spring mvc DispatcherServlet解读

DispatcherServlet的继承和实现

IDEA按快捷键：ctrl+alt+u，就可以查看DispatcherServlet的继承的类和实现的接口。

从继承图可以得知：

DispatcherServlet本质是一个HttpServlet
HttpServletBean，FrameworkServlet,DispatcherServlet是属于SpringMVC的类

DispatcherServlet初始化

DispatcherServlet

DispatcherServlet看initStrategies()，该方法用于初始化各类解析器。

onRefresh(ApplicationContext context)，该方法用于刷新策略

/*** This implementation calls {@link #initStrategies}.
*/
//刷新策略
@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.
*/
//初始化各类解析器
protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);    //初始化分段上传解析器initLocaleResolver(context);     //初始化地域解析器initThemeResolver(context);           //初始化主题解析器initHandlerMappings(context);     //初始化处理器映射initHandlerAdapters(context);     //初始化处理器适配器initHandlerExceptionResolvers(context);       //初始化处理器异常解析器initRequestToViewNameTranslator(context);     //初始化请求到视图名解析器initViewResolvers(context);        //初始化视图解析器initFlashMapManager(context);      //初始化快闪映射
}

FrameworkServlet

onRefresh(ApplicationContext context)重写于父类FrameworkServlet，点开FrameworkServlet，我们主要看看它initServletBean()方法，FrameworkServlet的onRefresh(ApplicationContext context)是个空实现，因为在DispatcherServlet实现了。

initServletBean()是重写HttpServletBean的initServletBean()，先看看initServletBean()干了什么：

调用了initWebApplicationContext()，初始化了WebApplicationContext，而WebApplicationContext是继承于ApplicationContext接口的，所以我们知道了initWebApplicationContext是初始化了一个上下文。
调用了initFrameworkServlet()，点开发现是个空实现。

@Overrideprotected final void initServletBean() throws ServletException {//*******查看核心部分**********try {//这里初始化WebApplicationContextthis.webApplicationContext = initWebApplicationContext();initFrameworkServlet();   //空实现，给其他继承类实现}catch (ServletException | RuntimeException ex) {logger.error("Context initialization failed", ex);throw ex;}//*******其他日志的不看********}

点开initWebApplicationContext()，因为太长了，所以放图片。

上半段：

通过servlet上下文中创建了一个rootcontext
中间的if代码块在上下文刷新时才会运行，初始化状态时，webapplicationcontext是null，所以初始化时是不进入的。该段代码主要用于刷新上下文

下半段：

查找已经存在的注册的上下文，如果没有就用rootContex去创建一样上下文。
refreshEventReceived默认是false

/** Flag used to detect whether onRefresh has already been called. */
private volatile boolean refreshEventReceived = false;

onRefresh()方法被重写了，所以我们知道在Dispatchservlet中，会初始化各种解析器（如：视图解析器等）

HttpServletBean

假设WEB-INF 目录下查找配置文件信息如下：

<!-- 部署 DispatcherServlet -->
<servlet><servlet-name>springmvc</servlet-name><servlet-class>org.springframework.web.servlet.DispatcherServlet </servlet-class><init-param><param-name>contextConfigLocation</param-name><param-value>classpath:springmvc-servlet.xml</param-value></init-param><!-- 表示容器再启动时立即加载servlet --><load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping><servlet-name>springmvc</servlet-name><url-pattern>/</url-pattern>
</servlet-mapping>

HttpServletBean是抽象类，ctrl+F12查看方法。

找到init()方法，这个是我们要找到的重要方法

init()方法是重写了GenericServlet的init()方法，所以这个是最根本的初始化方法。

让我们看看init()干了什么：

ServletConfigPropertyValues是用来加载Servlet配置，ServletConfigPropertyValues是HttpServletBean的内部静态类，它负责取到web.xml中init-param（如：contextConfigLocation）

@Overridepublic final void init() throws ServletException {//加载web.xml中init-param（如：contextConfigLocation）// Set bean properties from init parameters.PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);if (!pvs.isEmpty()) {try {//BeanWrapper将HttpServletBean进行包装BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);//ServletContext的资源加载器ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());//注册资源编辑器bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));//为HttpServletBean初始化包装，默认实现是空的initBeanWrapper(bw);//给包装类传递web.xml中init-parambw.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()是交给子类实现的，FrameworkServlet实现了这个方法initServletBean();}

初始化总结

各类的初始化工作：

HttpServletBean 主要初始化web.xml资源
FrameworkServlet 获取servlet上下文创建WebApplicationContext
DispatcherServlet 初始化各类解析器

调用过程

HttpServletBean ---- init()
FrameworkServlet ---- initServletBean()
FrameworkServlet ---- initWebApplicationContext()
DispatcherServlet ---- onRefresh(ApplicationContext context)
DispatcherServlet ---- initStrategies(ApplicationContext context)
DispatcherServlet ---- initXXX(ApplicationContext context)各种解析器组件的初始化

DispatcherServlet处理请求

核心成员变量

DispatcherServlet的核心成员变量就是上面DispatcherServlet初始化中initStrategies(ApplicationContext context)初始化的各类处理器。

/** MultipartResolver used by this servlet. */@Nullableprivate MultipartResolver multipartResolver;/** LocaleResolver used by this servlet. */@Nullableprivate LocaleResolver localeResolver;/** ThemeResolver used by this servlet. */@Nullableprivate ThemeResolver themeResolver;/** List of HandlerMappings used by this servlet. */@Nullableprivate List<HandlerMapping> handlerMappings;/** List of HandlerAdapters used by this servlet. */@Nullableprivate List<HandlerAdapter> handlerAdapters;/** List of HandlerExceptionResolvers used by this servlet. */@Nullableprivate List<HandlerExceptionResolver> handlerExceptionResolvers;/** RequestToViewNameTranslator used by this servlet. */@Nullableprivate RequestToViewNameTranslator viewNameTranslator;/** FlashMapManager used by this servlet. */@Nullableprivate FlashMapManager flashMapManager;/** List of ViewResolvers used by this servlet. */@Nullableprivate List<ViewResolver> viewResolvers;

doService()

在浏览器输入位置时，会触发DispatcherServlet的doService(HttpServletRequest request, HttpServletResponse response)去处理请求。

doService(HttpServletRequest request, HttpServletResponse response)又会去调用doDispatch(HttpServletRequest request, HttpServletResponse response)去处理请求

doDispatch()

doDispatch比较多代码分成两段截图

上半段代码

下半段代码

还有

getHandler()

getHandler()会遍历所有的handlerMapping，并找出匹配的handler。

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) {for (HandlerMapping mapping : this.handlerMappings) {HandlerExecutionChain handler = mapping.getHandler(request);if (handler != null) {return handler;}}}return null;}

关于handlerMapping接口的实现类

handlerMapping接口的实现类有许多，RequestMappingHandlerMapping，该类用于创建@RequestMapping和@Controller注解的实例