DispatcherServlet的工作原理

一、MVC程序

我们使用全注解的方式来使用MVC，所以先写段演示程序。

/**
 * 系统初始化入口。【代替web.xml】
 */
public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    /**
     * Spring配置(设置spring容器启动的入口。配置ContextLoaderListener)
     */
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class<?>[]{AppConfig.class};
    }

    /**
     * SpringMVC配置文件。(配置DispatcherServlet)
     */
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class<?>[]{WebMvcConfig.class};
    }

    /**
     * 设置DispatcherServlet的拦截路径
     */
    @Override
    protected String[] getServletMappings() {
        // /：拦截所有请求，包括静态资源，但不包括jsp
        // /*:拦截所有请求，包括静态资源和jsp文件
        return new String[]{"/"};
    }
}

/**
 * Spring父容器扫描.【@Contoller注解由MVC子容器来扫描,MVC中的C就表示Controller】
 */
@ComponentScan(value = "com.mvc", excludeFilters = {//排除扫描@Controller
        @Filter(type = FilterType.ANNOTATION, classes = {Controller.class})
})
public class AppConfig {
}

/**
 * SpringMVC子容器
 */
@EnableWebMvc
//@ComponentScan("com.mvc")
@ComponentScan(value = "com.mvc", includeFilters = {//扫描@Controller
        @Filter(type = FilterType.ANNOTATION, classes = {Controller.class})
}, useDefaultFilters = false)//禁用默认的过滤规则
public class WebMvcConfig extends WebMvcConfigurerAdapter {

    /**
     * 继承默认的转换器。【导入jackson包后会自动添加json转换器，不需显式添加】
     */
    /*
    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        System.out.println(converters);
        converters.add(new MappingJackson2HttpMessageConverter());
        System.out.println(converters);
    }*/

    /**
     * 拦截器
     */
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new MyAuthInterceptor());
    }

    /**
     * 视图解析器
     */
    @Bean
    public ViewResolver viewResolver() {
        //jsp视图解析器
        InternalResourceViewResolver resolver = new InternalResourceViewResolver();
        resolver.setPrefix("/WEB-INF/view/");
        resolver.setSuffix(".jsp");
        resolver.setExposeContextBeansAsAttributes(true);
        return resolver;
    }

    /**
     * 配置静态资源的处理
     * 将请求交由Servlet处理,不经过DispatchServlet
     */
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
        configurer.enable();
    }

}

/**
 * 返回视图
 */
@Controller
public class UserController {

    @Autowired
    private UserService userService;

    @GetMapping("/user")
    public String getUser() {
        userService.getUser();
        return "success";
    }

}

/**
 * 返回rest数据
 */
@RestController
public class UserRestController {

    @Autowired
    private UserService userService;

    @ResponseBody
    @GetMapping("/user-rest")
    public User getUser() {
        return userService.getUser();
    }

}

二、源码分析

1.MVC工作原理

下面是DispatcherServlet的工作原理图，图片来源于网络。

DispatcherServlet的工作流程如下：

1.请求到达后，调用HandlerMapping来查找对应的处理器Handler

2.查找能调用上面Handler的HandlerAdapter。(并非直接调用Handler，而是使用的适配器模式)

3.预处理操作。比如执行拦截器

4.真正的处理请求的操作。(比如执行controller中的某方法来处理请求)

5.后处理操作。重走拦截器栈。(跟strut2中的拦截器栈相似)

6.进行视图渲染输出。如果前面步骤发生了异常，则这里要处理异常。

    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 {
　　　　　　　　　 //0.检查是否为文件上传请求
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // 1.查找能处理当前请求的处理器 Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // 2.查找能处理当前请求的处理器适配器 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 = "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;
                    }
                }
　　　　　　　　　　// 3.预处理：比如调用拦截器
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // 4.真正的处理请求(调用Controller的处理方法) Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
　　　　　　　　　 // 5.处理后操作：比如再次重走拦截器栈
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
　　　　　　　// 6.处理最终结果：返回视图或处理异常
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
                }
            }
        }
    }

下面来看看每个步骤的源码

1.查找处理器-getHandler

遍历处理器HandlerMapping，查找出能够处理该请求的处理器，显然可以是多个，这些处理器组成了一个处理器执行链。

    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        //遍历SpringMVC内置的几种HandlerMapping
        for (HandlerMapping hm : this.handlerMappings) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
            }

            HandlerExecutionChain handler = hm.getHandler(request);
            //只要找到一种能处理我们的程序的HandlerMapping就返回。
            //
            if (handler != null) {
                return handler;
            }
        }
        return null;
    }

由于我们可以使用不同的方式来开发SpringMVC应用，每种方式SpringMVC都能找到对应的处理器来进行处理。比如我们使用@RequestMapping注解方式，则SpringMVC就利用RequestMappingHandler来处理。常见的几种HandlerMapping有RequestMappingHandlerMapping，BeanNameUrlHandlerMapping，SimpleUrlHandlerMaping。

SpringMVC只会选用其中一种HandlerMapping来处理。因为我们编写的程序使用了@RequestMapping，实际上就是@RequestMapping方式，所以SpringMVC对应使用RequestMappingHandlerMapping来处理。

SpringMVC是怎么判断RequestMappingHandlerMapping能处理我们的程序呢？实际上每种HandlerMapping对应一些默认的拦截器，也可能没有对应的拦截器。SpringMVC通过查找所有的拦截器，如果拦截器能够处理当前请求，则将这些拦截器串起来组成拦截器链。只要该拦截器链不为null，就表明能够处理我们的程序。

    @Override
    public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Object handler = getHandlerInternal(request);
        if (handler == null) {
            handler = getDefaultHandler();
        }
        if (handler == null) {
            return null;
        }
        // Bean name or resolved handler?
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }
        //获取能处理当前Controller的拦截器链
        HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
        if (CorsUtils.isCorsRequest(request)) {
            CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
            CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
            CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
            executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
        }
        return executionChain;
    }

    //AbstractHandlerMapping#getHandlerExecutionChain()
    protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
        HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
                (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
        //查找所有能处理当前url的拦截器，组成拦截器链
        for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
            if (interceptor instanceof MappedInterceptor) {
                MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
                if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
                    chain.addInterceptor(mappedInterceptor.getInterceptor());
                }
            }
            else {
                chain.addInterceptor(interceptor);
            }
        }
        return chain;
    }

下面是能处理我们UserController的拦截器链，除了我们自定义的MyAuthInterceptor外，还有两个默认拦截器。

2.查找能调用Handler的HandlerAdapter

因为系统中有不同类型的controller，每种controller的使用方式都不一样。必然会出现这样的代码。

if(handler instanceof A类型){

　　//处理A类型的controller

}else (handler instanceof B类型){　　

　　//处理B类型的controller

}

显然这样的处理方式不利于扩展，因此使用适配器模式来调用不同的处理器。

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        //遍历所有的HandlerMethodAdapter
        for (HandlerAdapter ha : this.handlerAdapters) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing handler adapter [" + ha + "]");
            }
            //只要该HandlerMethodAdapter支持处理handler就返回
            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");
    }

    //AbstractHandlerMethodAdapter.supports()
    @Override
    public final boolean supports(Object handler) {
        return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
    }

    //RequestMappingHandlerAdapter.supportsInternal()
    @Override
    protected boolean supportsInternal(HandlerMethod handlerMethod) {
        //RequestMappingHandlerAdapter默认是支持处理handler的
        return true;
    }

RequestMappingHandlerAdapter支持处理我们程序中的UserController。

3.预处理操作

在处理请求之前会先执行预处理操作，比如执行拦截器。具体逻辑是遍历拦截器栈，找出能处理该请求的拦截器(每个拦截器都要配置拦截映射)，然后调用该拦截器的preHandler()方法进行前置处理。在之后的第5步会逆序重走拦截器栈进行后置处理(同struts2)。

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        //获取所有的拦截器
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            //依次执行每个拦截器(前置处理)
            for (int i = 0; i < interceptors.length; i++) {
                HandlerInterceptor interceptor = interceptors[i];
                if (!interceptor.preHandle(request, response, this.handler)) {//前置处理
                    triggerAfterCompletion(request, response, null);
                    return false;
                }
                this.interceptorIndex = i;
            }
        }
        return true;
    }

4.真正的处理请求

这里会真正的执行controller中的方法来处理请求。下面来看看我们常用的RequestMappingHandlerAdatper是如何处理请求的。

    @Override
    public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        return handleInternal(request, response, (HandlerMethod) handler);
    }

    protected ModelAndView handleInternal(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        checkRequest(request);

        if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
            applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
        }
        else {
            prepareResponse(response);
        }

        // Execute invokeHandlerMethod in synchronized block if required.
　　　　　//同步调用
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized (mutex) {//加锁
　　　　　　　　　　　　//调用controller中的方法
                    return invokeHandlerMethod(request, response, handlerMethod);
                }
            }
        }
　　　　　//非同步调用
        return invokeHandlerMethod(request, response, handlerMethod);
    }

5.执行后处理操作

第3步已经提到了，这里会逆序重新走一遍拦截器栈，也就是调用拦截器的postHandler()方法进行后置处理。

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                interceptor.postHandle(request, response, this.handler, mv);//后置处理
            }
        }
    }

6.视图渲染和异常处理

这一步主要是进行视图的渲染，如果前面步骤有异常，这里就进行异常处理。

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

        boolean errorView = false;

        //发生了异常，则处理异常
        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                //处理异常
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }

        //执行到这里，说明没有发生异常

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, request, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }

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

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

处理异常的过程如下：

遍历异常解析器来解析异常。通常我们会配置SpringMVC的全局异常解析器来进行统一异常处理，就是在这里使用它来处理异常的。

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;
    }