Spring MVC工作原理及源码解析（三） HandlerMapping和HandlerAdapter实现原理及源码解析

1、HandlerMapping实现原理及源码解析

在前面讲解Spring MVC工作流程的时候我们说过，前端控制器收到请求后会调⽤处理器映射器（HandlerMapping），处理器映射器根据请求Url找到具体的处理器（Handler），生成处理器对象及处理器拦截器（如果有）一并返回给前端控制器。HandlerMapping就是处理器映射器，其作用就是根据当前请求的找到对应的 Handler，并将 Handler（执行程序）与一堆 HandlerInterceptor（拦截器）封装到HandlerExecutionChain 对象中。

HandlerMapping 具体的表现形式可以是类，也可以是⽅法。⽐如，标注了@RequestMapping的每个⽅法都可以看成是⼀个Handler。Handler负责具体实际的请求处理，在请求到达后，HandlerMapping 的作用便是找到请求相应的处理器Handler 和 Interceptor。

在 HandlerMapping 接口的内部只有一个方法：HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception; 这个方法由 AbstractHandlerMapping 实现，实现方法如下：

　　/**
     * 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
    @Nullable
    public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        // 根据请求获取执行程序，具体的获取方式由子类决定
        // getHandlerInternal() 是抽象方法
        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 = obtainApplicationContext().getBean(handlerName);
        }

        // 将 Handler 与一堆拦截器包装到 HandlerExecutionChain 对象中
        HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

        if (logger.isTraceEnabled()) {
            logger.trace("Mapped to " + handler);
        }
        else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
            logger.debug("Mapped to " + executionChain.getHandler());
        }

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

        return executionChain;
    }

可以看到在这个方法中调用了 getHandlerInternal() 方法获取到了 Handler 对象，而 Handler 对象具体内容是由它的子类去定义的。

HandlerMapping 有很多个实现类，如下图所示：

它主要有两个分支，分别继承自 AbstractHandlerMethodMapping（得到 HandlerMethod）和 AbstractUrlHandlerMapping（得到 HttpRequestHandler、Controller 或 Servlet），它们又统一继承于 AbstractHandlerMapping，如下图所示：

下面就来看一下 AbstractHandlerMapping 的两个分支子类（AbstractUrlHandlerMapping和AbstractHandlerMethodMapping）。

1.1、AbstractUrlHandlerMapping

AbstractUrlHandlerMapping 这个分支获取的 Handler 的类型实际就是一个 Controller 类，所以一个 Controller 只能对应一个请求（或者像 Struts2 那样定位到方法，使同一个业务的方法放在同一个类里）.

上文中getHandler的实现方法中调用的getHandlerInternal方法（根据请求获取执行程序）就是在AbstractUrlHandlerMapping中，源码如下所示：

/**
     * Look up a handler for the URL path of the given request.
     * @param request current HTTP request
     * @return the handler instance, or {@code null} if none found
     */
    @Override
    @Nullable
    protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
        // 根据当前请求获取 查找路径
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        // 根据路径获取 Handler（即Controller），先尝试直接匹配，再尝试模式匹配
        Object handler = lookupHandler(lookupPath, request);
        if (handler == null) {
            // We need to care for the default handler directly, since we need to
            // expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
            Object rawHandler = null;
            if ("/".equals(lookupPath)) {
                rawHandler = getRootHandler();
            }
            if (rawHandler == null) {
                rawHandler = getDefaultHandler();
            }
            if (rawHandler != null) {
                // Bean name or resolved handler?
                if (rawHandler instanceof String) {
                    String handlerName = (String) rawHandler;
                    rawHandler = obtainApplicationContext().getBean(handlerName);
                }
                validateHandler(rawHandler, request);
                handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
            }
        }
        return handler;
    }

1.2、AbstractHandlerMethodMapping

AbstractHandlerMethodMapping 这个分支获取的 Handler 的类型是 HandlerMethod，即这个 Handler 是一个方法，它保存了方法的信息（如Method），这样一个 Controller 就可以处理多个请求了，源码如下所示：

/**
     * Look up a handler method for the given request.
     */
    @Override
    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
        // 根据当前请求获取 查找路径
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        // 内部类对象获取锁
        this.mappingRegistry.acquireReadLock();
        try {
            // 获取当前请求最佳匹配的处理方法（即Controller类的方法）
            HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
            return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
        }
        finally {
            // 内部类对象释放锁
            this.mappingRegistry.releaseReadLock();
        }
    }

上述代码中 lookupHandlerMethod() 方法的主要工作是在 Map<T, HandlerMethod> handlerMethods 中找到 HandlerMethod，这里的 T 是 HandlerMappingInfo，它封装了 @RequestMapping 注解中的信息。lookupHandlerMethod的源码如下图所示：

/**
     * 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)
     */
    @Nullable
    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        List<Match> matches = new ArrayList<>();
        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));
            matches.sort(comparator);
            Match bestMatch = matches.get(0);
            if (matches.size() > 1) {
                if (logger.isTraceEnabled()) {
                    logger.trace(matches.size() + " matching mappings: " + matches);
                }
                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();
                    String uri = request.getRequestURI();
                    throw new IllegalStateException(
                            "Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
                }
            }
            request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);
            handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        }
        else {
            return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
        }
    }

那么 HandlerMethod 是怎么创建的（即怎么把 Controller 的方法变成了它）？继续看一下源码找到 initHandlerMethods() 方法，这个方法是在这个类创建后调用的，如下所示是它的源码（包含其调用的子方法的源码）：

    /**
     * Scan beans in the ApplicationContext, detect and register handler methods.
     * @see #getCandidateBeanNames()
     * @see #processCandidateBean
     * @see #handlerMethodsInitialized
     */
    protected void initHandlerMethods() {
        for (String beanName : getCandidateBeanNames()) {
            if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
                processCandidateBean(beanName);
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }

    /**
     * Determine the names of candidate beans in the application context.
     * @since 5.1
     * @see #setDetectHandlerMethodsInAncestorContexts
     * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors
     */
    protected String[] getCandidateBeanNames() {
        // 从容器中获取所有 Bean 的名称，detectHandlerMethodsInAncestorContexts 默认false，不从父容器中查找
        // 即默认只查找 SpringMVC 的 IOC 容器，不查找它的父容器 Spring 的 IOC 容器
        return (this.detectHandlerMethodsInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
                obtainApplicationContext().getBeanNamesForType(Object.class));
    }

    /**
     * Determine the type of the specified candidate bean and call
     * {@link #detectHandlerMethods} if identified as a handler type.
     * <p>This implementation avoids bean creation through checking
     * {@link org.springframework.beans.factory.BeanFactory#getType}
     * and calling {@link #detectHandlerMethods} with the bean name.
     * @param beanName the name of the candidate bean
     * @since 5.1
     * @see #isHandler
     * @see #detectHandlerMethods
     */
    protected void processCandidateBean(String beanName) {
        Class<?> beanType = null;
        try {
            beanType = obtainApplicationContext().getType(beanName);
        }
        catch (Throwable ex) {
            // An unresolvable bean type, probably from a lazy bean - let's ignore it.
            if (logger.isTraceEnabled()) {
                logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
            }
        }
        // 这里的 isHandler()方法由子类实现，判断是否拥有 @Controller 注解或 @RequestMapping 注解
        if (beanType != null && isHandler(beanType)) {
            // 利用反射得到 Bean 中的 Method 并包装成 HandlerMethod，然后放入 Map 中
            detectHandlerMethods(beanName);
        }
    }

    /**
     * Look for handler methods in the specified handler bean.
     * @param handler either a bean name or an actual handler instance
     * @see #getMappingForMethod
     */
    protected void detectHandlerMethods(Object handler) {
        // 获取这个 Bean 的 Class 对象
        Class<?> handlerType = (handler instanceof String ?
                obtainApplicationContext().getType((String) handler) : handler.getClass());

        if (handlerType != null) {
            // 获取被代理前的原始类型
            Class<?> userType = ClassUtils.getUserClass(handlerType);
            
            // 获取 Method
            // 这里的 T 就是 RequestMappingInfo，它封装了 @RequestMapping 信息
            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                    (MethodIntrospector.MetadataLookup<T>) method -> {
                        try {
                            // 根据 Method 和它的 @RequestMapping 注解，创建 RequestMappingInfo 对象。
                            return getMappingForMethod(method, userType);
                        }
                        catch (Throwable ex) {
                            throw new IllegalStateException("Invalid mapping on handler class [" +
                                    userType.getName() + "]: " + method, ex);
                        }
                    });
            if (logger.isTraceEnabled()) {
                logger.trace(formatMappings(userType, methods));
            }
            methods.forEach((method, mapping) -> {
                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                // 注册 Method 和它的映射，RequestMappingInfo 储存着映射信息
                registerHandlerMethod(handler, invocableMethod, mapping);
            });
        }
    }

看完上述代码后，可以知道是在 detectHandlerMethods() 方法中将 Bean 的方法转换为 HandlerMethod 对象。

最后在 registerHandlerMethod() 方法中，将 RequestMappingInfo 作为 key，把 Method 包装成 HandlerMethod 作为 value 添加到了 Map<T, HandlerMethod> handlerMethods 中。registerHandlerMethod的源码如下所示：

    /**
     * Register a handler method and its unique mapping. Invoked at startup for
     * each detected handler method.
     * @param handler the bean name of the handler or the handler instance
     * @param method the method to register
     * @param mapping the mapping conditions associated with the handler method
     * @throws IllegalStateException if another method was already registered
     * under the same mapping
     */
    protected void registerHandlerMethod(Object handler, Method method, T mapping) {
        this.mappingRegistry.register(mapping, handler, method);
    }

　　public void register(T mapping, Object handler, Method method) {
        this.readWriteLock.writeLock().lock();
        try {
　　　　　　　　HandlerMethod handlerMethod = createHandlerMethod(handler, method);
             assertUniqueMethodMapping(handlerMethod, mapping);
             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<>(mapping, handlerMethod, directUrls, name));
         } finally {
　　　　　　　　this.readWriteLock.writeLock().unlock();
         }
　　}

1.3、AbstractHandlerMapping 实现类及使用

AbstractHandlerMethodMapping<T> 只有一个实现类：RequestMappingHandlerMapping（下文中会介绍该类）。

2、HandlerAdapter实现原理及源码解析

HandlerAdapter的作用是根据 Handler 来找到支持它的 HandlerAdapter，通过 HandlerAdapter 执行这个 Handler 得到 ModelAndView 对象。HandlerAdapter 接口中的方法如下：

它有以下实现类：

2.1、RequestMappingHandlerAdapter

从上面的文章中可以知道，利用 RequestMappingHandlerMapping 获取的 Handler 是 HandlerMethod 类型，它代表 Controller 里要执行的方法，而 RequestMappingHandlerAdapter 可以执行 HandlerMethod 对象。

RequestMappingHandlerAdapter 的 handle() 方法是在它的父类 AbstractHandlerMethodAdapter 类中实现的，源码如下所示：

    /**
     * This implementation expects the handler to be an {@link HandlerMethod}.
     */
    @Override
    @Nullable
    public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        return handleInternal(request, response, (HandlerMethod) handler);
    }

handleInternal() 方法是由 RequestMappingHandlerAdapter 自己来实现的，源码如下所示：

    /**
     * Use the given handler method to handle the request.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handlerMethod handler method to use. This object must have previously been passed to the
     * {@link #supportsInternal(HandlerMethod)} this interface, which must have returned {@code true}.
     * @return a ModelAndView object with the name of the view and the required model data,
     * or {@code null} if the request has been handled directly
     * @throws Exception in case of errors
     */
    @Override
    protected ModelAndView handleInternal(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ModelAndView mav;
        checkRequest(request);

        // Execute invokeHandlerMethod in synchronized block if required.（是否需要在 synchronize 块中执行）
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized (mutex) {
                    // 执行 HandlerMethod
                    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)) {
            // 是否通过 @SessionAttributes 注释声明了 session 属性
            if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
                applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
            }
            else {
                prepareResponse(response);
            }
        }

        // 执行 HandlerMethod（invokeHandlerMethod(request, response, handlerMethod);），得到 ModelAndView
        return mav;
    }

继续来看一下如何得到 ModelAndView，invokeHandlerMethod() 方法如下：

    /**
     * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
     * if view resolution is required.
     * @since 4.2
     * @see #createInvocableHandlerMethod(HandlerMethod)
     */
    @Nullable
    protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

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

            // 绑定参数，执行方法
            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
            if (this.argumentResolvers != null) {
                invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
            }
            if (this.returnValueHandlers != null) {
                invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
            }
            invocableMethod.setDataBinderFactory(binderFactory);
            invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

            // 创建模型和视图容器
            ModelAndViewContainer mavContainer = new ModelAndViewContainer();
            // 设置FlasgMap中的值
            mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
            // 初始化模型
            modelFactory.initModel(webRequest, mavContainer, invocableMethod);
            mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

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

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

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

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

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

2.2、HttpRequestHandlerAdapter

HttpRequestHandlerAdapter 可以执行 HttpRequestHandler 类型的 Handler，源码如下：

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        ((HttpRequestHandler) handler).handleRequest(request, response);
        return null;
    }

2.3、SimpleControllerHandlerAdapter

SimpleControllerHandlerAdapter 可以执行 Controller 类型的 Handler，源码如下：

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        return ((Controller) handler).handleRequest(request, response);
    }

2.4、SimpleServletHandlerAdapter 

SimpleServletHandlerAdapter 可以执行 Servlet 类型的 Handler，源码如下：

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        ((Servlet) handler).service(request, response);
        return null;
    }