MVC

作为一个MVC框架，必须回答一下几个问题

• 请求映射到控制器
• 请求数据进行绑定
• 视图渲染

而spring mvc是如何解决这几个问题的呢？

首先，从DispatcherServlet这个入口开始分析，DispatcherServlet继承于FrameworkServlet，而FrameworkServlet继承HttpServletBean，HttpServletBean又继承与HttpServlet，先从
HttpServletBean的init方法开始分析

@Override
public final void init() throws ServletException {//省略// Let subclasses do whatever initialization they like. initServletBean();//这是个空方法，可以给其子类进行重写}

再从其子类FrameworkServlet重写的initServletBean()方法

@Override
protected final void initServletBean() throws ServletException {//日志块省略try {//初始化springMvc的ApplicationContext、并查找是否有spring的ApplicationContext 并将其设置为root applicationContext ,初始化spring mvc的beanthis.webApplicationContext = initWebApplicationContext();//空方法，可扩展initFrameworkServlet();}//catch块，日志省略
}

继续分析DispatcherServlet init()方法

protected void initStrategies(ApplicationContext context) {//初始化处理mine为multipart/form-data的处理器initMultipartResolver(context);//初始化本地化处理器，默认是以用户的http请求的AcceptHeaderLocaleResolverinitLocaleResolver(context);//初始化主题处理器initThemeResolver(context);//初始化HandlerMappingsinitHandlerMappings(context);//初始化处理器适配器initHandlerAdapters(context);//处理异常处理器initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);//处理视图转换initViewResolvers(context);initFlashMapManager(context);
}

从上面代码可以看出来，DispatcherServlet会初始化一些处理器，再分析其处理请求的方法

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 {//判断是否是multipart/form-dataprocessedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// 将请求映射到具体的处理器mappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}对处理器进行适配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;}}拦截器处理if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// 调用具体的控制器进行处理mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);拦截器处理mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}//处理异常、渲染视图processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}//省略
}

上述代码包括了请求到响应 spring做的过程。我们来细化分析一下。

首先我们来分析请求映射到逻辑处理器的过程

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;
}

从这段代码看，其会遍历所有的HandlerMapping，然后获取一个HandlerExecutionChain执行对象，而入参是HttpServletRequest，意味着这个HandlerMapping可以根据http请求的所有信息获取到一个控制器。这是第一个扩展点，我们可以实现一个HandlerMapping来处理请求映射。再看HandlerExecutionChain这个类

public class HandlerExecutionChain {private final Object handler;private HandlerInterceptor[] interceptors;}

这个类最重要的两个属性，可以看出其对执行对象里面包含了两个部分，一个是控制器，另一个则是拦截器数组，这是第二个扩展点，我们可以实现HandlerInterceptor，在控制器处理前后进行一些操作。

我们来这个操作，其会获取一个控制器的适配器
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

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;}}
}

从这个方法的执行，我们可以看出来其依旧是遍历handlerAdapters后返回第一个HandlerAdapter进行处理Handler,所以我们可以实现HandlerAdapter这个接口，自定义对Handler进行处理，这是第三个扩展点

继续玩下走，在processDispatchResult()方法中，先判断是否有exception，若有且不是ModelAndViewDefiningException，则交给HandlerExceptionResolver处理，HandlerExceptionResolver是个接口，可以看出这是第四个扩展点。继续往下走，到render()方法，

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View是不是字符串if (mv.isReference()) {//交给ViewResolvers处理，将其转换成View对象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.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.if (logger.isDebugEnabled()) {logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");}//将数据渲染到视图中try {view.render(mv.getModelInternal(), request, response);}
}

至此springMVC从请求到响应的流程结束，可以看出springMVC玩的是开闭原则，对修改关闭，可扩展。这是个大体流程，下一篇文章将分析其细节化