面試官：你分析過SpringMVC的源碼嗎？

時間 2019-12-05

原文原文鏈接

1. MVC使用

在研究源碼以前，先來回顧如下springmvc 是如何配置的，這將能使咱們更容易理解源碼。前端

1.1 web.xml

<servlet>
    <servlet-name>mvc-dispatcher</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <!-- 配置springMVC須要加載的配置文件
        spring-dao.xml,spring-service.xml,spring-web.xml
        Mybatis - > spring -> springmvc
     -->
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:spring/spring-*.xml</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>mvc-dispatcher</servlet-name>
    <!-- 默認匹配全部的請求 -->
    <url-pattern>/</url-pattern>
</servlet-mapping>
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值的注意的是contextConfigLocation和DispatcherServlet(用此類來攔截請求)的引用和配置。web

1.2 spring-web.xml

<!-- 配置SpringMVC -->
<!-- 1.開啓SpringMVC註解模式 -->
<!-- 簡化配置： 
    (1)自動註冊DefaultAnootationHandlerMapping,AnotationMethodHandlerAdapter 
    (2)提供一些列：數據綁定，數字和日期的format @NumberFormat, @DateTimeFormat, xml,json默認讀寫支持 
-->
<mvc:annotation-driven />

<!-- 2.靜態資源默認servlet配置
    (1)加入對靜態資源的處理：js,gif,png
    (2)容許使用"/"作總體映射
 -->
 <mvc:default-servlet-handler/>

 <!-- 3.配置jsp 顯示ViewResolver -->
 <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
     <property name="viewClass" value="org.springframework.web.servlet.view.JstlView" />
     <property name="prefix" value="/WEB-INF/jsp/" />
     <property name="suffix" value=".jsp" />
 </bean>

 <!-- 4.掃描web相關的bean -->
 <context:component-scan base-package="com.xxx.fantj.web" />
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值的注意的是InternalResourceViewResolver，它會在ModelAndView返回的試圖名前面加上prefix前綴，在後面加載suffix指定後綴。面試

SpringMvc主支源碼分析

引用《Spring in Action》中的一張圖來更好的瞭解執行過程： spring

上圖流程整體來講可分爲三大塊：json

Map的創建(並放入WebApplicationContext)後端
HttpRequest請求中Url的請求攔截處理(DispatchServlet處理)api
反射調用Controller中對應的處理方法，並返回視圖數組

本文將圍繞這三塊進行分析。bash

1. Map的創建

在容器初始化時會創建全部 url 和 Controller 的對應關係,保存到 Map中，那是如何保存的呢。架構

ApplicationObjectSupport #setApplicationContext方法

// 初始化ApplicationContext
@Override
public void initApplicationContext() throws ApplicationContextException {
    super.initApplicationContext();
    detectHandlers();
}
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AbstractDetectingUrlHandlerMapping #detectHandlers()方法：

/**
 * 創建當前ApplicationContext 中的 全部Controller 和url 的對應關係
 * Register all handlers found in the current ApplicationContext.
 * <p>The actual URL determination for a handler is up to the concrete
 * {@link #determineUrlsForHandler(String)} implementation. A bean for
 * which no such URLs could be determined is simply not considered a handler.
 * @throws org.springframework.beans.BeansException if the handler couldn't be registered * @see #determineUrlsForHandler(String) */ protected void detectHandlers() throws BeansException { if (logger.isDebugEnabled()) { logger.debug("Looking for URL mappings in application context: " + getApplicationContext()); } // 獲取容器中的beanNames String[] beanNames = (this.detectHandlersInAncestorContexts ? BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) : getApplicationContext().getBeanNamesForType(Object.class)); // 遍歷 beanNames 並找到對應的 url // Take any bean name that we can determine URLs for. for (String beanName : beanNames) { // 獲取bean上的url(class上的url + method 上的 url) String[] urls = determineUrlsForHandler(beanName); if (!ObjectUtils.isEmpty(urls)) { // URL paths found: Let's consider it a handler.
            // 保存url 和 beanName 的對應關係
            registerHandler(urls, beanName);
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");
            }
        }
    }
}
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determineUrlsForHandler()方法：

該方法在不一樣的子類有不一樣的實現，我這裏分析的是DefaultAnnotationHandlerMapping類的實現，該類主要負責處理@RequestMapping註解形式的聲明。

/**
 * 獲取@RequestMaping註解中的url
 * Checks for presence of the {@link org.springframework.web.bind.annotation.RequestMapping}
 * annotation on the handler class and on any of its methods.
 */
@Override
protected String[] determineUrlsForHandler(String beanName) {
    ApplicationContext context = getApplicationContext();
    Class<?> handlerType = context.getType(beanName);
    // 獲取beanName 上的requestMapping
    RequestMapping mapping = context.findAnnotationOnBean(beanName, RequestMapping.class);
    if (mapping != null) {
        // 類上面有@RequestMapping 註解
        this.cachedMappings.put(handlerType, mapping);
        Set<String> urls = new LinkedHashSet<String>();
        // mapping.value()就是獲取@RequestMapping註解的value值
        String[] typeLevelPatterns = mapping.value();
        if (typeLevelPatterns.length > 0) {
            // 獲取Controller 方法上的@RequestMapping
            String[] methodLevelPatterns = determineUrlsForHandlerMethods(handlerType);
            for (String typeLevelPattern : typeLevelPatterns) {
                if (!typeLevelPattern.startsWith("/")) {
                    typeLevelPattern = "/" + typeLevelPattern;
                }
                for (String methodLevelPattern : methodLevelPatterns) {
                    // controller的映射url+方法映射的url
                    String combinedPattern = getPathMatcher().combine(typeLevelPattern, methodLevelPattern);
                    // 保存到set集合中
                    addUrlsForPath(urls, combinedPattern);
                }
                addUrlsForPath(urls, typeLevelPattern);
            }
            // 以數組形式返回controller上的全部url
            return StringUtils.toStringArray(urls);
        }
        else {
            // controller上的@RequestMapping映射url爲空串,直接找方法的映射url
            return determineUrlsForHandlerMethods(handlerType);
        }
    }
    // controller上沒@RequestMapping註解
    else if (AnnotationUtils.findAnnotation(handlerType, Controller.class) != null) {
        // 獲取controller中方法上的映射url
        return determineUrlsForHandlerMethods(handlerType);
    }
    else {
        return null;
    }
}
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更深的細節代碼就比較簡單了，有興趣的能夠繼續深刻。

到這裏，Controller和Url的映射就裝配完成，下來就分析請求的處理過程。

2. url的請求處理

咱們在xml中配置了DispatcherServlet爲調度器，因此咱們就來看它的代碼，能夠
從名字上看出它是個Servlet,那麼它的核心方法就是doService()

DispatcherServlet #doService():

/**
 * 將DispatcherServlet特定的請求屬性和委託 公開給{@link #doDispatch}以進行實際調度。
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
 * for the actual dispatching.
 */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    if (logger.isDebugEnabled()) {
        String requestUri = new UrlPathHelper().getRequestUri(request);
        logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() +
                " request for [" + requestUri + "]");
    }

    //在包含request的狀況下保留請求屬性的快照，
    //可以在include以後恢復原始屬性。
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        logger.debug("Taking snapshot of request attributes before include");
        attributesSnapshot = new HashMap<String, Object>();
        Enumeration attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
    }

    // 使得request對象能供 handler處理和view處理 使用
    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());

    try {
        doDispatch(request, response);
    }
    finally {
        // 若是不爲空，則還原原始屬性快照。
        if (attributesSnapshot != null) {
            restoreAttributesAfterInclude(request, attributesSnapshot);
        }
    }
}
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能夠看到，它將請求拿到後，主要是給request設置了一些對象，以便於後續工做的處理(Handler處理和view處理)。好比WebApplicationContext，它裏面就包含了咱們在第一步完成的controller與url映射的信息。

DispatchServlet # doDispatch()

/**
 * 控制請求轉發
 * Process the actual dispatching to the handler.
 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order. * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
 * to find the first that supports the handler class.
 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers * themselves to decide which methods are acceptable. * @param request current HTTP request * @param response current HTTP response * @throws Exception in case of any kind of processing failure */ protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; int interceptorIndex = -1; try { ModelAndView mv; boolean errorView = false; try { // 1. 檢查是不是上傳文件 processedRequest = checkMultipart(request); // Determine handler for the current request. // 2. 獲取handler處理器，返回的mappedHandler封裝了handlers和interceptors mappedHandler = getHandler(processedRequest, false); if (mappedHandler == null || mappedHandler.getHandler() == null) { // 返回404 noHandlerFound(processedRequest, response); return; } // Apply preHandle methods of registered interceptors. // 獲取HandlerInterceptor的預處理方法 HandlerInterceptor[] interceptors = mappedHandler.getInterceptors(); if (interceptors != null) { for (int i = 0; i < interceptors.length; i++) { HandlerInterceptor interceptor = interceptors[i]; if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) { triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null); return; } interceptorIndex = i; } } // Actually invoke the handler. // 3. 獲取handler適配器 Adapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // 4. 實際的處理器處理並返回 ModelAndView 對象 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); // Do we need view name translation? if (mv != null && !mv.hasView()) { mv.setViewName(getDefaultViewName(request)); } // HandlerInterceptor 後處理 if (interceptors != null) { for (int i = interceptors.length - 1; i >= 0; i--) { HandlerInterceptor interceptor = interceptors[i]; // 結束視圖對象處理 interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv); } } } catch (ModelAndViewDefiningException ex) { logger.debug("ModelAndViewDefiningException encountered", ex); mv = ex.getModelAndView(); } catch (Exception ex) { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); mv = processHandlerException(processedRequest, response, handler, ex); errorView = (mv != null); } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { render(mv, processedRequest, 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"); } } // Trigger after-completion for successful outcome. // 請求成功響應以後的方法 triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null); } catch (Exception ex) { // Trigger after-completion for thrown exception. triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex); throw ex; } catch (Error err) { ServletException ex = new NestedServletException("Handler processing failed", err); // Trigger after-completion for thrown exception. triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex); throw ex; } finally { // Clean up any resources used by a multipart request. if (processedRequest != request) { cleanupMultipart(processedRequest); } } } 複製代碼

該方法主要是

經過request對象獲取到HandlerExecutionChain，HandlerExecutionChain對象裏面包含了攔截器interceptor和處理器handler。若是獲取到的對象是空，則交給noHandlerFound返回404頁面。
攔截器預處理，若是執行成功則進行3
獲取handler適配器 Adapter
實際的處理器處理並返回 ModelAndView 對象

下面是該方法中的一些核心細節：

DispatchServlet #doDispatch # noHandlerFound核心源碼：

response.sendError(HttpServletResponse.SC_NOT_FOUND);
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DispatchServlet #doDispatch #getHandler方法事實上調用的是AbstractHandlerMapping #getHandler方法,我貼出一個核心的代碼：

// 拿處處理對象
Object handler = getHandlerInternal(request);
...
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
...
// 返回HandlerExecutionChain對象
return getHandlerExecutionChain(handler, request);
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能夠看到，它先從request裏獲取handler對象，這就證實了以前DispatchServlet #doService爲何要吧WebApplicationContext放入request請求對象中。

最終返回一個HandlerExecutionChain對象.

3. 反射調用處理請求的方法，返回結果視圖

在上面的源碼中，實際的處理器處理並返回 ModelAndView 對象調用的是mv = ha.handle(processedRequest, response, mappedHandler.getHandler());這個方法。該方法由AnnotationMethodHandlerAdapter #handle() #invokeHandlerMethod()方法實現.

`AnnotationMethodHandlerAdapter #handle() #invokeHandlerMethod()`

/**
 * 獲取處理請求的方法,執行並返回結果視圖
 */
protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler)
        throws Exception {

    // 1.獲取方法解析器
    ServletHandlerMethodResolver methodResolver = getMethodResolver(handler);
    // 2.解析request中的url,獲取處理request的方法
    Method handlerMethod = methodResolver.resolveHandlerMethod(request);
    // 3. 方法調用器
    ServletHandlerMethodInvoker methodInvoker = new ServletHandlerMethodInvoker(methodResolver);
    ServletWebRequest webRequest = new ServletWebRequest(request, response);
    ExtendedModelMap implicitModel = new BindingAwareModelMap();
    // 4.執行方法（獲取方法的參數）
    Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel);
    // 5. 封裝成mv視圖
    ModelAndView mav =
            methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest);
    methodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest);
    return mav;
}
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這個方法有兩個重要的地方，分別是resolveHandlerMethod和invokeHandlerMethod。

resolveHandlerMethod 方法

methodResolver.resolveHandlerMethod(request):獲取controller類和方法上的@requestMapping value,與request的url進行匹配,找處處理request的controller中的方法.最終拼接的具體實現是org.springframework.util.AntPathMatcher#combine方法。

invokeHandlerMethod方法

從名字就能看出來它是基於反射，那它作了什麼呢。

解析該方法上的參數,並調用該方法。

//上面全都是爲解析方法上的參數作準備
...
// 解析該方法上的參數
Object[] args = resolveHandlerArguments(handlerMethodToInvoke, handler, webRequest, implicitModel);
// 真正執行解析調用的方法
return doInvokeMethod(handlerMethodToInvoke, handler, args);
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invokeHandlerMethod方法#resolveHandlerArguments方法

代碼有點長，我就簡介下它作了什麼事情吧。

若是這個方法的參數用的是註解，則解析註解拿到參數名，而後拿到request中的參數名，二者一致則進行賦值(詳細代碼在HandlerMethodInvoker#resolveRequestParam)，而後將封裝好的對象放到args[]的數組中並返回。
若是這個方法的參數用的不是註解，則須要asm框架(底層是讀取字節碼)來幫助獲取到參數名，而後拿到request中的參數名，二者一致則進行賦值，而後將封裝好的對象放到args[]的數組中並返回。

invokeHandlerMethod方法#doInvokeMethod方法

private Object doInvokeMethod(Method method, Object target, Object[] args) throws Exception {
    // 將一個方法設置爲可調用，主要針對private方法
    ReflectionUtils.makeAccessible(method);
    try {
        // 反射調用
        return method.invoke(target, args);
    }
    catch (InvocationTargetException ex) {
        ReflectionUtils.rethrowException(ex.getTargetException());
    }
    throw new IllegalStateException("Should never get here");
}
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到這裏,就能夠對request請求中url對應的controller的某個對應方法進行調用了。