下面例子,key就是stringList

    @ModelAttribute
    public List<String> baseTest1() {
        List<String> list = new ArrayList<>();
        list.add("1");
        return list;
    }

    @ModelAttribute
    public String[] baseTest2() {
        String[] strings = new String[1];
        strings[0] = "1";
        return strings;
    }

如果返回类型是String或者是Map,那么key就是string、map

    @ModelAttribute
    public String baseTest() {
        return "1";
    }

    @ModelAttribute
    public Map<String, String> baseTest3() {
        Map<String, String> map = new HashMap<>();
        map.put("username", "password");
        return map;
    }

为什么会是这样呢,可以看到ModelFactory中的getNameForReturnType(),首先判断@ModelAttribute注解value属性是不是为空。如果不为空,取value属性的值。如果为空,进行Conventions.getVariableNameForReturnType(method, resolvedType, returnValue)操作。

    public static String getNameForReturnValue(Object returnValue, MethodParameter returnType) {
        ModelAttribute ann = returnType.getMethodAnnotation(ModelAttribute.class);
        if (ann != null && StringUtils.hasText(ann.value())) {
            return ann.value();
        }
        else {
            Method method = returnType.getMethod();
            Class<?> containingClass = returnType.getContainingClass();
            Class<?> resolvedType = GenericTypeResolver.resolveReturnType(method, containingClass);
            return Conventions.getVariableNameForReturnType(method, resolvedType, returnValue);
        }
    }

看到getVariableNameForReturnType(),心中的疑惑应该解开了把。如果返回类型是Object类型,我们会通过返回值来得出它实际返回类型,再通过实际返回类型推出所在类的简称,再进行格式化返回其短名称(也就是首字母小写,也可以说小驼峰)。如果返回类型是Array类型或者是Collection类型,就在其返回基础上再拼接"List"字符串。

    public static String getVariableNameForReturnType(Method method, Class<?> resolvedType, Object value) {
        Assert.notNull(method, "Method must not be null");

        if (Object.class == resolvedType) {
            if (value == null) {
                throw new IllegalArgumentException("Cannot generate variable name for an Object return type with null value");
            }
            return getVariableName(value);
        }

        Class<?> valueClass;
        boolean pluralize = false;

        if (resolvedType.isArray()) {
            valueClass = resolvedType.getComponentType();
            pluralize = true;
        }
        else if (Collection.class.isAssignableFrom(resolvedType)) {
            valueClass = ResolvableType.forMethodReturnType(method).asCollection().resolveGeneric();
            if (valueClass == null) {
                if (!(value instanceof Collection)) {
                    throw new IllegalArgumentException(
                            "Cannot generate variable name for non-typed Collection return type and a non-Collection value");
                }
                Collection<?> collection = (Collection<?>) value;
                if (collection.isEmpty()) {
                    throw new IllegalArgumentException(
                            "Cannot generate variable name for non-typed Collection return type and an empty Collection value");
                }
                Object valueToCheck = peekAhead(collection);
                valueClass = getClassForValue(valueToCheck);
            }
            pluralize = true;
        }
        else {
            valueClass = resolvedType;
        }

        String name = ClassUtils.getShortNameAsProperty(valueClass);
        return (pluralize ? pluralize(name) : name);
    }

    private static final String PLURAL_SUFFIX = "List";

    private static String pluralize(String name) {
        return name + PLURAL_SUFFIX;
    }

回到ModelFactory中的initModel()上,把目光集中到下面这行代码上。

        for (String name : findSessionAttributeArguments(handlerMethod)) {
            if (!container.containsAttribute(name)) {
                Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
                if (value == null) {
                    throw new HttpSessionRequiredException("Expected session attribute '" + name + "'", name);
                }
                container.addAttribute(name, value);
            }
        }

遍历HandlerMethod的参数,判断参数是否被@ModelAttribute注解,如果有,继续判断这个参数和参数类型是否和当前handlerMethod所在类中的@SessionAttributes注解中的参数和类型是否保持一致。

    /**
     * Find {@code @ModelAttribute} arguments also listed as {@code @SessionAttributes}.
     */
    private List<String> findSessionAttributeArguments(HandlerMethod handlerMethod) {
        List<String> result = new ArrayList<String>();
        for (MethodParameter parameter : handlerMethod.getMethodParameters()) {
            if (parameter.hasParameterAnnotation(ModelAttribute.class)) {
                String name = getNameForParameter(parameter);
                Class<?> paramType = parameter.getParameterType();
                if (this.sessionAttributesHandler.isHandlerSessionAttribute(name, paramType)) {
                    result.add(name);
                }
            }
        }
        return result;
    }
    /**
     * Whether the attribute name or type match the names and types specified
     * via {@code @SessionAttributes} on the underlying controller.
     * <p>Attributes successfully resolved through this method are "remembered"
     * and subsequently used in {@link #retrieveAttributes(WebRequest)} and
     * {@link #cleanupAttributes(WebRequest)}.
     * @param attributeName the attribute name to check
     * @param attributeType the type for the attribute
     */
    public boolean isHandlerSessionAttribute(String attributeName, Class<?> attributeType) {
        Assert.notNull(attributeName, "Attribute name must not be null");
        if (this.attributeNames.contains(attributeName) || this.attributeTypes.contains(attributeType)) {
            this.knownAttributeNames.add(attributeName);
            return true;
        }
        else {
            return false;
        }
    }

通过findSessionAttributeArguments(handlerMethod)方法,我们得到了合适的参数名称集合。遍历这个集合,我们要判断ModelAndViewContainer容器中是否存在相同名称的参数。如果不存在,我们从sessionAttributeStore根据名称中获得这个参数的值,最后将参数绑定到容器中。

        for (String name : findSessionAttributeArguments(handlerMethod)) {
            if (!container.containsAttribute(name)) {
                Object value = this.sessionAttributesHandler.retrieveAttribute(request, name);
                if (value == null) {
                    throw new HttpSessionRequiredException("Expected session attribute '" + name + "'", name);
                }
                container.addAttribute(name, value);
            }
        }

设置ModelAndViewContainer容器使用defaultModel(默认模型),而不是redirectModel(重定向模型)。

   mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

   //ModelAndViewContainer类中的方法
    public ModelMap getModel() {
        if (useDefaultModel()) {
            return this.defaultModel;
        }
        else {
            if (this.redirectModel == null) {
                this.redirectModel = new ModelMap();
            }
            return this.redirectModel;
        }
    }

    private boolean useDefaultModel() {
        return (!this.redirectModelScenario || (this.redirectModel == null && !this.ignoreDefaultModelOnRedirect));
    }

处理一些异步请求。

            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();
                if (logger.isDebugEnabled()) {
                    logger.debug("Found concurrent result value [" + result + "]");
                }
                invocableMethod = invocableMethod.wrapConcurrentResult(result);
            }

接着invocableMethod去调用invokeAndHandle这个方法。invokeAndHandle是ServletInvocableHandlerMethod中的方法。

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

            return getModelAndView(mavContainer, modelFactory, webRequest);

ServletInvocableMethod中的invokeAndHandle()其实是间接调用handlerMethod,然后处理handlerMethod的返回值。

    /**
     * Invoke the method and handle the return value through one of the
     * configured {@link HandlerMethodReturnValueHandler}s.
     * @param webRequest the current request
     * @param mavContainer the ModelAndViewContainer for this request
     * @param providedArgs "given" arguments matched by type (not resolved)
     */
    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()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        }
        else if (StringUtils.hasText(getResponseStatusReason())) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);
        try {
            this.returnValueHandlers.handleReturnValue(
                    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
        }
        catch (Exception ex) {
            if (logger.isTraceEnabled()) {
                logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
            }
            throw ex;
        }
    }

我们可以看到invokeForRequest(webRequest, mavContainer, providedArgs)这个方法会返回handlerMethod的返回值。这个方法在给定请求的上下文中解析handlerMethod的方法参数后,然后去调用handlerMethod。参数的解析是通过 {@link HandlerMethodArgumentResolver}完成的。

    public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "' with arguments " + Arrays.toString(args));
        }
        Object returnValue = doInvoke(args);
        if (logger.isTraceEnabled()) {
            logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "] returned [" + returnValue + "]");
        }
        return returnValue;
    }

InvocableHandlerMethod中的getMethodArgumentValues()这个方法是获取handlerMethod的参数。首先获取handlerMethod中的所有参数数组,数组类型是MethodParameter。遍历参数数组,给每一个参数初始化parameterNameDisconverer(参数名称发现器)。

    private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        MethodParameter[] parameters = getMethodParameters();
        Object[] args = new Object[parameters.length];
        for (int i = 0; i < parameters.length; i++) {
            MethodParameter parameter = parameters[i];
            parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
            args[i] = resolveProvidedArgument(parameter, providedArgs);
            if (args[i] != null) {
                continue;
            }
            if (this.argumentResolvers.supportsParameter(parameter)) {
                try {
                    args[i] = this.argumentResolvers.resolveArgument(
                            parameter, mavContainer, request, this.dataBinderFactory);
                    continue;
                }
                catch (Exception ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                    }
                    throw ex;
                }
            }
            if (args[i] == null) {
                throw new IllegalStateException("Could not resolve method parameter at index " +
                        parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
                        ": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
            }
        }
        return args;
    }

看到resolveProvidedArgument(parameter, providedArgs)这行代码,里面会对提供的providedArgs参数进行类型判断,判断它是否和MethodParameter类型匹配。如果类型匹配,返回提供的参数。如果不匹配,返回null。

   args[i] = resolveProvidedArgument(parameter, providedArgs);
    /**
     * Attempt to resolve a method parameter from the list of provided argument values.
     */
    private Object resolveProvidedArgument(MethodParameter parameter, Object... providedArgs) {
        if (providedArgs == null) {
            return null;
        }
        for (Object providedArg : providedArgs) {
            if (parameter.getParameterType().isInstance(providedArg)) {
                return providedArg;
            }
        }
        return null;
    }

接着看,如果提供的参数值不为空,那么跳出当前循环,继续下一次循环。

            args[i] = resolveProvidedArgument(parameter, providedArgs);
            if (args[i] != null) {
                continue;
            }

我们通过supportParameter()方法判断argumentResolvers(这是HandlerMethodArgumentResovlerComposite对象,参数解析器处理链)是否支持parameter这种类型的参数解析。

            if (this.argumentResolvers.supportsParameter(parameter)) {
                try {
                    args[i] = this.argumentResolvers.resolveArgument(
                            parameter, mavContainer, request, this.dataBinderFactory);
                    continue;
                }
                catch (Exception ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                    }
                    throw ex;
                }
            }

HandlerMethodArgumentResovlerComposite中的supportsParamter()方法,通过parameter参数类型去获得合适的HandlerMethodArgumentResolver(参数解析器)。如果没有合适的参数解析器,那么就说明HandlerMethodArgumentResolverComposite中没有合适的参数解析器能解析这种类型的参数。

    /**
     * Whether the given {@linkplain MethodParameter method parameter} is supported by any registered
     * {@link HandlerMethodArgumentResolver}.
     */
    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return (getArgumentResolver(parameter) != null);
    }

首先通过parameter参数类型从argumentResolverCache缓存中获得合适的参数解析器。 如果没有找到,那么遍历HandlerMethodArgumentResolverComposite中所有的参数器,直到找到能够解析该parameter类型的参数解析器为止,且放入到argumentResolverCache缓存中,缓存的初始容量是256。

    /**
     * Find a registered {@link HandlerMethodArgumentResolver} that supports the given method parameter.
     */
    private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
        HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
        if (result == null) {
            for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Testing if argument resolver [" + methodArgumentResolver + "] supports [" +
                            parameter.getGenericParameterType() + "]");
                }
                if (methodArgumentResolver.supportsParameter(parameter)) {
                    result = methodArgumentResolver;
                    this.argumentResolverCache.put(parameter, result);
                    break;
                }
            }
        }
        return result;
    }

如果最后的参数还为空的话,那么很遗憾抛IllegalStateException异常,没有合适的参数解析器能够解析这个参数。

            if (args[i] == null) {
                throw new IllegalStateException("Could not resolve method parameter at index " +
                        parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
                        ": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
            }

到了最为关键的一步,开始解析参数。

                try {
                    args[i] = this.argumentResolvers.resolveArgument(
                            parameter, mavContainer, request, this.dataBinderFactory);
                    continue;
                }
                catch (Exception ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                    }
                    throw ex;
                }

进入到HandlerMethodArgumentResolverComposite中的resolveArgument()放到,老样子从缓存中获得合适的参数解析器。并且由这个参数解析器来解析这个参数。

    /**
     * Iterate over registered {@link HandlerMethodArgumentResolver}s and invoke the one that supports it.
     * @throws IllegalStateException if no suitable {@link HandlerMethodArgumentResolver} is found.
     */
    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
        if (resolver == null) {
            throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
        }
        return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }

HandlerMethod所需的方法参数都已经解析完毕,那么就可以开始调用HandlerMethod了。回到InvocableHandlerMethod中的invokeForRequest()方法。

    public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "' with arguments " + Arrays.toString(args));
        }
        Object returnValue = doInvoke(args);
        if (logger.isTraceEnabled()) {
            logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "] returned [" + returnValue + "]");
        }
        return returnValue;
    }

调用doInvoke(args)方法,我们可以看到通过ReflectionUtils.makeAccessible(getBridgedMethod())让handlerMethod方法具有访问性,必要要显式的设置它具有访问性。然后准备好handlerMethod所在类的实例和方法参数,反射调用handlerMethod。

    /**
     * Invoke the handler method with the given argument values.
     */
    protected Object doInvoke(Object... args) throws Exception {
        ReflectionUtils.makeAccessible(getBridgedMethod());
        try {
            return getBridgedMethod().invoke(getBean(), args);
        }
        catch (IllegalArgumentException ex) {
            assertTargetBean(getBridgedMethod(), getBean(), args);
            String text = (ex.getMessage() != null ? ex.getMessage() : "Illegal argument");
            throw new IllegalStateException(getInvocationErrorMessage(text, args), ex);
        }
        catch (InvocationTargetException ex) {
            // Unwrap for HandlerExceptionResolvers ...
            Throwable targetException = ex.getTargetException();
            if (targetException instanceof RuntimeException) {
                throw (RuntimeException) targetException;
            }
            else if (targetException instanceof Error) {
                throw (Error) targetException;
            }
            else if (targetException instanceof Exception) {
                throw (Exception) targetException;
            }
            else {
                String text = getInvocationErrorMessage("Failed to invoke handler method", args);
                throw new IllegalStateException(text, targetException);
            }
        }
    }

handlerMethod调用完毕后,可以要对返回值进行处理的操作。这时候可以关注ServletInvocableHandlerMethod中的invokeAndHandle()方法。首先是设置ResponseStatus的状态,如果有用到{@link ResponseStatus}注解来设置响应状态。mavContainer.setRequestHandled(false)只是初始化时默认采用view的解决方案,设置为true表示response直接处理,不需要view的解决方案。

    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()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        }
        else if (StringUtils.hasText(getResponseStatusReason())) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);
        try {
            this.returnValueHandlers.handleReturnValue(
                    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
        }
        catch (Exception ex) {
            if (logger.isTraceEnabled()) {
                logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
            }
            throw ex;
        }
    }

实际上handlerMethod的返回值处理是通过HandlerMethodReturnValueHandlerComposite的handleReturnValue()方法,还是老样子,遍历所有的返回值处理器,通过supportsReturnType()判断是否支持该返回值的类型。如果类型支持的话,那么就让合适的HandlerMethodReturnValueHandler去处理handlerMethod的返回值。这里的returnType其实是ReturnValueMethodParameter类型的。(有兴趣,可以看HandlerMethod这个类)

   // 调用HandlerMethodReturnValueHandlerComposite的handleReturnValue()方法
   this.returnValueHandlers.handleReturnValue(
                    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);

    /**
     * Iterate over registered {@link HandlerMethodReturnValueHandler}s and invoke the one that supports it.
     * @throws IllegalStateException if no suitable {@link HandlerMethodReturnValueHandler} is found.
     */
    @Override
    public void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

        HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
        if (handler == null) {
            throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
        }
        handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
    }

    private HandlerMethodReturnValueHandler selectHandler(Object value, MethodParameter returnType) {
        boolean isAsyncValue = isAsyncReturnValue(value, returnType);
        for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
            if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
                continue;
            }
            if (handler.supportsReturnType(returnType)) {
                return handler;
            }
        }
        return null;
    }

回到RequestMappingHandlerAdapter中的invokeHandlerMethod()方法中

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

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

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

首先调用modelFactory.updateModel(webRequest, mavContainer)将被@SessionAtrributes注解的模型属性上升到会话级别。如果mavContainer采用的是response直接处理策略,说明没有采用view的解决方案,直接返回null即可。将ModelAndViewContainer中的model、视图的名称、HttpStatus填充到ModelAndView中。如果mavContainer没有指定逻辑视图(或者说 view不是String类型的)的话,那么就设置视图对象。如果model是RedirectAttributes的实例,那么说明是model是重定向所需要的属性,我们把model填充到FlashMap即可。

    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);
            RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
        }
        return mav;
    }

这里的updateModel()首先获得defaultModel,然后判断当前会话是否处理完毕。如果处理完毕,进行资源清理操作。如果没有处理完毕,把当前request中的model对象保存在SessionAttributesHandler中的sessionAttributeStore中,方便下次请求。如果container采用的是view策略并且使用的是默认model模型,那么就调用updateBindingResult(request, defaultModel)方法,为需要它的属性添加到BindingResult属性到defaultModel中。

    /**
     * Promote model attributes listed as {@code @SessionAttributes} to the session.
     * Add {@link BindingResult} attributes where necessary.
     * @param request the current request
     * @param container contains the model to update
     * @throws Exception if creating BindingResult attributes fails
     */
    public void updateModel(NativeWebRequest request, ModelAndViewContainer container) throws Exception {
        ModelMap defaultModel = container.getDefaultModel();
        if (container.getSessionStatus().isComplete()){
            this.sessionAttributesHandler.cleanupAttributes(request);
        }
        else {
            this.sessionAttributesHandler.storeAttributes(request, defaultModel);
        }
        if (!container.isRequestHandled() && container.getModel() == defaultModel) {
            updateBindingResult(request, defaultModel);
        }
    }

回到RequestMappingHandlerAdapter中的hanlderInternal()方法中,如果response的响应条没有设置Cache-control属性的话,如果handlerMethod对应的SessionAttributesHandler中维护了被@SessionAtrribute注解的model,那么设置Cache-control为no store模式。否则设置Cache-control为-1。

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

        ModelAndView mav;
        checkRequest(request);

         ...
            // No synchronization on session demanded at all...
            mav = invokeHandlerMethod(request, response, handlerMethod);

        if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
            if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
             // 调用的是WebContentGeneratorl类中的方法
                applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
            }
            else {
                prepareResponse(response);
            }
        }

        return mav;
    }

回到DispatcherServlet中的doDispatch()方法,我们通过适配器调用HandlerExecutionChain中的handler返回ModelAndView,如果ModelAndView中没有视图引用,那么申请设置默认的视图名称。然后调用HandlerExecutionChain中所有的拦截器中的postHandle()方法,对handlerMethod返回的结果进行加强处理。

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

                // Actually invoke the handler.
                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);
            }
            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);
                }
            }
        }
    }

接着调用processDispatchResult()方法,开始对处理程序调用返回的结构进行处理。要么是ModelAndView,要么解析成ModelAndView的异常。如果异常不为空且是ModelAndViewDefiningException类型的异常,那么把视图解析成ModelAndViewDefiningException特定的视图。如果异常不为空且不是ModelAndViewDefiningException类型的异常,那么调用 processHandlerException()让HandlerExceptionResovlers中的异常处理器来处理。

    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

    /**
     * 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);
        }
    }

processHandlerException()内部实现是遍历DispatcherServlet中的handlerExceptionResolvers(程序异常解析器集合),如果当前有异常解析器能够处理这个异常且处理完毕后返回的ModelAndView不为空,那么跳出循环。然后继续判断,如果ModelAndView中的model属性并且view属性都为空的话,把异常信息放到request中(EXCEPTION_ATTRIBUTE的值是 DispatcherServlet.class.getName() + ".EXCEPTION"),直接返回null。

    protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
            Object handler, Exception ex) throws Exception {

        // Check registered HandlerExceptionResolvers...
        ModelAndView exMv = null;
        for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
            exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
            if (exMv != null) {
                break;
            }
        }
        if (exMv != null) {
            if (exMv.isEmpty()) {
                request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
                return null;
            }
            // We might still need view name translation for a plain error model...
            if (!exMv.hasView()) {
                exMv.setViewName(getDefaultViewName(request));
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
            }
            WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
            return exMv;
        }

        throw ex;
    }

同时我们也可以看看HandlerExceptionResolverComposite中的resolveException()方法。通过遍历已配置的异常解析器列表来解决处理异常,如果处理返回的ModelAndView实例不为空,那么直接返回ModelAndView实例。

    @Override
    public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response,
            Object handler,Exception ex) {

        if (this.resolvers != null) {
            for (HandlerExceptionResolver handlerExceptionResolver : this.resolvers) {
                ModelAndView mav = handlerExceptionResolver.resolveException(request, response, handler, ex);
                if (mav != null) {
                    return mav;
                }
            }
        }
        return null;
    }

回到DispatcherServlet中的processDispatchResult(),如果ModelAndView实例不为空且modelAndView中的model和view属性不为空,那么进行render()操作。render()操作之后,判断errorView是否为true,如果为true,代表已经有错误视图去响应错误,那么就可以清理request中一些关于错误的属性(status_code、exception_type、message、exception、request_uri、servlet_name)。

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

        boolean errorView = false;
            ....
        // 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);
        }
    }
    //WebUtils

    /**
     * Standard Servlet 2.3+ spec request attributes for error pages.
     * <p>To be exposed to JSPs that are marked as error pages, when forwarding
     * to them directly rather than through the servlet container's error page
     * resolution mechanism.
     */
    public static final String ERROR_STATUS_CODE_ATTRIBUTE = "javax.servlet.error.status_code";
    public static final String ERROR_EXCEPTION_TYPE_ATTRIBUTE = "javax.servlet.error.exception_type";
    public static final String ERROR_MESSAGE_ATTRIBUTE = "javax.servlet.error.message";
    public static final String ERROR_EXCEPTION_ATTRIBUTE = "javax.servlet.error.exception";
    public static final String ERROR_REQUEST_URI_ATTRIBUTE = "javax.servlet.error.request_uri";
    public static final String ERROR_SERVLET_NAME_ATTRIBUTE = "javax.servlet.error.servlet_name";

    /**
     * Clear the Servlet spec's error attributes as {@link javax.servlet.http.HttpServletRequest}
     * attributes under the keys defined in the Servlet 2.3 specification:
     * {@code javax.servlet.error.status_code},
     * {@code javax.servlet.error.exception_type},
     * {@code javax.servlet.error.message},
     * {@code javax.servlet.error.exception},
     * {@code javax.servlet.error.request_uri},
     * {@code javax.servlet.error.servlet_name}.
     * @param request current servlet request
     */
    public static void clearErrorRequestAttributes(HttpServletRequest request) {
        request.removeAttribute(ERROR_STATUS_CODE_ATTRIBUTE);
        request.removeAttribute(ERROR_EXCEPTION_TYPE_ATTRIBUTE);
        request.removeAttribute(ERROR_MESSAGE_ATTRIBUTE);
        request.removeAttribute(ERROR_EXCEPTION_ATTRIBUTE);
        request.removeAttribute(ERROR_REQUEST_URI_ATTRIBUTE);
        request.removeAttribute(ERROR_SERVLET_NAME_ATTRIBUTE);
    }

看看render()里面是干嘛的,它实际上是为了呈现ModelAndView,这是处理请求的最后一步,里面涉及到将逻辑视图转换成真正的物理视图。

    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()) {
            // We need to resolve the view name.
            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 {
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }
            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;
        }
    }

resolveViewName()其实将视图名称转换成真正的视图对象。通过遍历当前所有的viewResolvers,如果视图解析器解析后的view对象不为空的话,那么直接返回当前view对象。

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

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

进入到ContentNegotiatingViewResolver。ContentNegotiatingViewResolver根据请求头中的Accept属性或者是请求文件名来解析视图。它本身不提供解析视图,而是通过viewResolvers集合中的视图解析器来解析视图。resolveViewName()这个方法,通过请求头中的Accept属性获取requestMediaTypes,由此获取与之兼容的view对象集合。再通过兼容的view对象集合获得最佳匹配的view对象(AbstractMessageConverterMethodProcessor也用了类似的方法,获得最佳的selectedMediaType)。

    @Override
    public View resolveViewName(String viewName, Locale locale) throws Exception {
        RequestAttributes attrs = RequestContextHolder.getRequestAttributes();
        Assert.state(attrs instanceof ServletRequestAttributes, "No current ServletRequestAttributes");
        List<MediaType> requestedMediaTypes = getMediaTypes(((ServletRequestAttributes) attrs).getRequest());
        if (requestedMediaTypes != null) {
            List<View> candidateViews = getCandidateViews(viewName, locale, requestedMediaTypes);
            View bestView = getBestView(candidateViews, requestedMediaTypes, attrs);
            if (bestView != null) {
                return bestView;
            }
        }
        if (this.useNotAcceptableStatusCode) {
            if (logger.isDebugEnabled()) {
                logger.debug("No acceptable view found; returning 406 (Not Acceptable) status code");
            }
            return NOT_ACCEPTABLE_VIEW;
        }
        else {
            logger.debug("No acceptable view found; returning null");
            return null;
        }
    }

获取到合适的view对象,那么调用其本身的render()方法,将model对象填充到到view对象中,完成渲染操作。

            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }
            view.render(mv.getModelInternal(), request, response);

尾言

对于一个框架,我们不仅要做到熟练使用,还用知其然知其所以然。以后我再写关于框架源码分析的文章,会尽量切割,缩小篇幅。

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