DispatcherServlet源码注解分析

DispatcherServlet的介绍与工作流程

DispatcherServlet是SpringMVC的前端分发控制器，用于处理客户端请求，然后交给对应的handler进行处理，返回对应的模型和视图，视图解析器根据视图名称进行视图渲染，然后返回给DispatcherServlet，该分发servlet将渲染好的视图页面呈现给用户。

工作流程图

下面是对DispatcherServlet的源码注解分析，后期将会不断完善

 /*
  * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 package org.springframework.web.servlet;

 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Properties;
 import java.util.Set;
 import javax.servlet.ServletContext;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;

 import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.BeanInitializationException;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.i18n.LocaleContext;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.support.PropertiesLoaderUtils;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.ui.context.ThemeSource;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.web.context.WebApplicationContext;
 import org.springframework.web.context.request.ServletWebRequest;
 import org.springframework.web.context.request.async.WebAsyncManager;
 import org.springframework.web.context.request.async.WebAsyncUtils;
 import org.springframework.web.multipart.MultipartException;
 import org.springframework.web.multipart.MultipartHttpServletRequest;
 import org.springframework.web.multipart.MultipartResolver;
 import org.springframework.web.util.NestedServletException;
 import org.springframework.web.util.WebUtils;

 /**
  * Central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers
  * or HTTP-based remote service exporters. Dispatches to registered handlers for processing
  * a web request, providing convenient mapping and exception handling facilities.
  *
  * <p>This servlet is very flexible: It can be used with just about any workflow, with the
  * installation of the appropriate adapter classes. It offers the following functionality
  * that distinguishes it from other request-driven web MVC frameworks:
  *
  * <ul>
  * <li>It is based around a JavaBeans configuration mechanism.
  *
  * <li>It can use any {@link HandlerMapping} implementation - pre-built or provided as part
  * of an application - to control the routing of requests to handler objects. Default is
  * {@link org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping} and
  * {@link org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping}.
  * HandlerMapping objects can be defined as beans in the servlet's application context,
  * implementing the HandlerMapping interface, overriding the default HandlerMapping if
  * present. HandlerMappings can be given any bean name (they are tested by type).
  *
  * <li>It can use any {@link HandlerAdapter}; this allows for using any handler interface.
  * Default adapters are {@link org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter},
  * {@link org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter}, for Spring's
  * {@link org.springframework.web.HttpRequestHandler} and
  * {@link org.springframework.web.servlet.mvc.Controller} interfaces, respectively. A default
  * {@link org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter}
  * will be registered as well. HandlerAdapter objects can be added as beans in the
  * application context, overriding the default HandlerAdapters. Like HandlerMappings,
  * HandlerAdapters can be given any bean name (they are tested by type).
  *
  * <li>The dispatcher's exception resolution strategy can be specified via a
  * {@link HandlerExceptionResolver}, for example mapping certain exceptions to error pages.
  * Default are
  * {@link org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver},
  * {@link org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver}, and
  * {@link org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver}.
  * These HandlerExceptionResolvers can be overridden through the application context.
  * HandlerExceptionResolver can be given any bean name (they are tested by type).
  *
  * <li>Its view resolution strategy can be specified via a {@link ViewResolver}
  * implementation, resolving symbolic view names into View objects. Default is
  * {@link org.springframework.web.servlet.view.InternalResourceViewResolver}.
  * ViewResolver objects can be added as beans in the application context, overriding the
  * default ViewResolver. ViewResolvers can be given any bean name (they are tested by type).
  *
  * <li>If a {@link View} or view name is not supplied by the user, then the configured
  * {@link RequestToViewNameTranslator} will translate the current request into a view name.
  * The corresponding bean name is "viewNameTranslator"; the default is
  * {@link org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator}.
  *
  * <li>The dispatcher's strategy for resolving multipart requests is determined by a
  * {@link org.springframework.web.multipart.MultipartResolver} implementation.
  * Implementations for Apache Commons FileUpload and Servlet 3 are included; the typical
  * choice is {@link org.springframework.web.multipart.commons.CommonsMultipartResolver}.
  * The MultipartResolver bean name is "multipartResolver"; default is none.
  *
  * <li>Its locale resolution strategy is determined by a {@link LocaleResolver}.
  * Out-of-the-box implementations work via HTTP accept header, cookie, or session.
  * The LocaleResolver bean name is "localeResolver"; default is
  * {@link org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver}.
  *
  * <li>Its theme resolution strategy is determined by a {@link ThemeResolver}.
  * Implementations for a fixed theme and for cookie and session storage are included.
  * The ThemeResolver bean name is "themeResolver"; default is
  * {@link org.springframework.web.servlet.theme.FixedThemeResolver}.
  * </ul>
  *
  * <p><b>NOTE: The {@code @RequestMapping} annotation will only be processed if a
  * corresponding {@code HandlerMapping} (for type-level annotations) and/or
  * {@code HandlerAdapter} (for method-level annotations) is present in the dispatcher.</b>
  * This is the case by default. However, if you are defining custom {@code HandlerMappings}
  * or {@code HandlerAdapters}, then you need to make sure that a corresponding custom
  * {@code DefaultAnnotationHandlerMapping} and/or {@code AnnotationMethodHandlerAdapter}
  * is defined as well - provided that you intend to use {@code @RequestMapping}.
  *
  * <p><b>A web application can define any number of DispatcherServlets.</b>
  * Each servlet will operate in its own namespace, loading its own application context
  * with mappings, handlers, etc. Only the root application context as loaded by
  * {@link org.springframework.web.context.ContextLoaderListener}, if any, will be shared.
  *
  * <p>As of Spring 3.1, {@code DispatcherServlet} may now be injected with a web
  * application context, rather than creating its own internally. This is useful in Servlet
  * 3.0+ environments, which support programmatic registration of servlet instances.
  * See the {@link #DispatcherServlet(WebApplicationContext)} javadoc for details.
  *
  * @author Rod Johnson
  * @author Juergen Hoeller
  * @author Rob Harrop
  * @author Chris Beams
  * @author Rossen Stoyanchev
  * @see org.springframework.web.HttpRequestHandler
  * @see org.springframework.web.servlet.mvc.Controller
  * @see org.springframework.web.context.ContextLoaderListener
  */
 @SuppressWarnings("serial")
 public class DispatcherServlet extends FrameworkServlet {

     /** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
     public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";

     /** Well-known name for the LocaleResolver object in the bean factory for this namespace. */
     public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";

     /** Well-known name for the ThemeResolver object in the bean factory for this namespace. */
     public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";

     /**
      * Well-known name for the HandlerMapping object in the bean factory for this namespace.
      * Only used when "detectAllHandlerMappings" is turned off.
      * @see #setDetectAllHandlerMappings
      */
     public static final String HANDLER_MAPPING_BEAN_NAME = "handlerMapping";

     /**
      * Well-known name for the HandlerAdapter object in the bean factory for this namespace.
      * Only used when "detectAllHandlerAdapters" is turned off.
      * @see #setDetectAllHandlerAdapters
      */
     public static final String HANDLER_ADAPTER_BEAN_NAME = "handlerAdapter";

     /**
      * Well-known name for the HandlerExceptionResolver object in the bean factory for this namespace.
      * Only used when "detectAllHandlerExceptionResolvers" is turned off.
      * @see #setDetectAllHandlerExceptionResolvers
      */
     public static final String HANDLER_EXCEPTION_RESOLVER_BEAN_NAME = "handlerExceptionResolver";

     /**
      * Well-known name for the RequestToViewNameTranslator object in the bean factory for this namespace.
      */
     public static final String REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME = "viewNameTranslator";

     /**
      * Well-known name for the ViewResolver object in the bean factory for this namespace.
      * Only used when "detectAllViewResolvers" is turned off.
      * @see #setDetectAllViewResolvers
      */
     public static final String VIEW_RESOLVER_BEAN_NAME = "viewResolver";

     /**
      * Well-known name for the FlashMapManager object in the bean factory for this namespace.
      */
     public static final String FLASH_MAP_MANAGER_BEAN_NAME = "flashMapManager";

     /**
      * Request attribute to hold the current web application context.
      * Otherwise only the global web app context is obtainable by tags etc.
      * @see org.springframework.web.servlet.support.RequestContextUtils#findWebApplicationContext
      */
     public static final String WEB_APPLICATION_CONTEXT_ATTRIBUTE = DispatcherServlet.class.getName() + ".CONTEXT";

     /**
      * Request attribute to hold the current LocaleResolver, retrievable by views.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getLocaleResolver
      */
     public static final String LOCALE_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".LOCALE_RESOLVER";

     /**
      * Request attribute to hold the current ThemeResolver, retrievable by views.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeResolver
      */
     public static final String THEME_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_RESOLVER";

     /**
      * Request attribute to hold the current ThemeSource, retrievable by views.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getThemeSource
      */
     public static final String THEME_SOURCE_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_SOURCE";

     /**
      * Name of request attribute that holds a read-only {@code Map<String,?>}
      * with "input" flash attributes saved by a previous request, if any.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getInputFlashMap(HttpServletRequest)
      */
     public static final String INPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".INPUT_FLASH_MAP";

     /**
      * Name of request attribute that holds the "output" {@link FlashMap} with
      * attributes to save for a subsequent request.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getOutputFlashMap(HttpServletRequest)
      */
     public static final String OUTPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".OUTPUT_FLASH_MAP";

     /**
      * Name of request attribute that holds the {@link FlashMapManager}.
      * @see org.springframework.web.servlet.support.RequestContextUtils#getFlashMapManager(HttpServletRequest)
      */
     public static final String FLASH_MAP_MANAGER_ATTRIBUTE = DispatcherServlet.class.getName() + ".FLASH_MAP_MANAGER";

     /**
      * Name of request attribute that exposes an Exception resolved with an
      * {@link HandlerExceptionResolver} but where no view was rendered
      * (e.g. setting the status code).
      */
     public static final String EXCEPTION_ATTRIBUTE = DispatcherServlet.class.getName() + ".EXCEPTION";

     /** Log category to use when no mapped handler is found for a request. */
     public static final String PAGE_NOT_FOUND_LOG_CATEGORY = "org.springframework.web.servlet.PageNotFound";

     /**
      * Name of the class path resource (relative to the DispatcherServlet class)
      * that defines DispatcherServlet's default strategy names.
      */
     private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

     /**
      * Common prefix that DispatcherServlet's default strategy attributes start with.
      */
     private static final String DEFAULT_STRATEGIES_PREFIX = "org.springframework.web.servlet";

     /** Additional logger to use when no mapped handler is found for a request. */
     protected static final Log pageNotFoundLogger = LogFactory.getLog(PAGE_NOT_FOUND_LOG_CATEGORY);

     private static final Properties defaultStrategies;

     static {
         // Load default strategy implementations from properties file.
         // This is currently strictly internal and not meant to be customized
         // by application developers.
         try {
             ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
             defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
         }
         catch (IOException ex) {
             throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
         }
     }

     /** Detect all HandlerMappings or just expect "handlerMapping" bean? */
     private boolean detectAllHandlerMappings = true;

     /** Detect all HandlerAdapters or just expect "handlerAdapter" bean? */
     private boolean detectAllHandlerAdapters = true;

     /** Detect all HandlerExceptionResolvers or just expect "handlerExceptionResolver" bean? */
     private boolean detectAllHandlerExceptionResolvers = true;

     /** Detect all ViewResolvers or just expect "viewResolver" bean? */
     private boolean detectAllViewResolvers = true;

     /** Throw a NoHandlerFoundException if no Handler was found to process this request? **/
     private boolean throwExceptionIfNoHandlerFound = false;

     /** Perform cleanup of request attributes after include request? */
     private boolean cleanupAfterInclude = true;

     /** MultipartResolver used by this servlet */
     private MultipartResolver multipartResolver;

     /** LocaleResolver used by this servlet */
     private LocaleResolver localeResolver;

     /** ThemeResolver used by this servlet */
     private ThemeResolver themeResolver;

     /** List of HandlerMappings used by this servlet */
     private List<HandlerMapping> handlerMappings;

     /** List of HandlerAdapters used by this servlet */
     private List<HandlerAdapter> handlerAdapters;

     /** List of HandlerExceptionResolvers used by this servlet */
     private List<HandlerExceptionResolver> handlerExceptionResolvers;

     /** RequestToViewNameTranslator used by this servlet */
     private RequestToViewNameTranslator viewNameTranslator;

     /** FlashMapManager used by this servlet */
     private FlashMapManager flashMapManager;

     /** List of ViewResolvers used by this servlet */
     private List<ViewResolver> viewResolvers;

     /**
      * Create a new {@code DispatcherServlet} that will create its own internal web
      * application context based on defaults and values provided through servlet
      * init-params. Typically used in Servlet 2.5 or earlier environments, where the only
      * option for servlet registration is through {@code web.xml} which requires the use
      * of a no-arg constructor.
      * <p>Calling {@link #setContextConfigLocation} (init-param 'contextConfigLocation')
      * will dictate which XML files will be loaded by the
      * {@linkplain #DEFAULT_CONTEXT_CLASS default XmlWebApplicationContext}
      * <p>Calling {@link #setContextClass} (init-param 'contextClass') overrides the
      * default {@code XmlWebApplicationContext} and allows for specifying an alternative class,
      * such as {@code AnnotationConfigWebApplicationContext}.
      * <p>Calling {@link #setContextInitializerClasses} (init-param 'contextInitializerClasses')
      * indicates which {@code ApplicationContextInitializer} classes should be used to
      * further configure the internal application context prior to refresh().
      * @see #DispatcherServlet(WebApplicationContext)
      */
     public DispatcherServlet() {
         super();
         setDispatchOptionsRequest(true);
     }

     /**
      * Create a new {@code DispatcherServlet} with the given web application context. This
      * constructor is useful in Servlet 3.0+ environments where instance-based registration
      * of servlets is possible through the {@link ServletContext#addServlet} API.
      * <p>Using this constructor indicates that the following properties / init-params
      * will be ignored:
      * <ul>
      * <li>{@link #setContextClass(Class)} / 'contextClass'</li>
      * <li>{@link #setContextConfigLocation(String)} / 'contextConfigLocation'</li>
      * <li>{@link #setContextAttribute(String)} / 'contextAttribute'</li>
      * <li>{@link #setNamespace(String)} / 'namespace'</li>
      * </ul>
      * <p>The given web application context may or may not yet be {@linkplain
      * ConfigurableApplicationContext#refresh() refreshed}. If it has <strong>not</strong>
      * already been refreshed (the recommended approach), then the following will occur:
      * <ul>
      * <li>If the given context does not already have a {@linkplain
      * ConfigurableApplicationContext#setParent parent}, the root application context
      * will be set as the parent.</li>
      * <li>If the given context has not already been assigned an {@linkplain
      * ConfigurableApplicationContext#setId id}, one will be assigned to it</li>
      * <li>{@code ServletContext} and {@code ServletConfig} objects will be delegated to
      * the application context</li>
      * <li>{@link #postProcessWebApplicationContext} will be called</li>
      * <li>Any {@code ApplicationContextInitializer}s specified through the
      * "contextInitializerClasses" init-param or through the {@link
      * #setContextInitializers} property will be applied.</li>
      * <li>{@link ConfigurableApplicationContext#refresh refresh()} will be called if the
      * context implements {@link ConfigurableApplicationContext}</li>
      * </ul>
      * If the context has already been refreshed, none of the above will occur, under the
      * assumption that the user has performed these actions (or not) per their specific
      * needs.
      * <p>See {@link org.springframework.web.WebApplicationInitializer} for usage examples.
      * @param webApplicationContext the context to use
      * @see #initWebApplicationContext
      * @see #configureAndRefreshWebApplicationContext
      * @see org.springframework.web.WebApplicationInitializer
      */
     public DispatcherServlet(WebApplicationContext webApplicationContext) {
         super(webApplicationContext);
         setDispatchOptionsRequest(true);
     }

     /**
      * Set whether to detect all HandlerMapping beans in this servlet's context. Otherwise,
      * just a single bean with name "handlerMapping" will be expected.
      * <p>Default is "true". Turn this off if you want this servlet to use a single
      * HandlerMapping, despite multiple HandlerMapping beans being defined in the context.
      */
     public void setDetectAllHandlerMappings(boolean detectAllHandlerMappings) {
         this.detectAllHandlerMappings = detectAllHandlerMappings;
     }

     /**
      * Set whether to detect all HandlerAdapter beans in this servlet's context. Otherwise,
      * just a single bean with name "handlerAdapter" will be expected.
      * <p>Default is "true". Turn this off if you want this servlet to use a single
      * HandlerAdapter, despite multiple HandlerAdapter beans being defined in the context.
      */
     public void setDetectAllHandlerAdapters(boolean detectAllHandlerAdapters) {
         this.detectAllHandlerAdapters = detectAllHandlerAdapters;
     }

     /**
      * Set whether to detect all HandlerExceptionResolver beans in this servlet's context. Otherwise,
      * just a single bean with name "handlerExceptionResolver" will be expected.
      * <p>Default is "true". Turn this off if you want this servlet to use a single
      * HandlerExceptionResolver, despite multiple HandlerExceptionResolver beans being defined in the context.
      */
     public void setDetectAllHandlerExceptionResolvers(boolean detectAllHandlerExceptionResolvers) {
         this.detectAllHandlerExceptionResolvers = detectAllHandlerExceptionResolvers;
     }

     /**
      * Set whether to detect all ViewResolver beans in this servlet's context. Otherwise,
      * just a single bean with name "viewResolver" will be expected.
      * <p>Default is "true". Turn this off if you want this servlet to use a single
      * ViewResolver, despite multiple ViewResolver beans being defined in the context.
      */
     public void setDetectAllViewResolvers(boolean detectAllViewResolvers) {
         this.detectAllViewResolvers = detectAllViewResolvers;
     }

     /**
      * Set whether to throw a NoHandlerFoundException when no Handler was found for this request.
      * This exception can then be caught with a HandlerExceptionResolver or an
      * {@code @ExceptionHandler} controller method.
      * <p>Note that if {@link org.springframework.web.servlet.resource.DefaultServletHttpRequestHandler}
      * is used, then requests will always be forwarded to the default servlet and a
      * NoHandlerFoundException would never be thrown in that case.
      * <p>Default is "false", meaning the DispatcherServlet sends a NOT_FOUND error through the
      * Servlet response.
      * @since 4.0
      */
     public void setThrowExceptionIfNoHandlerFound(boolean throwExceptionIfNoHandlerFound) {
         this.throwExceptionIfNoHandlerFound = throwExceptionIfNoHandlerFound;
     }

     /**
      * Set whether to perform cleanup of request attributes after an include request, that is,
      * whether to reset the original state of all request attributes after the DispatcherServlet
      * has processed within an include request. Otherwise, just the DispatcherServlet's own
      * request attributes will be reset, but not model attributes for JSPs or special attributes
      * set by views (for example, JSTL's).
      * <p>Default is "true", which is strongly recommended. Views should not rely on request attributes
      * having been set by (dynamic) includes. This allows JSP views rendered by an included controller
      * to use any model attributes, even with the same names as in the main JSP, without causing side
      * effects. Only turn this off for special needs, for example to deliberately allow main JSPs to
      * access attributes from JSP views rendered by an included controller.
      */
     public void setCleanupAfterInclude(boolean cleanupAfterInclude) {
         this.cleanupAfterInclude = cleanupAfterInclude;
     }

     /**
      * This implementation calls {@link #initStrategies}.
      */
     @Override
     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);
     }

     /**
      * Initialize the MultipartResolver used by this class.
      * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
      * no multipart handling is provided.
      */
     private void initMultipartResolver(ApplicationContext context) {
         try {
             this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");
             }
         }
         catch (NoSuchBeanDefinitionException ex) {
             // Default is no multipart resolver.
             this.multipartResolver = null;
             if (logger.isDebugEnabled()) {
                 logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME +
                         "': no multipart request handling provided");
             }
         }
     }

     /**
      * Initialize the LocaleResolver used by this class.
      * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
      * we default to AcceptHeaderLocaleResolver.
      */
     private void initLocaleResolver(ApplicationContext context) {
         try {
             this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
             }
         }
         catch (NoSuchBeanDefinitionException ex) {
             // We need to use the default.
             this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME +
                         "': using default [" + this.localeResolver + "]");
             }
         }
     }

     /**
      * Initialize the ThemeResolver used by this class.
      * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
      * we default to a FixedThemeResolver.
      */
     private void initThemeResolver(ApplicationContext context) {
         try {
             this.themeResolver = context.getBean(THEME_RESOLVER_BEAN_NAME, ThemeResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Using ThemeResolver [" + this.themeResolver + "]");
             }
         }
         catch (NoSuchBeanDefinitionException ex) {
             // We need to use the default.
             this.themeResolver = getDefaultStrategy(context, ThemeResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Unable to locate ThemeResolver with name '" + THEME_RESOLVER_BEAN_NAME +
                         "': using default [" + this.themeResolver + "]");
             }
         }
     }

     /**
      * Initialize the HandlerMappings used by this class.
      * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
      * we default to BeanNameUrlHandlerMapping.
      */
     private void initHandlerMappings(ApplicationContext context) {
         this.handlerMappings = null;

         if (this.detectAllHandlerMappings) {
             // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
             Map<String, HandlerMapping> matchingBeans =
                     BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
             if (!matchingBeans.isEmpty()) {
                 this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                 // We keep HandlerMappings in sorted order.
                 AnnotationAwareOrderComparator.sort(this.handlerMappings);
             }
         }
         else {
             try {
                 HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                 this.handlerMappings = Collections.singletonList(hm);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, we'll add a default HandlerMapping later.
             }
         }

         // Ensure we have at least one HandlerMapping, by registering
         // a default HandlerMapping if no other mappings are found.
         if (this.handlerMappings == null) {
             this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
             }
         }
     }

     /**
      * Initialize the HandlerAdapters used by this class.
      * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace,
      * we default to SimpleControllerHandlerAdapter.
      */
     private void initHandlerAdapters(ApplicationContext context) {
         this.handlerAdapters = null;

         if (this.detectAllHandlerAdapters) {
             // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
             Map<String, HandlerAdapter> matchingBeans =
                     BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
             if (!matchingBeans.isEmpty()) {
                 this.handlerAdapters = new ArrayList<HandlerAdapter>(matchingBeans.values());
                 // We keep HandlerAdapters in sorted order.
                 AnnotationAwareOrderComparator.sort(this.handlerAdapters);
             }
         }
         else {
             try {
                 HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                 this.handlerAdapters = Collections.singletonList(ha);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, we'll add a default HandlerAdapter later.
             }
         }

         // Ensure we have at least some HandlerAdapters, by registering
         // default HandlerAdapters if no other adapters are found.
         if (this.handlerAdapters == null) {
             this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
             }
         }
     }

     /**
      * Initialize the HandlerExceptionResolver used by this class.
      * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
      * we default to no exception resolver.
      */
     private void initHandlerExceptionResolvers(ApplicationContext context) {
         this.handlerExceptionResolvers = null;

         if (this.detectAllHandlerExceptionResolvers) {
             // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
             Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
                     .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
             if (!matchingBeans.isEmpty()) {
                 this.handlerExceptionResolvers = new ArrayList<HandlerExceptionResolver>(matchingBeans.values());
                 // We keep HandlerExceptionResolvers in sorted order.
                 AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
             }
         }
         else {
             try {
                 HandlerExceptionResolver her =
                         context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
                 this.handlerExceptionResolvers = Collections.singletonList(her);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, no HandlerExceptionResolver is fine too.
             }
         }

         // Ensure we have at least some HandlerExceptionResolvers, by registering
         // default HandlerExceptionResolvers if no other resolvers are found.
         if (this.handlerExceptionResolvers == null) {
             this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("No HandlerExceptionResolvers found in servlet '" + getServletName() + "': using default");
             }
         }
     }

     /**
      * Initialize the RequestToViewNameTranslator used by this servlet instance.
      * <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
      */
     private void initRequestToViewNameTranslator(ApplicationContext context) {
         try {
             this.viewNameTranslator =
                     context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]");
             }
         }
         catch (NoSuchBeanDefinitionException ex) {
             // We need to use the default.
             this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Unable to locate RequestToViewNameTranslator with name '" +
                         REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator +
                         "]");
             }
         }
     }

     /**
      * Initialize the ViewResolvers used by this class.
      * <p>If no ViewResolver beans are defined in the BeanFactory for this
      * namespace, we default to InternalResourceViewResolver.
      * 通过该类初始化ViewResolvers，若BeanFactory里面未定义ViewResolver的beans，
      * 则默认使用InternalResourceViewResolver。
      */
     private void initViewResolvers(ApplicationContext context) {
         //初始化viewResolvers
         this.viewResolvers = null;
         //查找ApplicationContext（包括祖先上下文）中所有的ViewResolvers
         if (this.detectAllViewResolvers) {
             // Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
             Map<String, ViewResolver> matchingBeans =
                     BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
             //若ViewResolvers存在，则将其放入ArrayList中并进行排序操作
             if (!matchingBeans.isEmpty()) {
                 this.viewResolvers = new ArrayList<ViewResolver>(matchingBeans.values());
                 // We keep ViewResolvers in sorted order.
                 AnnotationAwareOrderComparator.sort(this.viewResolvers);
             }
         }
         else {   //若ApplicationContext（包括祖先上下文）中没有ViewResolvers，则使用viewResolver
             try {
                 ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
                 this.viewResolvers = Collections.singletonList(vr);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, we'll add a default ViewResolver later.
             }
         }
         // Ensure we have at least one ViewResolver, by registering
         // a default ViewResolver if no other resolvers are found.
         //确保至少有一个ViewResolver，若果没有resolvers查到的话就注册一个默认的ViewResolver
         if (this.viewResolvers == null) {
             this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default");
             }
         }
     }

     /**
      * Initialize the {@link FlashMapManager} used by this servlet instance.
      * <p>If no implementation is configured then we default to
      * {@code org.springframework.web.servlet.support.DefaultFlashMapManager}.
      */
     private void initFlashMapManager(ApplicationContext context) {
         try {
             this.flashMapManager = context.getBean(FLASH_MAP_MANAGER_BEAN_NAME, FlashMapManager.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Using FlashMapManager [" + this.flashMapManager + "]");
             }
         }
         catch (NoSuchBeanDefinitionException ex) {
             // We need to use the default.
             this.flashMapManager = getDefaultStrategy(context, FlashMapManager.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("Unable to locate FlashMapManager with name '" +
                         FLASH_MAP_MANAGER_BEAN_NAME + "': using default [" + this.flashMapManager + "]");
             }
         }
     }

     /**
      * Return this servlet's ThemeSource, if any; else return {@code null}.
      * <p>Default is to return the WebApplicationContext as ThemeSource,
      * provided that it implements the ThemeSource interface.
      * @return the ThemeSource, if any
      * @see #getWebApplicationContext()
      */
     public final ThemeSource getThemeSource() {
         if (getWebApplicationContext() instanceof ThemeSource) {
             return (ThemeSource) getWebApplicationContext();
         }
         else {
             return null;
         }
     }

     /**
      * Obtain this servlet's MultipartResolver, if any.
      * @return the MultipartResolver used by this servlet, or {@code null} if none
      * (indicating that no multipart support is available)
      */
     public final MultipartResolver getMultipartResolver() {
         return this.multipartResolver;
     }

     /**
      * Return the default strategy object for the given strategy interface.
      * <p>The default implementation delegates to {@link #getDefaultStrategies},
      * expecting a single object in the list.
      * @param context the current WebApplicationContext
      * @param strategyInterface the strategy interface
      * @return the corresponding strategy object
      * @see #getDefaultStrategies
      */
     protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
         List<T> strategies = getDefaultStrategies(context, strategyInterface);
         if (strategies.size() != 1) {
             throw new BeanInitializationException(
                     "DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
         }
         return strategies.get(0);
     }

     /**
      * Create a List of default strategy objects for the given strategy interface.
      * <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
      * package as the DispatcherServlet class) to determine the class names. It instantiates
      * the strategy objects through the context's BeanFactory.
      * @param context the current WebApplicationContext
      * @param strategyInterface the strategy interface
      * @return the List of corresponding strategy objects
      */
     @SuppressWarnings("unchecked")
     protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
         String key = strategyInterface.getName();
         String value = defaultStrategies.getProperty(key);
         if (value != null) {
             String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
             List<T> strategies = new ArrayList<T>(classNames.length);
             for (String className : classNames) {
                 try {
                     Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                     Object strategy = createDefaultStrategy(context, clazz);
                     strategies.add((T) strategy);
                 }
                 catch (ClassNotFoundException ex) {
                     throw new BeanInitializationException(
                             "Could not find DispatcherServlet's default strategy class [" + className +
                                     "] for interface [" + key + "]", ex);
                 }
                 catch (LinkageError err) {
                     throw new BeanInitializationException(
                             "Error loading DispatcherServlet's default strategy class [" + className +
                                     "] for interface [" + key + "]: problem with class file or dependent class", err);
                 }
             }
             return strategies;
         }
         else {
             return new LinkedList<T>();
         }
     }

     /**
      * Create a default strategy.
      * <p>The default implementation uses
      * {@link org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean}.
      * @param context the current WebApplicationContext
      * @param clazz the strategy implementation class to instantiate
      * @return the fully configured strategy instance
      * @see org.springframework.context.ApplicationContext#getAutowireCapableBeanFactory()
      * @see org.springframework.beans.factory.config.AutowireCapableBeanFactory#createBean
      */
     protected Object createDefaultStrategy(ApplicationContext context, Class<?> clazz) {
         return context.getAutowireCapableBeanFactory().createBean(clazz);
     }

     /**
      * 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 resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
             logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
                     " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
         }

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

         // Make framework objects available to handlers and view objects.
         request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
         request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
         request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
         request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

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

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

     /**
      * 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.
      * 处理分发请求。处理程序将通过应用servlet的HandlerMappings来获得。HandlerAdapter将通过查询servlet
      * 安装的HandlerAdapter获得。所有的HTTP方法都被该方法处理。这取决于handleradapter或handler。
      * 自己决定哪些方法是可以接受的。
      * @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;//获取请求
         HandlerExecutionChain mappedHandler = null;//初始化Hadler处理链
         boolean multipartRequestParsed = false;
         //实例化web异步管理类
         WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

         try {
             ModelAndView mv = null;//初始化ModelAndView
             Exception dispatchException = null;//初始化分发异常

             try {
                 processedRequest = checkMultipart(request);
                 multipartRequestParsed = (processedRequest != request);

                 // Determine handler for the current request.确定当前请求的处理程序
                 mappedHandler = getHandler(processedRequest);
                 if (mappedHandler == null || mappedHandler.getHandler() == null) {
                     noHandlerFound(processedRequest, response);
                     return;
                 }

                 // Determine handler adapter for the current request.确定当前请求的处理程序适配器
                 HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                 // Process last-modified header, if supported by the handler.
                 String method = request.getMethod();
                 boolean isGet = "GET".equals(method);
                 if (isGet || "HEAD".equals(method)) {
                     long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                     if (logger.isDebugEnabled()) {
                         logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                     }
                     if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                         return;
                     }
                 }

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

                 // Actually invoke the handler.实际调用处理程序。
                 mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                 //当前请求对应的handler选择使用异步的方式处理当前请求
                 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.清理文件上传用到的任何资源
                 if (multipartRequestParsed) {
                     cleanupMultipart(processedRequest);
                 }
             }
         }
     }

     /**
      * Do we need view name translation?
      */
     private void applyDefaultViewName(HttpServletRequest request, ModelAndView mv) throws Exception {
         if (mv != null && !mv.hasView()) {
             mv.setViewName(getDefaultViewName(request));
         }
     }

     /**
      * Handle the result of handler selection and handler invocation, which is
      * either a ModelAndView or an Exception to be resolved to a ModelAndView.
      * 处理选择的handler或者回调的handler的结果，要么是一个ModelAndView，要么是一个异常。
      */
     private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
             HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

         boolean errorView = false;
         //若发生了异常，根据异常的类型做不同处理
         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);
             }
         }

         // Did the handler return a view to render?
         //判断handler是否返回了用于渲染的视图
         if (mv != null && !mv.wasCleared()) {
             render(mv, request, response);//渲染给定的ModelAndView
             if (errorView) {
                 WebUtils.clearErrorRequestAttributes(request);
             }
         }
         else {  //无ModelAndView返回
             if (logger.isDebugEnabled()) {
                 logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                         "': assuming HandlerAdapter completed request handling");
             }
         }
         //若handler以异步方式处理请求，则返回
         if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
             // Concurrent handling started during a forward
             return;
         }
         //handler执行链不为空，则在完成后触发映射的handler拦截器
         if (mappedHandler != null) {
             mappedHandler.triggerAfterCompletion(request, response, null);
         }
     }

     /**
      * Build a LocaleContext for the given request, exposing the request's primary locale as current locale.
      * <p>The default implementation uses the dispatcher's LocaleResolver to obtain the current locale,
      * which might change during a request.
      * @param request current HTTP request
      * @return the corresponding LocaleContext
      */
     @Override
     protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
         if (this.localeResolver instanceof LocaleContextResolver) {
             return ((LocaleContextResolver) this.localeResolver).resolveLocaleContext(request);
         }
         else {
             return new LocaleContext() {
                 @Override
                 public Locale getLocale() {
                     return localeResolver.resolveLocale(request);
                 }
             };
         }
     }

     /**
      * Convert the request into a multipart request, and make multipart resolver available.
      * <p>If no multipart resolver is set, simply use the existing request.
      * 将请求转成文件上传请求并且可以获取文件上传解析器
      * 如果没有设置文件上传解析器，那就使用已存在的请求。
      * @param request current HTTP request
      * @return the processed request (multipart wrapper if necessary)
      * @see MultipartResolver#resolveMultipart
      */
     protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
         //存在multipartResolver并且是multipart请求
         if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
             if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                 logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
                         "this typically results from an additional MultipartFilter in web.xml");
             }
             else if (hasMultipartException(request) ) {
                 logger.debug("Multipart resolution failed for current request before - " +
                         "skipping re-resolution for undisturbed error rendering");
             }
             else {
                 try {
                     return this.multipartResolver.resolveMultipart(request);
                 }
                 catch (MultipartException ex) {
                     if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                         logger.debug("Multipart resolution failed for error dispatch", ex);
                         // Keep processing error dispatch with regular request handle below
                     }
                     else {
                         throw ex;
                     }
                 }
             }
         }
         // If not returned before: return original request.
         return request;
     }

     /**
      * Check "javax.servlet.error.exception" attribute for a multipart exception.
      * 检查是否有MultipartException
      */
     private boolean hasMultipartException(HttpServletRequest request) {
         Throwable error = (Throwable) request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE);
         while (error != null) {
             if (error instanceof MultipartException) {
                 return true;
             }
             error = error.getCause();
         }
         return false;
     }

     /**
      * Clean up any resources used by the given multipart request (if any).
      * @param request current HTTP request
      * @see MultipartResolver#cleanupMultipart
      */
     protected void cleanupMultipart(HttpServletRequest request) {
         MultipartHttpServletRequest multipartRequest =
                 WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class);
         if (multipartRequest != null) {
             this.multipartResolver.cleanupMultipart(multipartRequest);
         }
     }

     /**
      * Return the HandlerExecutionChain for this request.
      * 返回请求对应的handler执行链
      * <p>Tries all handler mappings in order.
      * @param request current HTTP request
      * @return the HandlerExecutionChain, or {@code null} if no handler could be found
      */
     protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         for (HandlerMapping hm : this.handlerMappings) {
             if (logger.isTraceEnabled()) {
                 logger.trace(
                         "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
             }
             HandlerExecutionChain handler = hm.getHandler(request);
             if (handler != null) {
                 return handler;
             }
         }
         return null;
     }

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

     /**
      * Return the HandlerAdapter for this handler object.
      * 返回handler对象的handler适配器
      * @param handler the handler object to find an adapter for
      * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
      */
     protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
         for (HandlerAdapter ha : this.handlerAdapters) {
             if (logger.isTraceEnabled()) {
                 logger.trace("Testing handler adapter [" + ha + "]");
             }
             if (ha.supports(handler)) {
                 return ha;
             }
         }
         throw new ServletException("No adapter for handler [" + handler +
                 "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
     }

     /**
      * Determine an error ModelAndView via the registered HandlerExceptionResolvers.
      * @param request current HTTP request
      * @param response current HTTP response
      * @param handler the executed handler, or {@code null} if none chosen at the time of the exception
      * (for example, if multipart resolution failed)
      * @param ex the exception that got thrown during handler execution
      * @return a corresponding ModelAndView to forward to
      * @throws Exception if no error ModelAndView found
      */
     protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
             Object handler, Exception ex) throws Exception {

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

         throw ex;
     }

     /**
      * Render the given ModelAndView.
      * <p>This is the last stage in handling a request. It may involve resolving the view by name.
      * 根据给定的ModelAndView进行渲染。这是处理请求的最后一步，涉及到通过名称解析视图。
      * @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.resolveLocale(request);
         response.setLocale(locale);

         View view;
         //判断是否是视图引用
         if (mv.isReference()) {
             // We need to resolve the view name.
             //将视图名称解析为一个view对象（等待渲染），为null则抛出无法解析异常
             view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
             if (view == null) {
                 throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                         "' in servlet with name '" + getServletName() + "'");
             }
         }
         else {
             // No need to lookup: the ModelAndView object contains the actual View object.
             //若是非视图引用，则直接可以从ModelAndView对象中获取真实的View对象
             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.
         //委托到视图对象进行呈现。
         if (logger.isDebugEnabled()) {
             logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
         }
         try {
             if (mv.getStatus() != null) {
                 response.setStatus(mv.getStatus().value());//设置http响应状态
             }
             /**
              * The first step will be preparing the request: In the JSP case, this would mean
              * setting model objects as request attributes. The second step will be the actual
              * rendering of the view, for example including the JSP via a RequestDispatcher.
              * 第一步是准备request：如果是jsp页面，则意味着将model对象设置为request属性。第二步
              * 将是真正的渲染视图，举例来说，JSP通过一个RequestDispatcher(RequestDispatcher是一个
              * Web资源的包装器，可以用来把当前request传递到该资源，或者把新的资源包括到当前响应中。)。
             */
             view.render(mv.getModelInternal(), request, response);//根据特定的model渲染视图
         }
         catch (Exception ex) {
             if (logger.isDebugEnabled()) {
                 logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
                         getServletName() + "'", ex);
             }
             throw ex;
         }
     }

     /**
      * Translate the supplied request into a default view name.
      * 根据提供的请求得到默认的视图名称
      * @param request current HTTP servlet request
      * @return the view name (or {@code null} if no default found)
      * @throws Exception if view name translation failed
      */
     protected String getDefaultViewName(HttpServletRequest request) throws Exception {
         return this.viewNameTranslator.getViewName(request);
     }

     /**
      * Resolve the given view name into a View object (to be rendered).
      * 根据给定的视图名称将其变成一个view对象（等待渲染）
      * <p>The default implementations asks all ViewResolvers of this dispatcher.
      * Can be overridden for custom resolution strategies, potentially based on
      * specific model attributes or request parameters.
      * @param viewName the name of the view to resolve
      * @param model the model to be passed to the view
      * @param locale the current locale
      * @param request current HTTP servlet request
      * @return the View object, or {@code null} if none found
      * @throws Exception if the view cannot be resolved
      * (typically in case of problems creating an actual View object)
      * @see ViewResolver#resolveViewName
      */
     protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
             HttpServletRequest request) throws Exception {
         //遍历所有的视图解析器，解析出视图名称
         for (ViewResolver viewResolver : this.viewResolvers) {
             View view = viewResolver.resolveViewName(viewName, locale);
             if (view != null) {
                 return view;
             }
         }
         return null;
     }

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

         if (mappedHandler != null) {
             mappedHandler.triggerAfterCompletion(request, response, ex);
         }
         throw ex;
     }

     /**
      * Restore the request attributes after an include.
      * 重新存储请求中的属性
      * @param request current HTTP request
      * @param attributesSnapshot the snapshot of the request attributes before the include
      */
     @SuppressWarnings("unchecked")
     private void restoreAttributesAfterInclude(HttpServletRequest request, Map<?,?> attributesSnapshot) {
         // Need to copy into separate Collection here, to avoid side effects
         // on the Enumeration when removing attributes.
         Set<String> attrsToCheck = new HashSet<String>();
         Enumeration<?> attrNames = request.getAttributeNames();
         while (attrNames.hasMoreElements()) {
             String attrName = (String) attrNames.nextElement();
             if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                 attrsToCheck.add(attrName);
             }
         }

         // Add attributes that may have been removed
         attrsToCheck.addAll((Set<String>) attributesSnapshot.keySet());

         // Iterate over the attributes to check, restoring the original value
         // or removing the attribute, respectively, if appropriate.
         for (String attrName : attrsToCheck) {
             Object attrValue = attributesSnapshot.get(attrName);
             if (attrValue == null){
                 request.removeAttribute(attrName);
             }
             else if (attrValue != request.getAttribute(attrName)) {
                 request.setAttribute(attrName, attrValue);
             }
         }
     }

     private static String getRequestUri(HttpServletRequest request) {
         String uri = (String) request.getAttribute(WebUtils.INCLUDE_REQUEST_URI_ATTRIBUTE);
         if (uri == null) {
             uri = request.getRequestURI();
         }
         return uri;
     }

 }