之前分析過SpringMVC中的DispatcherServlet，分析了SpringMVC處理請(qǐng)求的過程。但忽略了一些DispatcherServlet協(xié)助請(qǐng)求處理的組件，例如SpringMVC中的HandlerMapping、HandlerAdapter、ViewResolvers等等。

HandlerMappings

HandlerMappings在DispathServlet中主要作用是為請(qǐng)求的urlpath匹配對(duì)應(yīng)的Controller，建立一個(gè)映射關(guān)系，根據(jù)請(qǐng)求查找Handler、Interceptor。HandlerMappings將請(qǐng)求傳遞到HandlerExecutionChain上，HandlerExecutionChain包含了一個(gè)能夠處理該請(qǐng)求的處理器，還可以包含攔截改請(qǐng)求的攔截器。

在沒有處理器映射相關(guān)配置情況下，DispatcherServlet會(huì)為你創(chuàng)建一個(gè)BeanNameUrlHandlerMapping作為默認(rèn)映射的配置。在DispatchServlet.properties文件中對(duì)于HandlerMapping的默認(rèn)配置是：

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

HandlerMapping的配置策略一般分為配置式BeanNameUrlHandlerMapping和注解式DefaultAnnotationHandlerMapping。不過DefaultAnnotationHandlerMapping已經(jīng)被放棄了，取代它的是RequestMappingHandlerMapping，不知道為啥SpringMVC這個(gè)默認(rèn)配置尚未做修改。

AbstractHandlerMapping

AbstractHandlerMapping是HandlerMapping的抽象實(shí)現(xiàn)，是所有HandlerMapping實(shí)現(xiàn)類的父類。

AbstractHandlerMapping的作用是是為了初始化Interceptors。AbstractHandlerMapping重寫了WebApplicationObjectSupport的initApplicationContext方法。

    protected void initApplicationContext() throws BeansException {
        extendInterceptors(this.interceptors);
        detectMappedInterceptors(this.adaptedInterceptors);
        initInterceptors();
    }

• extendInterceptors方法，Springmvc并沒有做出具體實(shí)現(xiàn)，這里留下一個(gè)拓展，子類可以重寫這個(gè)模板方法，為子類添加或者修改Interceptors。

• detectMappedInterceptors方法將SpringMVC容器中所有MappedInterceptor類的bean添加到adaptedInterceptors中。

• 最后調(diào)用initInterceptors初始化攔截器。遍歷interceptors將WebRequestInterceptor和HandlerInterceptor類型的攔截器添加到adaptedInterceptors中。

HandlerMapping通過getHandler方法來獲取請(qǐng)求的處理器Handler和攔截器Interceptor。在getHandlerExecutionChain方法中將遍歷之前初始化的adaptedInterceptors，為當(dāng)前的請(qǐng)求選擇對(duì)應(yīng)的MappedInterceptors和adaptedInterceptors。

AbstractUrlHandlerMapping

AbstractUrlHandlerMapping

AbstractUrlHandlerMapping繼承于AbstractHandlerMapping，它是通過URL來匹配具體的Handler。AbstractUrlHandlerMapping維護(hù)一個(gè)handlerMap來存儲(chǔ)Url和Handler的映射關(guān)系。

AbstractUrlHandlerMapping重寫了AbstractHandlerMapping類中的getHandlerInternal方法。HandlerMapping通過getHandler方法，就會(huì)調(diào)用這里的getHandlerInternal方法來獲取Handler。getHandlerInternal方法中關(guān)鍵調(diào)用lookupHandler方法去獲取handler。

    protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
        // Direct match?
        Object handler = this.handlerMap.get(urlPath);
        if (handler != null) {
            // Bean name or resolved handler?
            if (handler instanceof String) {
                String handlerName = (String) handler;
                handler = getApplicationContext().getBean(handlerName);
            }
            validateHandler(handler, request);
            return buildPathExposingHandler(handler, urlPath, urlPath, null);
        }

        // Pattern match?
        List<String> matchingPatterns = new ArrayList<String>();
        for (String registeredPattern : this.handlerMap.keySet()) {
            if (getPathMatcher().match(registeredPattern, urlPath)) {
                matchingPatterns.add(registeredPattern);
            }
            else if (useTrailingSlashMatch()) {
                if (!registeredPattern.endsWith("/") && getPathMatcher().match(registeredPattern + "/", urlPath)) {
                    matchingPatterns.add(registeredPattern +"/");
                }
            }
        }

        String bestMatch = null;
        Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
        if (!matchingPatterns.isEmpty()) {
            Collections.sort(matchingPatterns, patternComparator);
            if (logger.isDebugEnabled()) {
                logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
            }
            bestMatch = matchingPatterns.get(0);
        }
        if (bestMatch != null) {
            handler = this.handlerMap.get(bestMatch);
            if (handler == null) {
                if (bestMatch.endsWith("/")) {
                    handler = this.handlerMap.get(bestMatch.substring(0, bestMatch.length() - 1));
                }
                if (handler == null) {
                    throw new IllegalStateException(
                            "Could not find handler for best pattern match [" + bestMatch + "]");
                }
            }
            // Bean name or resolved handler?
            if (handler instanceof String) {
                String handlerName = (String) handler;
                handler = getApplicationContext().getBean(handlerName);
            }
            validateHandler(handler, request);
            String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestMatch, urlPath);

            // There might be multiple 'best patterns', let's make sure we have the correct URI template variables
            // for all of them
            Map<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();
            for (String matchingPattern : matchingPatterns) {
                if (patternComparator.compare(bestMatch, matchingPattern) == 0) {
                    Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
                    Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
                    uriTemplateVariables.putAll(decodedVars);
                }
            }
            if (logger.isDebugEnabled()) {
                logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
            }
            return buildPathExposingHandler(handler, bestMatch, pathWithinMapping, uriTemplateVariables);
        }

        // No handler found...
        return null;
    }

• 首先調(diào)用lookupHandler方法來獲取handler。在lookupHandler方法中，先通過URL在handlerMap查找是否有合適的handler。
• 如果沒有獲取到handler，遍歷handlerMap利用正則匹配的方法，找到符合要求的handlers(有可能是多個(gè))。
• 正則匹配是采用Ant風(fēng)格，將會(huì)通過排序篩選出一個(gè)匹配程度最高的Handler。
• 最后調(diào)用buildPathExposingHandler方法構(gòu)建一個(gè)handler,添加PathExposingHandlerInterceptor和UriTemplateVariablesHandlerInterceptor兩個(gè)攔截器并返回。

上面介紹獲取handler的過程中，會(huì)先從handlerMap查找。下面看一下handlerMap是如何初始化的。AbstractUrlHandlerMapping是通過registerHandler初始化handlerMap的。AbstractUrlHandlerMapping共有兩個(gè)registerHandler方法。分別是注冊(cè)多個(gè)url到一個(gè)handler和注冊(cè)一個(gè)url到一個(gè)handler。首先判斷handlerMap是否有此handler。如果存在的話，判斷是否一致，不一致則拋出異常，如果不存在的話，如果url是/、/*，則，返回root handler、default handler，如果不是將添加到handlerMap中。

SimpleUrlHandlerMapping

SimpleUrlHandlerMapping繼承于AbstractUrlHandlerMapping。SimpleUrlHandlerMapping重寫了父類AbstractHandlerMapping中的初始化方法initApplicationContext。在initApplicationContext方法中調(diào)用registerHandlers方法。

    protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {
        if (urlMap.isEmpty()) {
            logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
        }
        else {
            for (Map.Entry<String, Object> entry : urlMap.entrySet()) {
                String url = entry.getKey();
                Object handler = entry.getValue();
                // Prepend with slash if not already present.
                if (!url.startsWith("/")) {
                    url = "/" + url;
                }
                // Remove whitespace from handler bean name.
                if (handler instanceof String) {
                    handler = ((String) handler).trim();
                }
                registerHandler(url, handler);
            }
        }
    }

判斷是url是否以/開頭，如果不是，默認(rèn)補(bǔ)齊/,確保所有的url都是以/開頭，然后依次調(diào)用父類的registerHandler方法注冊(cè)到AbstractUrlHandlerMapping中的handlerMap。

在使用SimpleUrlHandlerMapping時(shí)，需要在注冊(cè)的時(shí)候配置其urlmap否則會(huì)拋異常。

AbstractDetectingUrlHandlerMapping

AbstractDetectingUrlHandlerMapping類繼承于AbstractUrlHandlerMapping類，重寫了initApplicationContext方法，在initApplicationContext方法中調(diào)用了detectHandlers方法。

    protected void detectHandlers() throws BeansException {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking for URL mappings in application context: " + getApplicationContext());
        }
        String[] beanNames = (this.detectHandlersInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
                getApplicationContext().getBeanNamesForType(Object.class));

        // Take any bean name that we can determine URLs for.
        for (String beanName : beanNames) {
            String[] urls = determineUrlsForHandler(beanName);
            if (!ObjectUtils.isEmpty(urls)) {
                // URL paths found: Let's consider it a handler.
                registerHandler(urls, beanName);
            }
            else {
                if (logger.isDebugEnabled()) {
                    logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");
                }
            }
        }
    }

獲取所有容器的beanNames，遍歷所有的beanName，調(diào)用determineUrlsForHandler方法解析url，這里的determineUrlsForHandler也是運(yùn)用了模板方法設(shè)計(jì)模式，具體的實(shí)現(xiàn)在其子類中，如果解析到子類，將注冊(cè)到父類的handlerMap中。

BeanNameUrlHandlerMapping

BeanNameUrlHandlerMapping類的類圖大致如下：

BeanNameUrlHandlerMapping類繼承于AbstractDetectingUrlHandlerMapping類。重寫了父類中的determineUrlsForHandler方法。

    protected String[] determineUrlsForHandler(String beanName) {
        List<String> urls = new ArrayList<String>();
        if (beanName.startsWith("/")) {
            urls.add(beanName);
        }
        String[] aliases = getApplicationContext().getAliases(beanName);
        for (String alias : aliases) {
            if (alias.startsWith("/")) {
                urls.add(alias);
            }
        }
        return StringUtils.toStringArray(urls);
    }

其通過beanName解析Url規(guī)則也很簡(jiǎn)單，判斷beanName是否以/開頭。

BeanNameUrlHandlerMapping是SpringMVC的默認(rèn)映射配置。

AbstractHandlerMethodMapping

通常我們也習(xí)慣于用@Controller、@Re questMapping來定義Handler，AbstractHandlerMethodMapping可以將method作為Handler來使用。

AbstractHandlerMethodMapping實(shí)現(xiàn)了InitializingBean接口，實(shí)現(xiàn)了afterPropertiesSet方法。當(dāng)容器啟動(dòng)的時(shí)候會(huì)調(diào)用initHandlerMethods注冊(cè)委托handler中的方法。

    public void afterPropertiesSet() {
        initHandlerMethods();
    }
    
    protected void initHandlerMethods() {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking for request mappings in application context: " + getApplicationContext());
        }
        String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
                getApplicationContext().getBeanNamesForType(Object.class));

        for (String beanName : beanNames) {
            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                Class<?> beanType = null;
                try {
                    beanType = getApplicationContext().getType(beanName);
                }
                catch (Throwable ex) {
                    // An unresolvable bean type, probably from a lazy bean - let's ignore it.
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
                    }
                }
                if (beanType != null && isHandler(beanType)) {
                    detectHandlerMethods(beanName);
                }
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }

在initHandlerMethods方法中，做了以下工作：

• 首先通過BeanFactoryUtils掃描應(yīng)用上下文，獲取所有的bean。
• 遍歷所有的beanName，調(diào)用isHandler方法判斷是目標(biāo)bean是否包含@Controller或@RequestMapping注解。
• 對(duì)于帶有@Controller或@RequestMapping注解的類，調(diào)用detectHandlerMethods委托處理,獲取所有的method，并調(diào)用registerHandlerMethod注冊(cè)所有的方法。

在detectHandlerMethods方法負(fù)責(zé)將Handler保存到Map中。

    protected void detectHandlerMethods(final Object handler) {
       //  獲取handler的類型
        Class<?> handlerType = (handler instanceof String ?
                getApplicationContext().getType((String) handler) : handler.getClass());
        final Class<?> userType = ClassUtils.getUserClass(handlerType);
        
        Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                new MethodIntrospector.MetadataLookup<T>() {
                    @Override
                    public T inspect(Method method) {
                        try {
                            return getMappingForMethod(method, userType);
                        }
                        catch (Throwable ex) {
                            throw new IllegalStateException("Invalid mapping on handler class [" +
                                    userType.getName() + "]: " + method, ex);
                        }
                    }
                });

        if (logger.isDebugEnabled()) {
            logger.debug(methods.size() + " request handler methods found on " + userType + ": " + methods);
        }
        for (Map.Entry<Method, T> entry : methods.entrySet()) {
            Method invocableMethod = AopUtils.selectInvocableMethod(entry.getKey(), userType);
            T mapping = entry.getValue();
            registerHandlerMethod(handler, invocableMethod, mapping);
        }
    }
    
    

selectMethods方法中重寫了MetadataLookup中的inspect方法，inspect方法中調(diào)用了子類RequestMappingHandlerMapping實(shí)現(xiàn)了getMappingForMethod模板方法，用于構(gòu)建RequestMappingInfo。

public static <T> Map<Method, T> selectMethods(Class<?> targetType, final MetadataLookup<T> metadataLookup) {
        final Map<Method, T> methodMap = new LinkedHashMap<Method, T>();
        Set<Class<?>> handlerTypes = new LinkedHashSet<Class<?>>();
        Class<?> specificHandlerType = null;

        if (!Proxy.isProxyClass(targetType)) {
            handlerTypes.add(targetType);
            specificHandlerType = targetType;
        }
        handlerTypes.addAll(Arrays.asList(targetType.getInterfaces()));

        for (Class<?> currentHandlerType : handlerTypes) {
            final Class<?> targetClass = (specificHandlerType != null ? specificHandlerType : currentHandlerType);

            ReflectionUtils.doWithMethods(currentHandlerType, new ReflectionUtils.MethodCallback() {
                @Override
                public void doWith(Method method) {
                    Method specificMethod = ClassUtils.getMostSpecificMethod(method, targetClass);
                    T result = metadataLookup.inspect(specificMethod);
                    if (result != null) {
                        Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
                        if (bridgedMethod == specificMethod || metadataLookup.inspect(bridgedMethod) == null) {
                            methodMap.put(specificMethod, result);
                        }
                    }
                }
            }, ReflectionUtils.USER_DECLARED_METHODS);
        }

        return methodMap;
    }

在selectMethods通過反射獲取所有的方法，重寫了doWith方法，將handler中的method和請(qǐng)求對(duì)應(yīng)的RequestMappingInfo保存到methodMap中。

最終detectHandlerMethods將遍歷這個(gè)methodMap，調(diào)用registerHandlerMethod注冊(cè)HandlerMethod到MappingRegistry。

在AbstractHandlerMethodMapping類中，有個(gè)內(nèi)部類MappingRegistry，用來存儲(chǔ)mapping和 handler methods注冊(cè)關(guān)系，并提供了并發(fā)訪問方法。

AbstractHandlerMethodMapping通過getHandlerInternal來為一個(gè)請(qǐng)求選擇對(duì)應(yīng)的handler。

    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
       // 根據(jù)request獲取對(duì)應(yīng)的urlpath
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        if (logger.isDebugEnabled()) {
            logger.debug("Looking up handler method for path " + lookupPath);
        }
        // 獲取讀鎖
        this.mappingRegistry.acquireReadLock();
        try {
          // 調(diào)用lookupHandlerMethod方法獲取請(qǐng)求對(duì)應(yīng)的HandlerMethod
            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();
        }
    }

lookupHandlerMethod的具體實(shí)現(xiàn)如下：

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        List<Match> matches = new ArrayList<Match>();
        // 通過lookupPath獲取所有匹配到的path
        List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
        if (directPathMatches != null) {
           // 將匹配條件添加到matches
            addMatchingMappings(directPathMatches, matches, request);
        }
        if (matches.isEmpty()) {
            // 如果沒有匹配條件，將所有的匹配條件都加入matches
            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);
            }
            // 選取排序后的第一個(gè)作為最近排序條件
            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                if (CorsUtils.isPreFlightRequest(request)) {
                    return PREFLIGHT_AMBIGUOUS_MATCH;
                }
                Match secondBestMatch = matches.get(1);
                // 前兩個(gè)匹配條件排序一樣拋出異常
                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 + "}");
                }
            }
            // 將lookupPath設(shè)為請(qǐng)求request的PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE屬性
            handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        }
        else {
            return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
        }
    }

整個(gè)過程以Match作為載體，Match是個(gè)內(nèi)部類，封裝了匹配條件和handlerMethod兩個(gè)屬性，默認(rèn)的實(shí)現(xiàn)是將lookupPath設(shè)置為請(qǐng)求的屬性。

總結(jié)

本文從源碼角度上分析了HandlerMapping的各種實(shí)現(xiàn)。主要功能是為請(qǐng)求找到合適的handler和interceptors，并組合成HandlerExecutionChain。查找handler的過程通過getHandlerInternal方法實(shí)現(xiàn)，每個(gè)子類都其不同的實(shí)現(xiàn)。

所有的HandlerMapping的實(shí)現(xiàn)都繼承于AbstarctHandlerMapping，AbstarctHandlerMapping主要作用是完成攔截器的初始化工作。而通過AbstarctHandlerMapping又衍生出兩個(gè)系列，AbstractUrlHandlerMapping和AbstractHandlerMethodMapping。

AbstractUrlHandlerMapping也有很多子類的實(shí)現(xiàn)，如SimpleUrlHandlerMapping、AbstractDetectingUrlHandlerMapping。總體來說，AbstractUrlHandlerMapping需要用到一個(gè)保存url和handler的對(duì)應(yīng)關(guān)系的map，map的初始化工作由子類實(shí)現(xiàn)。不同的子類會(huì)有自己的策略，可以在配置文件中注冊(cè)，也可以在spring容器中找。

AbstractHandlerMethodMapping系列則通常用于注解的方法，解析包含@Controller或者@RequestMapping注解的類，建立url和method的直接對(duì)應(yīng)關(guān)系，這也是目前使用最多的一種方式。