从源码来看 Spring MVC 的工作原理

 2022-07-26
原文作者:youzhibing2904

遗留问题

在关于利用maven搭建ssm的博客,我们一起来探讨下问的最多的问题中,我遗留了一个问题:Spring mvc是何时、何地、如何将Model中的属性绑定到哪个作用域,这里的作用域指的是Servlet的四大作用域;不了解问题背景的可以回过头去看看我的上篇博文。

明确的解答我会放到最后,在解答问题之前,我先和大家一起来捋一捋Spring mvc的工作原理。废话不多说,开始我们神秘的探险之旅!

应用示例

在讲工作原理之前,我们先看一个简单的spring mvc(ssm)示例,以及实现的效果

工程代码地址:ssm-web 

202207262306212591.png

工程结构与效果如上所示,我们不做过多的探究,我们打起精神往下看本篇的重点

工作原理

准备 - 资源的加载与初始化

1、DispatcherServlet 静态初始化

DispatcherServlet中有如下静态块

    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 'DispatcherServlet.properties': " + ex.getMessage());
     }
     }
    

这里会将
DispatcherServlet.properties中的内容读取到DispatcherServlet的属性:private static final Properties defaultStrategies中,
DispatcherServlet.properties内容如下

    # Default implementation classes for DispatcherServlet's strategy interfaces.
    # Used as fallback when no matching beans are found in the DispatcherServlet context.
    # Not meant to be customized by application developers.
    org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
    org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
    org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
     org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
    org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
     org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
     org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter
    org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
     org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
     org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
    org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
    org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
    org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
    

指定了DispatcherServlet策略接口的默认实现,后续DispatcherServlet初始化策略的时候会用到

2、interceptor定义的加载

spring启动过程中会调用
InterceptorsBeanDefinitionParser的parse方法来解析出我们自定义的interceptor定义,封装成MappedInterceptor类型的bean定义,并放到spring容器中;我们可以简单的认为spring容器中已经存在了我们自定义的interceptor的bean定义

3、DispatcherServlet初始化策略:initStrategies

DispatcherServlet的继承图如下

202207262306240662.png

DispatcherServlet是一个Servlet,tomcat启动过程中会调用其init方法,一串的调用后,会调用DispatcherServlet的initStrategies方法

protected void initStrategies(ApplicationContext context) {

initMultipartResolver(context);

initLocaleResolver(context);

initThemeResolver(context);

initHandlerMappings(context);

initHandlerAdapters(context);

initHandlerExceptionResolvers(context);

initRequestToViewNameTranslator(context);

initViewResolvers(context);

initFlashMapManager(context);

}

实例化步骤1中的默认实现,并填充到DispatcherServlet各个属性值中

4、 DefaultAnnotationHandlerMapping的拦截器初始化

DispatcherServlet.properties种指定了两个默认的HandlerMapping:BeanNameUrlHandlerMapping、
DefaultAnnotationHandlerMapping,这两者的类继承图如下(我们暂时只关注
DefaultAnnotationHandlerMapping)

202207262306245943.png

DefaultAnnotationHandlerMapping间接实现了ApplicationContextAware,那么在
DefaultAnnotationHandlerMapping实例初始化过程中,会调用setApplicationContext(ApplicationContext applicationContext)方法,一串调用后,会来到AbstractUrlHandlerMapping的initApplicationContext()

    @Override
    protected void initApplicationContext() throws BeansException {
     extendInterceptors(this.interceptors);
     detectMappedInterceptors(this.mappedInterceptors);
     initInterceptors();
    }
    

初始化了 DefaultAnnotationHandlerMapping的拦截器:interceptor

我们来看下具体的初始化过程,看看上面的顺序是否只是我个人的臆想?

202207262306251694.png

可以看到,初始化顺序就是我们上面说的,不是我个人的意淫;此时的
DefaultAnnotationHandlerMapping中有我们自定义的MyInterceptor。初始化过程我们需要关注的就是上述这些,下面我们一起看看具体请求的过程

请求的处理

请求从servlet的service开始,一路到DispatcherServlet的doDispatch,如下图

202207262306291965.png

    /**
     * Process the actual dispatching to the handler. 将请求分发到具体的handler,也就是我们的controller
     * <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.
     * @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;
     HandlerExecutionChain mappedHandler = null;
     boolean multipartRequestParsed = false;
     WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    
     try {
     ModelAndView mv = null;
     Exception dispatchException = null;
     try {
     processedRequest = checkMultipart(request);
     multipartRequestParsed = processedRequest != request;
     // Determine handler for the current request. 决定哪个handler来处理当前的请求
     // mappedHandler是由handler和interceptor集合组成的一个执行链,有点类似FilterChain
     mappedHandler = getHandler(processedRequest);
     if (mappedHandler == null || mappedHandler.getHandler() == null) {
     noHandlerFound(processedRequest, response);
     return;
     }
     // Determine handler adapter for the current request. 决定哪个adapter来处理当前的请求
     // handlerMapping是找出适配的handler,而真正回调handler的是adapter
     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()) {
     String requestUri = urlPathHelper.getRequestUri(request);
     logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
     }
     if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
     return;
     }
     }
     // handler的前置处理,也就是调用适配当前url的interceptor的preHandler方法
     if (!mappedHandler.applyPreHandle(processedRequest, response)) {
     return;
     }
     try {
     // Actually invoke the handler. 真正调用handler
     mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
     }
     finally {
     if (asyncManager.isConcurrentHandlingStarted()) {
     return;
     }
     }
     applyDefaultViewName(request, mv);
     // handler的后置处理,也就是调用适配当前url的interceptor的postHandler方法
     mappedHandler.applyPostHandle(processedRequest, response, mv);
     }
     catch (Exception ex) {
     dispatchException = ex;
     }
     // 处理handler返回的结果,会调用适配当前url的interceptor的afterCompletion方法
     // 这里会将响应结果返回给请求者
     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
     mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
     return;
     }
     // Clean up any resources used by a multipart request.
     if (multipartRequestParsed) {
     cleanupMultipart(processedRequest);
     }
     }
    }
    
    

handlerMapping具体如何找到匹配当前url的handler(一般而言就是我们的controller)、handlerAdapter具体如何回调真正的handler,有兴趣的可以自行去跟下,我就不跟了。我们具体看下processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); 这个与我们最初的疑问有关

processDispatchResult

202207262306365046.png

可以看到model中的persons会被设置到request的attributes中,然后转发请求到show_person.jsp,转发过程中request作用域的变量仍然有效,所以show_person.jsp中的jstl标签和el表达式能够取到persons变量,最后将show_person.jsp中的内容填充好之后的静态内容返回给请求者;至此就完成了一次请求的响应

问题解答

回到我们开篇的疑问:Spring mvc是何时、何地、如何将Model中的属性绑定到哪个作用域?想必大家已经知道答案了

Controller中的model、ModelMap的注入由spring mvc完成,这个不是请求传入的参数,用于绑定变量到Servlet作用域;默认情况下,在DispatcherServlet调用了真正的handler之后,将结果返回给请求者的过程中,将model、modelMap中的变量设置到了request的attributes中,转发的过程中,request中的变量仍然有效,所以show_person.jsp中能取到persons这个变量,自此疑问得到解答

总结

1、Spring MVC工作原理图

图是用的别人的,具体是谁的我也不记得了(捂脸)

202207262306410067.png

2、DefaultAnnotationHandlerMapping在spring3.2中被废弃,替换成了 RequestMappingHandlerMapping