DispatcherServlet 详细解析

首先看下 DispatcherServlet 源码：

package org.springframework.web.servlet;

@SuppressWarnings("serial")
public class DispatcherServlet extends FrameworkServlet {

    public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";
    public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";
    public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";
    public static final String HANDLER_MAPPING_BEAN_NAME = "handlerMapping";
    public static final String HANDLER_ADAPTER_BEAN_NAME = "handlerAdapter";
    public static final String HANDLER_EXCEPTION_RESOLVER_BEAN_NAME = "handlerExceptionResolver";
    public static final String REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME = "viewNameTranslator";
    public static final String VIEW_RESOLVER_BEAN_NAME = "viewResolver";
    public static final String FLASH_MAP_MANAGER_BEAN_NAME = "flashMapManager";
    public static final String WEB_APPLICATION_CONTEXT_ATTRIBUTE = DispatcherServlet.class.getName() + ".CONTEXT";
    public static final String LOCALE_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".LOCALE_RESOLVER";
    public static final String THEME_RESOLVER_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_RESOLVER";
    public static final String THEME_SOURCE_ATTRIBUTE = DispatcherServlet.class.getName() + ".THEME_SOURCE";
    public static final String INPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".INPUT_FLASH_MAP";
    public static final String OUTPUT_FLASH_MAP_ATTRIBUTE = DispatcherServlet.class.getName() + ".OUTPUT_FLASH_MAP";
    public static final String FLASH_MAP_MANAGER_ATTRIBUTE = DispatcherServlet.class.getName() + ".FLASH_MAP_MANAGER";
    public static final String EXCEPTION_ATTRIBUTE = DispatcherServlet.class.getName() + ".EXCEPTION";
    public static final String PAGE_NOT_FOUND_LOG_CATEGORY = "org.springframework.web.servlet.PageNotFound";
    private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
    protected static final Log pageNotFoundLogger = LogFactory.getLog(PAGE_NOT_FOUND_LOG_CATEGORY);
    private static final Properties defaultStrategies;

    static {
        try {
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + 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;

    private FlashMapManager flashMapManager;

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

    public DispatcherServlet() {
        super();
    }

    public DispatcherServlet(WebApplicationContext webApplicationContext) {
        super(webApplicationContext);
    }
    @Override
    protected void onRefresh(ApplicationContext context) {
        initStrategies(context);
    }

    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }
}

DispatcherServlet 类中的属性 beans：

• HandlerMapping：用于 handlers 映射请求和一系列的对于拦截器的前处理和后处理，大部分用 @Controller 注解。
• HandlerAdapter：帮助 DispatcherServlet 处理映射请求处理程序的适配器，而不用考虑实际调用的是哪个处理程序。
• ViewResolver：根据配置解析实际的 View 类型。
• ThemeResolver：解决 Web 应用程序可以使用的主题，例如提供个性化布局。
• MultipartResolver：解析多部分请求，以支持从 HTML 表单上传文件。
• FlashMapManager：存储并检索可用于将一个请求属性传递到另一个请求的 input 和 output 的 FlashMap，通常用于重定向。

在 Web MVC 框架中，每个 DispatcherServlet 都拥有自己的 WebApplicationContext，它继承了 ApplicationContext。WebApplicationContext 包含了其上下文和 Servlet 实例之间共享的所有 beans。

1. HandlerMapping 接口 - 处理请求的映射

• SimpleUrlHandlerMapping 类通过配置文件把 URL 映射到 Controller 类。
• DefaultAnnotationHandlerMapping 类通过注解把 URL 映射到 Controller 类。

HandlerMapping

2. HandlerAdapter 接口 - 处理请求映射

AnnotationMethodHandlerAdapter：通过注解，把请求 URL 映射到 Controller 类的方法上。

HandlerAdapter

3. HandlerExceptionResolver 接口 - 异常处理

• SimpleMappingExceptionResolver：通过配置文件进行异常处理。
• AnnotationMethodHandlerExceptionResolver：通过注解进行异常处理。

HandlerExceptionResolver

4. ViewResolver 接口 - 解析 View 视图

UrlBasedViewResolver 类: 通过配置文件，把一个视图名交给一个 View 来处理。

ViewResolver