Spring MVC

后端 / Spring
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Spring MVC是一款基于Java的Web MVC框架，是目前主流的Web MVC框架之一，本文结合源码，详细介绍下Spring MVC的相关知识。

Spring MVC概述

这一章节和下一章节内容主要搬运自这里。

Spring MVC的由来

早期 Java Web 的开发中，把显示层、控制层、数据层的操作全部交给 JSP 或者 JavaBean 来进行处理，我们称之为 Model1：

然而出现了很多的弊端，比如：

  • JSP 和 Java Bean 之间严重耦合，Java 代码和 HTML 代码也耦合在了一起
  • 要求开发者不仅要掌握 Java ，还要有高超的前端水平
  • 前端和后端相互依赖，前端需要等待后端完成，后端也依赖前端完成，才能进行有效的测试

随后出现servlet，就有了早期的MVC模式

首先用户请求到servlet，然后根据请求调用响应的JavaBean，并把所有的显示交给Jsp去处理，这样就称之为mvc模式：

  • M代表模型（Model）：数据、bean
  • V代表视图（View）：网页，jsp…展示模型中的数据
  • C代表控制器（Controller）：把不同的数据(Model)，显示在不同的视图(View)上，Servlet 扮演的就是这样的角色

为解决持久层中一直未处理好的数据库事务的编程，又为了迎合 NoSQL 的强势崛起，Spring MVC 给出了方案

Spring MVC启动流程

当一个web应用部署tomcat时，在接收用户请求之前，会进行以下初始化过程

  • 部署在web.xml文件里由<listener>元素标记事件的监听器会被创建和初始化

  • 对于所有事件监听器，如果实现了ServletContextListener接口，将会执行其实现的contextInitialized()方法

  • 部署描述文件由元素标记的过滤器会被创建和初始化，并调用其init()方法

  • 部署在描述文件由元素标记的servlet会根据的权值按顺序创建并初始化，并调用其init()方法

Listener初始化

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<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

在web.xml里面我们定义了一个<listener>，用于配置监听器，这里我们定义了一个ContextLoaderListener的listener，监听器的初始化就是从这里开始的。

ContextLoaderListener类核心就2个方法：初始化和销毁。

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public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    // 初始化方法
    @Override
    public void contextInitialized(ServletContextEvent event) {
    	initWebApplicationContext(event.getServletContext());
    }
    // 销毁方法
    @Override
    public void contextDestroyed(ServletContextEvent event) {
    	closeWebApplicationContext(event.getServletContext());
    	ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

ContextLoaderListener类实现了ServletContextListener，本质上是一个servlet监听器，tomcat将会优先加载servlet监听器组件，并调用contextInitialized方法,在contextInitialized方法中调用initWebApplicationContext方法初始化Spring web上下文。

initWebApplicationContext

这个方法核心功能是下面这段：

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//判断context是否为空，为空，则创建，默认创建XmlWebApplicationContext
if (this.context == null) {
    this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
    ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
    // 如果该容器还没有刷新过
    if (!cwac.isActive()) {
        // The context has not yet been refreshed -> provide services such as
        // setting the parent context, setting the application context id, etc
        if (cwac.getParent() == null) {
            // The context instance was injected without an explicit parent ->
            // determine parent for root web application context, if any.
            ApplicationContext parent = loadParentContext(servletContext);
            cwac.setParent(parent);
        }
        // 配置并刷新容器
        configureAndRefreshWebApplicationContext(cwac, servletContext);
    }
}

该方法的主要目的就是创建root WebApplicationContext对象即IOC容器，其中比较重要的就是，整个Web应用如果存在IOC容器则有且只能有一个，根IOC容器作为全局变量存储在ServletContext即application对象中。将根IOC容器放入到application对象之前进行了IOC容器的配置和刷新操作，调用了configureAndRefreshWebApplicationContext()方法 。

createWebApplicationContext

这个方法比较简单，就是利用反射创建一个根IOC容器。

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protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    Class<?> contextClass = determineContextClass(sc);
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
                                              "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    }
    return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

configureAndRefreshWebApplicationContext

创建好根IOC容器后，调用该方法配置和刷新容器，其源码如下：

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protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
    if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
        // The application context id is still set to its original default value
        // -> assign a more useful id based on available information
        String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
        if (idParam != null) {
            wac.setId(idParam);
        }
        else {
            // Generate default id...
            wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                      ObjectUtils.getDisplayString(sc.getContextPath()));
        }
    }

    wac.setServletContext(sc);
    String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
    if (configLocationParam != null) {
        wac.setConfigLocation(configLocationParam);
    }

    // The wac environment's #initPropertySources will be called in any case when the context
    // is refreshed; do it eagerly here to ensure servlet property sources are in place for
    // use in any post-processing or initialization that occurs below prior to #refresh
    ConfigurableEnvironment env = wac.getEnvironment();
    if (env instanceof ConfigurableWebEnvironment) {
        ((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
    }

    customizeContext(sc, wac);
    wac.refresh();
}

比较重要的就是获取到了web.xml中的标签配置的全局变量contextConfigLocation，并最后一行调用了refresh()方法 。

refresh()方法

ConfigurableWebApplicationContext是一个接口，通过对常用实现类ClassPathXmlApplicationContext逐层查找后可以找到一个抽象类AbstractApplicationContext实现了refresh()方法，其源码如下 ：

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public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
        // Prepare this context for refreshing.
        prepareRefresh();
        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);
        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);
            StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);
            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);
            beanPostProcess.end();
            // Initialize message source for this context.
            initMessageSource();
            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();
            // Initialize other special beans in specific context subclasses.
            onRefresh();
            // Check for listener beans and register them.
            registerListeners();
            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);
            // Last step: publish corresponding event.
            finishRefresh();
        }

        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
            }

            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();
            // Reset 'active' flag.
            cancelRefresh(ex);
            // Propagate exception to caller.
            throw ex;
        }
        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            resetCommonCaches();
            contextRefresh.end();
        }
    }
}

该方法主要用于创建并初始化contextConfigLocation类配置的xml文件中的Bean，因此，如果我们在配置Bean时出错，在Web应用启动时就会抛出异常，而不是等到运行时才抛出异常
整个ContextLoaderListener类的启动过程到此就结束了，可以发现，创建ContextLoaderListener是比较核心的一个步骤，主要工作就是为了创建根IoC容器并使用特定的key将其放入到application对象中，供整个Web应用使用，由于在ContextLoaderListener类中构造的IOC容器配置的Bean是全局共享的，因此，在标识的contextConfigLocation的xml配置文件一般包括: 数据库DataSource、DAO层、Service层、事务等相关Bean。

Filter初始化

在监听器listener初始化完成后，接下来会进行filter的初始化操作，GenericFilterBean是任何类型的过滤器的一个比较方便的超类，这个类主要实现的就是从web.xml文件中取得init-param中设定的值，然后对Filter进行初始化（当然，其子类可以覆盖init()方法）。
Filter的生命周期如下：

  • void init(FilterConfig config): 用于完成Filter 的初始化
  • void destroy(): 用于Filter 销毁前，完成某些资源的回收
  • void doFilter(ServletRequest request, ServletResponse response,FilterChain chain): 实现过滤功能，该方法就是对每个请求及响应增加的额外处理。 过滤器Filter也具有生命周期：init()->doFilter()->destroy()，由部署文件中的filter元素驱动

Servlet初始化

Web应用启动的最后一个步骤就是创建和初始化相关Servlet，在开发中常用的Servlet就是DispatcherServlet类前端控制器，前端控制器作为中央控制器是整个Web应用的核心，用于获取分发用户请求并返回响应。

DispatcherServlet类的间接父类实现了Servlet接口，因此其本质上依旧是一个Servlet。DispatcherServlet设计很巧妙，上层父类不同程度的实现了相关接口的部分方法，并留出了相关方法用于子类覆盖，将不变的部分统一实现，将变化的部分预留方法用于子类实现 。

因为一个servlet的生命周期是init()->service()->destory()，DispatcherServlet 也是一样的。

init()

该方法解析初始化参数，并创建BeanWrapper，然后调用子类实现的initServletBean()进行后续操作。

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public final void init() throws ServletException {

    // Set bean properties from init parameters.
    PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
    if (!pvs.isEmpty()) {
        try {
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            if (logger.isErrorEnabled()) {
                logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            }
            throw ex;
        }
    }

    // Let subclasses do whatever initialization they like.
    initServletBean();
}

initServletBean()

该方法是重写了父类HttpServletBean抽象类的initServletBean()方法，HttpServletBean抽象类在执行init()方法时会调用initServletBean()方法，由于多态的特性，最终会调用其子类FrameworkServlet抽象类的initServletBean()方法。该方法由final标识，子类就不可再次重写了。该方法中比较重要的就是initWebApplicationContext()方法的调用，该方法仍由FrameworkServlet抽象类实现 。

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protected final void initServletBean() throws ServletException {
    getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
    if (logger.isInfoEnabled()) {
        logger.info("Initializing Servlet '" + getServletName() + "'");
    }
    long startTime = System.currentTimeMillis();

    try {
        this.webApplicationContext = initWebApplicationContext();
        initFrameworkServlet();
    }
    catch (ServletException | RuntimeException ex) {
        logger.error("Context initialization failed", ex);
        throw ex;
    }

    if (logger.isDebugEnabled()) {
        String value = this.enableLoggingRequestDetails ?
            "shown which may lead to unsafe logging of potentially sensitive data" :
        "masked to prevent unsafe logging of potentially sensitive data";
        logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
                     "': request parameters and headers will be " + value);
    }

    if (logger.isInfoEnabled()) {
        logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
    }
}

initWebApplicationContext()

该方法的主要作用同样是创建一个WebApplicationContext对象，即IOC容器，不过前文讲过每个Web应用最多只能存在一个IOC容器，这里创建的则是特定Servlet拥有的子IOC容器。

父子容器类似于类的继承关系，子类可以访问父类中的成员变量，而父类不可访问子类的成员变量，同样的，子容器可以访问父容器中定义的Bean，但父容器无法访问子容器定义的Bean，根IOC容器做为全局共享的IOC容器放入Web应用需要共享的Bean，而子IOC容器根据需求的不同，放入不同的Bean，这样能够做到隔离，保证系统的安全性。

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protected WebApplicationContext initWebApplicationContext() {
    //先获取root容器
    WebApplicationContext rootContext =
        WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;

    if (this.webApplicationContext != null) {
        // A context instance was injected at construction time -> use it
        wac = this.webApplicationContext;
        if (wac instanceof ConfigurableWebApplicationContext cwac && !cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, etc
            if (cwac.getParent() == null) {
                // The context instance was injected without an explicit parent -> set
                // the root application context (if any; may be null) as the parent
                cwac.setParent(rootContext);
            }
            configureAndRefreshWebApplicationContext(cwac);
        }
    }
    if (wac == null) {
        // No context instance was injected at construction time -> see if one
        // has been registered in the servlet context. If one exists, it is assumed
        // that the parent context (if any) has already been set and that the
        // user has performed any initialization such as setting the context id
        wac = findWebApplicationContext();
    }
    //创建一个继承自root容器的IOC容器
    if (wac == null) {
        // No context instance is defined for this servlet -> create a local one
        wac = createWebApplicationContext(rootContext);
    }

    if (!this.refreshEventReceived) {
        // Either the context is not a ConfigurableApplicationContext with refresh
        // support or the context injected at construction time had already been
        // refreshed -> trigger initial onRefresh manually here.
        synchronized (this.onRefreshMonitor) {
            onRefresh(wac);
        }
    }

    if (this.publishContext) {
        // Publish the context as a servlet context attribute.
        String attrName = getServletContextAttributeName();
        getServletContext().setAttribute(attrName, wac);
    }

    return wac;
}

DispatcherServlet类的子IOC容器创建过程，如果当前Servlet存在一个IOC容器则为其设置根IOC容器作为其父类，并配置刷新该容器，用于构造其定义的Bean，这里的方法与前文讲述的根IOC容器类似，同样会读取用户在web.xml中配置的中的值，用于查找相关的xml配置文件用于构造定义的Bean。如果当前Servlet不存在一个子IoC容器就去查找一个，如果仍然没有查找到则调用createWebApplicationContext()方法去创建。

createWebApplicationContext()

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protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    Class<?> contextClass = getContextClass();
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException(
            "Fatal initialization error in servlet with name '" + getServletName() +
            "': custom WebApplicationContext class [" + contextClass.getName() +
            "] is not of type ConfigurableWebApplicationContext");
    }
    ConfigurableWebApplicationContext wac =
        (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

    wac.setEnvironment(getEnvironment());
    wac.setParent(parent);
    String configLocation = getContextConfigLocation();
    if (configLocation != null) {
        wac.setConfigLocation(configLocation);
    }
    configureAndRefreshWebApplicationContext(wac);

    return wac;
}

该方法用于创建一个子IOC容器并将根IOC容器做为其父容器，接着进行配置和刷新操作用于构造相关的Bean。至此，根IOC容器以及相关Servlet的子IOC容器已经配置完成，子容器中管理的Bean一般只被该Servlet使用，因此，其中管理的Bean一般是局部的，如SpringMVC中需要的各种重要组件，包括Controller、Interceptor、Converter、ExceptionResolver等

onrefresh()

当IOC子容器构造完成后调用了onRefresh()方法，该方法的调用与initServletBean()方法的调用相同，由父类调用但具体实现由子类覆盖，调用onRefresh()方法时将前文创建的IOC子容器作为参数传入。

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protected void onRefresh(ApplicationContext context) {
	initStrategies(context);
}
/**
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}

onRefresh()方法直接调用了initStrategies()方法，源码如上，通过函数名可以判断，该方法用于初始化创建multipartResovle来支持图片等文件的上传、本地化解析器、主题解析器、HandlerMapping处理器映射器、HandlerAdapter处理器适配器、异常解析器、视图解析器、flashMap管理器等，这些组件都是SpringMVC开发中的重要组件，相关组件的初始化创建过程均在此完成 。 到此，初始化就全部结束了！

总结

在Spring的web容器启动时会去读取web.xml文件，相关启动顺序为：<context-param> --> <listener> --> <filter> --><servlet> ，具体为：

  • 1、解析<context-param>键值对
  • 2、创建一个application对象即ServletContext，servlet上下文，用于全局共享
  • 3、将<context-param>键值对放入ServletContext中，web应用全局共享
  • 4、读取<listener>标签，创建监听器，一般使用ContextLoaderListener，如果使用了ContextLoaderListener，Spring就会创建一个WebApplicationContext对象，这个就是IOC容器, ContextLoaderListener创建的IOC容器是全局共享的，并将其放在ServletContext中, 键名为WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, 读取web.xml文件里的contextConfigLocation配置中的xml文件来创建bean
  • 5、listener创建完毕后如果有Filter会去创建Filter
  • 6、初始化Servlet，一般使用DispatchServlet类
  • 7、DispatchServlet的父类FrameworkServlet会重写其父类的initServletBean方法，并调用initWebApplicationContext()以及onRefresh()方法
  • 8、initWebApplicationContext()方法会创建一个当前servlet的一个IOC子容器，如果存在上述的全局WebApplicationContext则将其设置为父容器，如果不存在上述全局的则父容器为null。
  • 9、读取标签的配置的xml文件并加载相关Bean
  • 10、onRefresh()方法创建Web应用相关组件

DispatcherServlet请求处理

这里内容主要搬运自这篇文章。

处理流程

Spring MVC的请求处理流程大致如下图所示：

核心架构的具体流程步骤如下：

  1. 首先用户发送请求——>DispatcherServlet，前端控制器收到请求后自己不进行处理，而是委托给其他的解析器进行 处理，作为统一访问点，进行全局的流程控制；
  2. DispatcherServlet——>HandlerMapping， HandlerMapping 将会把请求映射为 HandlerExecutionChain 对象（包含一 个Handler 处理器（页面控制器）对象、多个HandlerInterceptor 拦截器）对象，通过这种策略模式，很容易添加新 的映射策略；
  3. DispatcherServlet——>HandlerAdapter，HandlerAdapter 将会把处理器包装为适配器，从而支持多种类型的处理器， 即适配器设计模式的应用，从而很容易支持很多类型的处理器；
  4. HandlerAdapter——>处理器功能处理方法的调用，HandlerAdapter 将会根据适配的结果调用真正的处理器的功能处 理方法，完成功能处理；并返回一个ModelAndView 对象（包含模型数据、逻辑视图名）；
  5. ModelAndView 的逻辑视图名——> ViewResolver，ViewResolver 将把逻辑视图名解析为具体的View，通过这种策 略模式，很容易更换其他视图技术；
  6. View——>渲染，View 会根据传进来的Model 模型数据进行渲染，此处的Model 实际是一个Map 数据结构，因此 很容易支持其他视图技术；
  7. 返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户，到此一个流程结束。

doGet入口

我们以get请求为例，看下DispatcherServlet是如何处理请求的。servlet处理get请求是doGet方法，这个方法由FrameworkServlet类实现：

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protected final void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    processRequest(request, response);
}

这个方法很简单，调用processRequest()来处理请求。

processRequest()

这个方法还是由FrameworkServlet类实现：

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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());
	//初始化context
    initContextHolders(request, localeContext, requestAttributes);

    try {
        //调用doService方法，由子类实现，它是处理请求的核心
        doService(request, response);
    }
    catch (ServletException | 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();
        }
        logResult(request, response, failureCause, asyncManager);
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

doService()

该方法由DispatchServlet实现：

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protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(request);

    // Keep a snapshot of the request attributes in case of an include,
    // to be able to restore the original attributes after the include.
    //保存请求相关参数，供后续使用
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        attributesSnapshot = new HashMap<>();
        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());

    if (this.flashMapManager != null) {
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }
        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
    }

    RequestPath previousRequestPath = null;
    if (this.parseRequestPath) {
        previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
        ServletRequestPathUtils.parseAndCache(request);
    }

    try {
        //调用doDispatch，分发请求
        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);
            }
        }
        if (this.parseRequestPath) {
            ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
        }
    }
}

分发请求

在doService()方法中调用doDispatch()来分发请求，其源码如下：

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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) {
                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 = HttpMethod.GET.matches(method);
            if (isGet || HttpMethod.HEAD.matches(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }

            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);
            }
        }
    }
}

方法的核心点有3个：

  1. 调用getHandler(processedRequest)获取处理器
    • 这个方法很简单，就是从一个Map数据结构里面找到和请求对应的处理器
  2. 调用getHandlerAdapter(mappedHandler.getHandler())获取处理器适配器
    • 这个方法和getHandler类似，不过是从一个List中获取对应处理器适配器
  3. 调用handle()处理请求
    • 这个方法是核心，下面我们重点看看它的细节。

处理请求

handle()是HandlerAdapter接口定义的方法，有5个子类都实现了它。

这里我们以AbstractHandlerMethodAdapter为例，看下处理请求的具体细节。

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public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
    throws Exception {
	//调用handleInternal方法来处理请求
    return handleInternal(request, response, (HandlerMethod) handler);
}

handleInternal()

handleInternal方法由其子类RequestMappingHandlerAdapter实现：

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@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) {
    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;
}

这里再次调用invokeHandlerMethod(request, response, handlerMethod)来处理请求。

invokeHandlerMethod()

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/**
  * 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 {
    
    WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
    ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

    // 重要：设置handler(controller#list)方法上的参数，返回值处理，绑定databinder等
    ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
    if (this.argumentResolvers != null) {
      invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
    }
    if (this.returnValueHandlers != null) {
      invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
    }
    invocableMethod.setDataBinderFactory(binderFactory);
    invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

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

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

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

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

    // 执行controller中方法
    invocableMethod.invokeAndHandle(webRequest, mavContainer);
    if (asyncManager.isConcurrentHandlingStarted()) {
      return null;
    }

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

在invokeHandlerMethod()在通过调用invocableMethod.invokeAndHandle(webRequest, mavContainer);，通过反射，让具体Controller来处理该请求。

返回视图

Controller处理完请求，得到ModelAndView后，重新回到doDispatch方法，进行后续的视图解析和渲染工作。

视图渲染

视图渲染之前首先调用processDispatchResult处理返回的视图：

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/**
  * 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,
    @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
    @Nullable Exception exception) throws Exception {

  boolean errorView = false;

  // 如果处理过程有异常，则异常处理
  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);
    }
  }

  // 是否需要渲染视图
  if (mv != null && !mv.wasCleared()) {
    render(mv, request, response); // 渲染视图
    if (errorView) {
      WebUtils.clearErrorRequestAttributes(request);
    }
  }
  else {
    if (logger.isTraceEnabled()) {
      logger.trace("No view rendering, null ModelAndView returned.");
    }
  }

  if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
    // Concurrent handling started during a forward
    return;
  }

  if (mappedHandler != null) {
    // Exception (if any) is already handled..
    mappedHandler.triggerAfterCompletion(request, response, null);
  }
}

接下来显然就是渲染视图了, spring在initStrategies方法中初始化的组件（LocaleResovler等）就派上用场了。

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/**
  * Render the given ModelAndView.
  * <p>This is the last stage in handling a request. It may involve resolving the view by name.
  * @param mv the ModelAndView to render
  * @param request current HTTP servlet request
  * @param response current HTTP servlet response
  * @throws ServletException if view is missing or cannot be resolved
  * @throws Exception if there's a problem rendering the view
  */
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 != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
  response.setLocale(locale);

  View view;
  String viewName = mv.getViewName();
  if (viewName != null) {
    // We need to resolve the view name.
    view = resolveViewName(viewName, 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.isTraceEnabled()) {
    logger.trace("Rendering view [" + view + "] ");
  }
  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 + "]", ex);
    }
    throw ex;
  }
}

后续就是通过viewResolver进行解析了，这里就不再继续看代码了，上述流程基本上够帮助你构建相关的认知了。最后无非是返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户。

参考资料

  1. https://developer.aliyun.com/article/707995#slide-4
  2. https://juejin.cn/post/6844903645100638215#comment
  3. https://pdai.tech/md/spring/spring-x-framework-springmvc-source-1.html
  4. https://pdai.tech/md/spring/spring-x-framework-springmvc-source-2.html
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