spring MVC是现在最流行的MVC框架, 很多人说它是一个优秀的框架。实质上是由于Spring MVC加入注解,注解让Spring MVC质变, 这使得开发效率得到了飞速提升。而且它本身就是Spring的一小部分,所以让就不再像Struts2那样,需要插件的支持。也就是说,它们是无缝连接的。但无论多少多么优秀的框架,它们永远都是建立在listener, servlet, filter这些服务器容器组件之上的。可见基础尤其重要。
首先, 如果我们想在Web项目中添加Spring MVC的支持,要在web.xml中配置如下代码:
- <servlet>
- <servlet-name>springMVC</servlet-name>
- <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
- <load-on-startup>1</load-on-startup>
- </servlet>
其中,org.springframework.web.servlet.DispatcherServlet就是整个Spring MVC框架的切入点。与添加Struts类似。Struts是通过Filter作为切入点,而Spring MVC使用的是Servlet。原理其实是一样的,都起到拦截用户请求的作用。Filter是在服务器启动时,即生成一个实例。而Servlet若不设置load-on-startup,则为第一次访问时生成实例。DispatcherServlet需要在服务器启动时生成实例。下面我们通过源码的来解析Spring MVC的大体流程:
-
- public class DispatcherServlet extends FrameworkServlet {
- ...
- protected void initStrategies(ApplicationContext context) {
- ...
- }
-
- protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
- ...
- }
- protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
- ...
- }
- ...
-
- }
当我们发送请求时,DispatcherServlet会进行拦截,而对于Servlet来说。service方法是处理请求的核心,但DispatcherServlet中并没有定义此方法。我们通过父类FrameworkServlet中查看:
- protected void service(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
-
- String method = request.getMethod();
- if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
- processRequest(request, response);
- }
- else {
- super.service(request, response);
- }
- }
processRequest(request,response)方法调用,在这个方法中,调用的了doService(request,response)。而此时doService正是上述的DispatcherServlet中的doService方法。而doService中又调用了doDispatcher方法。那么可以看出,doDispatcher就是DispatcherServlet处理核心方法。
下面为doDispatcher方法的源码:
- <span style="white-space:pre"> </span>
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- 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;
-
-
- mappedHandler = getHandler(processedRequest, false);
- if (mappedHandler == null || mappedHandler.getHandler() == null) {
- noHandlerFound(processedRequest, response);
- return;
- }
-
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- HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
-
-
- 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;
- }
- }
-
- if (!mappedHandler.applyPreHandle(processedRequest, response)) {
- return;
- }
-
- try {
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- mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
- }
- finally {
- if (asyncManager.isConcurrentHandlingStarted()) {
- return;
- }
- }
-
- applyDefaultViewName(request, mv);
- mappedHandler.applyPostHandle(processedRequest, response, mv);
- }
- catch (Exception ex) {
- dispatchException = ex;
- }
- 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()) {
-
- mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
- return;
- }
-
- if (multipartRequestParsed) {
- cleanupMultipart(processedRequest);
- }
- }
- }
首先有几个类需要声明,ModelAndView、HandlerExecutionChain、HandlerMapping、HandlerMethod、HandlerAdapter。
1、HandlerMethod(org.springframework.web.method.HandlerMethod),这个类为中存放了某个bean对象和该bean对象的某个要处理的Method对象。
2、HandlerMapping,作用为通过request对象,获取对应的HandlerMethod对象。
3、HandlerExecutionChain作用为通过加入Interceptor拦截器包装HandlerMapping返回的HandlerMethod对象。让待处理的方法对象与拦截器称为一个整体,即执行链。
4、ModelAndView,顾名思义。Model即MVC的M,View即MVC的V。其对象存放的正是数据与视图信息。
5、HandlerAdapter:作用为具体处理HandlerMethod,即通过它调用某个方法。
下面分析正题:
doDispatcher方法中,首先通过getHandler方法获得handler,如下:
- <span style="white-space:pre"> </span>
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- 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;
- }
遍历所有HandlerMapping并调用每个HandlerMapping对象的getHandler方法,而HandlerMapping为接口,其中AbstractHandlerMapping这个抽象类中实现了HandlerMapping接口,并实现了getHandler方法,如下:
- public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
- Object handler = getHandlerInternal(request);
- if (handler == null) {
- handler = getDefaultHandler();
- }
- if (handler == null) {
- return null;
- }
-
- if (handler instanceof String) {
- String handlerName = (String) handler;
- handler = getApplicationContext().getBean(handlerName);
- }
- return getHandlerExecutionChain(handler, request);
- }
其中Object handler = getHandlerInternal(request);为获取HandlerMethod对象,此方法源码如下:
- <span style="white-space:pre"> </span>
-
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- @Override
- protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
- String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
- if (logger.isDebugEnabled()) {
- logger.debug("Looking up handler method for path " + lookupPath);
- }
- HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
- if (logger.isDebugEnabled()) {
- if (handlerMethod != null) {
- logger.debug("Returning handler method [" + handlerMethod + "]");
- }
- else {
- logger.debug("Did not find handler method for [" + lookupPath + "]");
- }
- }
- return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
- }
在获得HandlerMethod对象之后,通过getHandlerExecutionChain(handler, request)将HandlerMethod对象与Interceptor拦截器封装,成为HandlerExecutionChain的对象。这就是doDispatch方法中mappedHandler引用指向的对象的来源流程。
获得HandlerExecutionChain对象之后,可以通过这个对象获得之前封装进去的HandlerMethod对象。通过getHandler()即可。
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- HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());</span>
而HandlerAdapter接口的具体实现类的对象为真正调用方法,即通过反射调用HandlerMethod对象中封装的某个类的实例所对应的某个方法。
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- mv = ha.handle(processedRequest, response, mappedHandler.getHandler());</span>
返回值为ModelAndView类型的对象,作用就不必多说.而在hanle方法之前和之后有这么两个方法调用:- mappedHandler.applyPreHandle(processedRequest, response)</span>
- mappedHandler.applyPostHandle(processedRequest, response, mv);</span>
作用就是通过执行链中的Interceptor拦截器做拦截处理。功能与Filter相似。
总结如图:
另外应该注意的是,我们若想使用Spring MVC, 就必须有一个类似于Spring的applicationContext.xml那样的配置文件,(默认名为servletName-servlet.xml, 这里的servletName指在web.xml中配置的servlet的name属性)配置文件,。作用不用多说,与Spring的applicationContext.xml类似,作为IOC容器的核心文件, 所有bean组件都通过它来创建。也是IOC的核心。
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