TOC
Spring MVC
Spring MVC 中核心关键类是 DispatchServlet, 所有的请求都会通过这个类进行分派调用指定的业务逻辑代码；这个类中最重要的两个入口方法是 initStrategies() 和 doDispatch()
DispatchServlet.initStrategies()
从上图继承关系，可以发现 DispatchServlet 其实就是一个 HttpServlet；在 Web 容器初始化过程中，会自动调用 HttpServlet.init() 方法，下面看看 DispatchServlet 中 init 方法的大致调用；（spring-webmvc-5.1.4 版本）
// HttpServletBean
@Override
public final void init() throws ServletException {
// Set bean properties from init parameters.
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
...
// Let subclasses do whatever initialization they like.
initServletBean();
}
// FrameworkServlet
@Override
protected final void initServletBean() throws ServletException {
...
long startTime = System.currentTimeMillis();
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException | RuntimeException ex) {
logger.error("Context initialization failed", ex);
throw ex;
}
...
}
// FrameworkServlet
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
...
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
synchronized (this.onRefreshMonitor) {
onRefresh(wac);
}
}
...
return wac;
}
// DispatchServlet
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
// DispacthServlet
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
上述代码简化时序图如下
可以发现 DispacthServlet 最终调用了 initStrategies 方法，此方法就是初始化 SpringMVC 9 大组件的入口方法；
SpringMVC 9 大组件
- MultipartResolver：文件上传解析
- LocaleResolver：国际化处理
- ThemeResolver：视图（View）解析
- HandlerMapping：解析 URL 获取 Handler
- HandlerAdapter：适配不同的 Handler
- HandlerExceptionResolver：异常处理解析
- RequestToViewNameTranslator：解析 Request 获取视图名称(ViewName)
- ViewResolver：通过 ViewName 和 Locale 信息解析获取相应的 View
- FlashMapManager：存储一个请求的结果至 FlashMap, 可用于重定向场景；
DispatchServlet.doDispatch()
根据 J2EE 规范，我们知道 Http 请求分为 Get, Post, Put, Delete 请求;分别对应 HttpServlet 中的 doGet(), doPost, doPost, doDelete() 方法，下面我们看看 DispatchServlet 中是如何处理请求的; (spring-webmvc-5.1.4 版本)
// FrameworkServlet
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected final void doPut(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected final void doDelete(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// FrameworkServlet
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
...
try {
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
...
}
}
// DispatchServlet
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
...
try {
doDispatch(request, response);
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
上述代码简化时序图如下：
从中可以发现，SpringMVC 中所有的请求最终都是会通过 DispatchServlet 中的 doDispatch 方法进行处理，下面大概讲解 doDispatch 大致处理逻辑；
// DispatchServlet
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
...
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
...
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
...
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
...
// Set default Veiw Name
applyDefaultViewName(processedRequest, mv);
...
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
// Resolver the View
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
...
}
}
从代码中可以看出，doDispatch 的执行逻辑大致有以下几步：
- 获取 Request 相应的 Handler；
- 获取 Request 相应的 HandlerAdpater;
- 通过 HandlerAdpater 调用指定的 Handler 方法（即我们常写的 Controller 的指定某个方法）；并返回相应的 ModelAndView;
- 通过 RequestToViewNameTranslator 设置默认 ViewName;
- 通过 ViewResovle 解析获得 View;
- 处理 View 信息返回客户端;
实现简单 SpringMVC 功能
上面讲述了 SpringMVC 的基本实现，其中关键的入口类是 DispatchServlet，在容器初始化时，会调用 DispatchServlet 中的 initStrategies 初始 9 大组件；每个请求都会经过 DispatchServlet 中的 doDispatch 方法；下面演示如何自己实现一个SpringMVC;
( 项目 GitHub 地址：https://github.com/Pismery/Pis-Spring-MVC.git)
<!-- web.xml -->
<web-app>
<display-name>Archetype Created Web Application</display-name>
<servlet>
<servlet-name>PisSpringMVC</servlet-name>
<servlet-class>org.pismery.mvc.PisDispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:application.properties</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>PisSpringMVC</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
</web-app>
// DispacthServlet
public class PisDispatcherServlet extends HttpServlet {
private PisHandlerMapping handlerMapping;
private PisHandlerAdapter handlerAdapter;
private List<ViewResolver> viewResolvers;
@Override
public void init() throws ServletException {
// 初始化 Context
PisApplicationContext context = new PisApplicationContext("/application.properties");
// 文件上传
initMultipartResolver(context);
// 国际化
initLocaleResolver(context);
// View 解析
initThemeResolver(context);
// 解析url -> method
initHandlerMappings(context);
// 适配器
initHandlerAdapters(context);
// 异常处理
initHandlerExceptionResolvers(context);
// 从 request 中找到默认 viewName
initRequestToViewNameTranslator(context);
// 将 viewName 和 local (本地化信息) 解析成 View
initViewResolvers(context);
// 存储一个请求结果至 FlashMap ，可用于一个请求的输入是另一个请求的输出的场景；如重定向；
initFlashMapManager(context);
}
private void initHandlerMappings(PisApplicationContext context) {
this.handlerMapping = new PisUrlHandlerMapping(context);
}
private void initHandlerAdapters(PisApplicationContext context) {
if (this.handlerMapping == null) {
return;
}
this.handlerAdapter = new PisAnnotationHandlerAdapter();
}
private void initViewResolvers(PisApplicationContext context) {
String templateRoot = context.getProperties().getProperty("templateRoot");
String rootPath = this.getClass().getClassLoader().getResource(templateRoot).getFile();
File rootDir = new File(rootPath);
for (File templateFile : rootDir.listFiles()) {
if (viewResolvers == null) {
viewResolvers = new ArrayList<>();
}
viewResolvers.add(new PisViewResolver(templateFile.getName(), templateFile));
}
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doDispatch(req, resp);
}
/**
* 所有请求的处理入口
* @param req 请求
* @param resp 响应
* @throws IOException
*/
private void doDispatch(HttpServletRequest req, HttpServletResponse resp) throws IOException {
//1. 根据 url 获取 handler;
PisHandler handler = handlerMapping.getHandler(req);
if (handler == null) {
resp.getWriter().write("404 Pis No found!..");
}
//2. 通过 handler 调用目标方法；
PisModelAndView mv = handlerAdapter.handle(req, resp, handler);
if (mv == null || mv.getViewName() == null) {
resp.getWriter().write("404 Pis No found!..");
return;
}
//3. 解析 View，返回前端
for (ViewResolver viewResolver : viewResolvers) {
if (!mv.getViewName().equalsIgnoreCase(viewResolver.matchView())) {
continue;
}
String parse = viewResolver.parse(mv);
if (parse != null) {
resp.getWriter().write(parse);
break;
}
}
}
}